@article{jensen_genereux_solomon_knappe_gilmore_2024, title={Forecasting and Hindcasting PFAS Concentrations in Groundwater Discharging to Streams near a PFAS Production Facility}, volume={9}, ISSN={["1520-5851"]}, url={https://doi.org/10.1021/acs.est.4c06697}, DOI={10.1021/acs.est.4c06697}, abstractNote={Per- and polyfluoroalkyl substances (PFAS) are known to be highly persistent in groundwater, making it vital to develop new approaches to important practical questions such as the time scale for future persistence of PFAS in contaminated groundwater. In the approach presented here, groundwater from beneath streambeds was analyzed for PFAS and age-dated using SF}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Jensen, Craig R. and Genereux, David P. and Solomon, D. Kip and Knappe, Detlef R. U. and Gilmore, Troy E.}, year={2024}, month={Sep} }
 @article{humphrey_solomon_gilmore_macnamara_genereux_mittelstet_zeyrek_zlotnik_jensen_2024, title={Spatial Variation in Transit Time Distributions of Groundwater Discharge to a Stream Overlying the Northern High Plains Aquifer, Nebraska, USA}, volume={60}, ISSN={0043-1397 1944-7973}, url={http://dx.doi.org/10.1029/2022WR034410}, DOI={10.1029/2022WR034410}, abstractNote={AbstractGroundwater transit time distributions (TTDs) describe the spectrum of flow‐weighted apparent ages of groundwater from aquifer recharge to discharge. Regional‐scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age‐dating techniques. We investigate both local and regional‐scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km2) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow‐weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater‐dominated stream segment using 3H, noble gases, 14C, and groundwater flux measurements at the point‐scale (<7.6 cm diameter). Points were organized in transects across the stream width (3–10 points per transect) and transects were clustered in five sampling areas (10–610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial‐aged groundwater. Observations indicate: (a) TTDs can exhibit spatial variability within a watershed and (b) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed.}, number={2}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Humphrey, C. Eric and Solomon, D. Kip and Gilmore, Troy E. and MacNamara, Markus R. and Genereux, David P. and Mittelstet, Aaron R. and Zeyrek, Caner and Zlotnik, Vitaly A. and Jensen, Craig R.}, year={2024}, month={Feb} }
 @article{jensen_genereux_gilmore_solomon_2023, title={Modified Tracer Gas Injection for Measuring Stream Gas Exchange Velocity in the Presence of Significant Temperature Variation}, volume={59}, ISSN={["1944-7973"]}, DOI={10.1029/2023WR034495}, abstractNote={AbstractGas exchange between streams and overlying air is an important physical‐chemical environmental process that is typically determined by injecting a tracer gas into a stream at a steady rate and sampling steady‐state tracer gas concentrations in the stream water. Previous modes of tracer gas injection allow gas‐water partitioning of the tracer gas, making the rate of gas injection and thus the measured gas transfer velocity potentially sensitive to temperature variation. Presented here is a modification to the tracer solution injection method in which a tracer gas solution was prepared in Tedlar® bags from which all headspace was removed before injecting the solution into the stream. Along with four other strategies to prevent a headspace from forming in the bags during tracer injection in the field, this zero‐headspace tracer solution method prevents gas‐water partitioning anywhere in the injection system, allowing a steady delivery of tracer gas to the stream even in the presence of variation in air and/or stream water temperature. A field test of the method in Nebraska yielded a gas transfer velocity of 4.1 m/day, within the range found in the literature for similarly‐sized streams.}, number={6}, journal={WATER RESOURCES RESEARCH}, author={Jensen, Craig R. and Genereux, David P. and Gilmore, Troy E. and Solomon, D. Kip}, year={2023}, month={Jun} }
 @article{zlotnik_solomon_genereux_gilmore_humphrey_mittelstet_zlotnik_2023, title={Theory of an Automatic Seepage Meter and Ramifications for Applications}, volume={59}, ISSN={["1944-7973"]}, DOI={10.1029/2023WR034766}, abstractNote={AbstractA new approach for measuring fluxes across surface water—groundwater interfaces was recently proposed. The Automatic Seepage Meter (ASM) is equipped with a precise water level sensor and digital memory that analyzes water level time series in a vertical tube inserted into a streambed (Solomon et al., 2020, https://doi.org/10.1029/2019WR026983). The ability to infer flux values with high temporal resolution relies on an accurate interpretation of water level dynamics inside the tube. Here, we reduce the three‐dimensional hydrodynamic problem that describes the ASM water level in a variety of field conditions to a single ordinary differential equation. This novel general analytical solution for estimating ASM responses is more comprehensive and flexible than previous approaches and is applicable to the entire range of field conditions, including steady or transient stream stages, evaporation, rainfall, and noise. For example, our analysis determines the timing of the nonmonotonic ASM response to a monotonic linear stream stage variation and explains previously used empirical parabolic approximation for estimating fluxes. We present algorithms for simultaneous inference of vertical interface flux and hydraulic conductivity values together with an example code. We quantify how the accuracy of parameter estimation depends on test duration and noise amplitude and propose how our analysis can be used to optimize field test protocols. On this basis, changing the ASM geometry by increasing the radius and decreasing tube insertion depth may enable ASM field test protocols that estimate interface flux and hydraulic conductivity faster while maintaining desired accuracy. Potential applications of joint parameter estimation are suggested.}, number={10}, journal={WATER RESOURCES RESEARCH}, author={Zlotnik, Vitaly A. and Solomon, D. Kip and Genereux, David P. and Gilmore, Troy E. and Humphrey, C. Eric and Mittelstet, Aaron R. and Zlotnik, Anatoly V.}, year={2023}, month={Oct} }
 @article{rudd_neal_genereux_shea_nichols_2023, title={Vulnerability of wells in unconfined and confined aquifers to modern contamination from flood events}, volume={901}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2023.165729}, abstractNote={Groundwater is a primary potable water supply for coastal North Carolina (NC), but the increased intensity of extreme rainfall events and floods may exacerbate surface and subsurface processes that contribute anthropogenic chemicals to wells in the major confined aquifers of this region. We evaluated groundwater for organic chemicals of emerging concern (CEC) and the presence of tritium using flooded and not-flooded wells in the NC Department of Environmental Quality well monitoring network across the NC Coastal Plain. Flooded wells experienced standing water around the well casing at least once during the study period. Tritium concentrations, which indicate modern water presence (water recharged after 1953), were significantly greater in groundwater from flooded wells than not-flooded wells. In confined aquifers, modern water was detected at greater depths in flooded wells (206 m) than not-flooded wells (100 m). Suspect-screening high resolution mass spectrometry (HRMS) analysis of 150 groundwater samples yielded a total of 382 unique organic chemicals. Each groundwater sample contained, on average, 19 tentatively identified chemicals from the NIST 20 mass spectral database (M1) and 9 USEPA ToxCast chemicals. The number of tentatively-identified chemicals per sample was not significantly different among aquifers demonstrating the pervasive presence of the detected CECs in unconfined and confined aquifers. The presence of modern water in groundwater from flooded wells coincided with higher detection frequencies of certain organic contaminant classes, particularly pharmaceuticals, food additives, and regulated aromatic hydrocarbons. These results indicate that wells in both unconfined and confined aquifers are susceptible to modern water contamination during flood events; this finding has critical public health implications for coastal communities.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Rudd, Hayden and Neal, Andy and Genereux, David P. and Shea, Damian and Nichols, Elizabeth Guthrie}, year={2023}, month={Nov} }
 @article{genereux_2022, title={Addendum to "Quantifying Uncertainty in Tracer-Based Hydrograph Separations" for Three-Component Mixing Problems}, volume={58}, ISSN={["1944-7973"]}, DOI={10.1029/2022WR031987}, abstractNote={AbstractWater mixing problems are common in hydrology. A 1998 paper describing uncertainty analysis in tracer‐based mixing problems is widely cited but does not present the equations needed for three‐component problems. This Commentary presents the equations needed to compute the uncertainties in the mixing fractions for a three‐component water mixing problem based on tracer data.}, number={2}, journal={WATER RESOURCES RESEARCH}, author={Genereux, David P.}, year={2022}, month={Feb} }
 @article{jensen_genereux_gilmore_solomon_mittelstet_humphrey_macnamara_zeyrek_zlotnik_2022, title={Estimating groundwater mean transit time from SF6 in stream water: field example and planning metrics for a reach mass-balance approach}, ISSN={["1435-0157"]}, DOI={10.1007/s10040-021-02435-8}, journal={HYDROGEOLOGY JOURNAL}, author={Jensen, Craig R. and Genereux, David P. and Gilmore, Troy E. and Solomon, D. Kip and Mittelstet, Aaron R. and Humphrey, C. Eric and MacNamara, Markus R. and Zeyrek, Caner and Zlotnik, Vitaly A.}, year={2022}, month={Jan} }
 @article{nickels_genereux_knappe_2023, title={Improved Darcian streambed measurements to quantify flux and mass discharge of volatile organic compounds from a contaminated aquifer to an urban stream}, volume={253}, ISSN={["1873-6009"]}, DOI={10.1016/j.jconhyd.2022.104124}, abstractNote={Quantifying VOC transport from contaminated groundwater to streams is challenging and important for understanding off-site migration of VOCs, cross-media contamination (groundwater to surface water and eventually air), and potential impacts on downstream ecosystems and human populations. A streambed point sampling approach was used to quantify fluxes of water and 14 VOCs from groundwater to an urban stream in North Carolina, USA, during summer (June 2015) and winter (January 2016). The approach is unique in coupling measurements of vertical hydraulic conductivity, vertical hydraulic head gradient, and groundwater VOC concentration at each individual sampling point, reducing or eliminating some potential concerns with other Darcian methods for quantifying VOC inputs to streams. Most results were consistent with discharge of two main VOC plumes on opposite sides of the stream. Plume 1 from the west side was dominated by cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) at mean concentrations of 19 and 11 μg L−1, respectively. Plume 2 from the east side was dominated by benzene (mean concentration 56 μg L−1). Plume 2 was not previously known, and the improved sampling approach allowed VOC discharge from both plumes to be quantified simultaneously. For 13 of the 14 detected VOCs, the mean VOC flux from groundwater to the stream (fVOC) was higher in January 2016 than in June 2015, mainly because groundwater flux was higher in January. The only exception was cDCE, the most abundant VOC in Plume 1, which had mean fVOC values of 9.8 and 9.5 mg m−2 d−1 in June 2015 and January 2016, respectively. Benzene was the most abundant VOC in Plume 2 and had mean fVOC values of 11 and 37 mg m−2 d−1 in June 2015 and January 2016, respectively. High groundwater flux drove almost all the occurrences of high VOC flux. For a given VOC, the flow-weighted mean concentration (with each VOC concentration weighted by the upward groundwater flux at the VOC sampling point) was generally larger than the unweighted mean concentration. Thus, flow-weighting of concentrations gave a more accurate indication of the average VOC concentration in net groundwater discharge to the stream. An estimate of total VOC mass discharge from groundwater to the study reach of the stream, 3.6 kg of VOC per year, was based on the fVOC results and streambed area in the reach. The bulk of this discharge was due to benzene, cDCE, and VC, with individual mass discharges of 2.1, 0.83, and 0.40 kg yr−1, respectively. Estimates of maximum potential VOC degradation in the streambed suggest that the 3.6 kg yr−1 estimate of mass discharge was not sensitive to potential degradation of VOCs in the streambed sediments above the groundwater sampling depth.}, journal={JOURNAL OF CONTAMINANT HYDROLOGY}, author={Nickels, J. L. and Genereux, D. P. and Knappe, D. R. U.}, year={2023}, month={Feb} }
 @article{zeyrek_mittelstet_gilmore_zlotnik_solomon_genereux_humphrey_shrestha_2023, title={Modeling groundwater transit time distributions and means across a Nebraska watershed: Effects of heterogeneity in the aquifer, riverbed, and recharge parameters}, volume={617}, ISSN={["1879-2707"]}, DOI={10.1016/j.jhydrol.2022.128891}, abstractNote={Groundwater transit time distributions (TTDs), the travel times through the aquifer from recharge at the water table to discharge at the surface water body, provide critical information on the timescales of hydrologic response of subsurface flow systems. We investigated the effects of spatial patterns of recharge, aquifer heterogeneity, and systematic variation in riverbed hydraulic conductivity on the mean transit times (MTTs) and TTDs of groundwater discharge to the Upper Middle Loup River (UMLR) and headwaters using a 3D-steady-state MODFLOW USG-MODPATHD3DU model. This 5436 km2 watershed overlies the High Plains Aquifer in the Sand Hills of Nebraska, USA. Modeled MTTs differed by up to three orders of magnitude from upstream to downstream in a 158 km section of the UMLR under varying recharge, aquifer, and riverbed heterogeneity scenarios. The simulated MTTs ranged from 1 to 397 years for upstream sites and 820 to 7968 years for the downstream sites. The TTDs at upstream sites were dominated by young groundwater from shallow flow paths and were sensitive to changes in riverbed hydraulic conductivity. Recharge parameterization had greater influence on the shape of the TTDs and magnitude of MTTs at the downstream sites, where much older groundwater discharged to the UMLR. Overall, spatial trends in transit times under varying model scenarios provided important information for refined conceptualization and calibration of future numerical models.}, journal={JOURNAL OF HYDROLOGY}, author={Zeyrek, Caner and Mittelstet, Aaron R. and Gilmore, Troy E. and Zlotnik, Vitaly and Solomon, D. Kip and Genereux, David P. and Humphrey, C. Eric and Shrestha, Nawaraj}, year={2023}, month={Feb} }
 @article{marzolf_small_oviedo-vargas_ganong_duff_ramirez_pringle_genereux_ardon_2022, title={Partitioning inorganic carbon fluxes from paired O-2-CO2 gas measurements in a Neotropical headwater stream, Costa Rica}, volume={7}, ISSN={["1573-515X"]}, url={https://doi.org/10.1007/s10533-022-00954-4}, DOI={10.1007/s10533-022-00954-4}, abstractNote={The role of streams and rivers in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO2 evasion (FCO2) to the atmosphere is high. Stream C cycling is understudied in the tropics compared to temperate streams, and tropical streams may have among the highest FCO2 due to higher temperatures, continuous organic matter inputs, and high respiration rates both in-stream and in surrounding soils. In this paper, we present paired in-stream O2 and CO2 sensor data from a headwater stream in a lowland rainforest in Costa Rica to explore temporal variability in gas concentrations and ecosystem processes. Further, we estimate groundwater CO2 inputs (GWCO2) from riparian well CO2 measurements. Paired O2–CO2 data reveal stream CO2 supersaturation driven by groundwater CO2 inputs and large in-stream production of CO2. At short time scales, CO2 was diluted during storm events, but increased at longer seasonal scales. Areal fluxes in our study reach show that FCO2 is supported by greater in-stream metabolism compared to GWCO2. Our results underscore the importance of tropical headwater streams as large contributors of carbon dioxide to the atmosphere and show evaded C can be derived from both in-stream and terrestrial sources.}, journal={BIOGEOCHEMISTRY}, publisher={Springer Science and Business Media LLC}, author={Marzolf, Nicholas S. and Small, Gaston E. and Oviedo-Vargas, Diana and Ganong, Carissa N. and Duff, John H. and Ramirez, Alonso and Pringle, Catherine M. and Genereux, David P. and Ardon, Marcelo}, year={2022}, month={Jul} }
 @article{petre_salk_stapleton_ferguson_tait_obenour_knappe_genereux_2022, title={Per- and polyfluoroalkyl substances (PFAS) in river discharge: Modeling loads upstream and downstream of a PFAS manufacturing plant in the Cape Fear watershed, North Carolina}, volume={831}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2022.154763}, abstractNote={The Cape Fear River is an important source of drinking water in North Carolina, and many drinking water intakes in the watershed are affected by per- and polyfluoroalkyl substances (PFAS). We quantified PFAS concentrations and loads in river water upstream and downstream of a PFAS manufacturing plant that has been producing PFAS since 1980. River samples collected from September 2018 to February 2021 were analyzed for 13 PFAS at the upstream station and 43-57 PFAS downstream near Wilmington. Frequent PFAS sampling (daily to weekly) was conducted close to gauging stations (critical to load estimation), and near major drinking water intakes (relevant to human exposure). Perfluoroalkyl acids dominated upstream while fluoroethers associated with the plant made up about 47% on average of the detected PFAS downstream. Near Wilmington, Σ43PFAS concentration averaged 143 ng/L (range 40-377) and Σ43PFAS load averaged 3440 g/day (range 459-17,300), with 17-88% originating from the PFAS plant. LOADEST was a useful tool in quantifying individual and total quantified PFAS loads downstream, however, its use was limited at the upstream station where PFAS levels in the river were affected by variable inputs from a wastewater treatment plant. Long-term monitoring of PFAS concentrations is warranted, especially at the downstream station. Results suggest a slight downward trend in PFAS levels downstream, as indicated by a decrease in flow-weighted mean concentrations and the best-fitting LOADEST model. However, despite the cessation of PFAS process wastewater discharge from the plant in November 2017, and the phase-out of perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in North America, both fluoroethers and legacy PFAS continue to reach the river in significant quantities, reflecting groundwater discharge to the river and other continuing inputs. Persistence of PFAS in surface water and drinking water supplies suggests that up to 1.5 million people in the Cape Fear watershed might be exposed.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Petre, M-A and Salk, K. R. and Stapleton, H. M. and Ferguson, P. L. and Tait, G. and Obenour, D. R. and Knappe, D. R. U. and Genereux, D. P.}, year={2022}, month={Jul} }
 @article{humphrey_solomon_genereux_gilmore_mittelstet_zlotnik_zeyrek_jensen_macnamara_2022, title={Using Automated Seepage Meters to Quantify the Spatial Variability and Net Flux of Groundwater to a Stream}, volume={58}, ISSN={["1944-7973"]}, DOI={10.1029/2021WR030711}, abstractNote={AbstractWe utilized 251 measurements from a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA to investigate the small‐scale spatial variability of groundwater seepage flux (q) and the ability of the ASM to estimate mean q at larger scales. Small‐scale spatial variability of q was analyzed in five dense arrays, each covering an area of 13.5–28.0 m2 (169 total point measurements). Streambed vertical hydraulic conductivity (K) was also measured. Results provided: (a) high‐resolution contour plots of q and K, (b) anisotropic semi‐variograms demonstrating greater correlation scales of q and K along the stream length than across the stream width, and (c) the number of rows of points (perpendicular to streamflow) needed to represent the groundwater flux of areas up to 28.0 m2. The findings suggest that representative streambed measurements are best conducted perpendicular to streamflow to accommodate larger seepage flux heterogeneity in this direction and minimize sampling redundancy. To investigate the ASM's ability to produce accurate mean q at larger scales, seepage meters were deployed in four stream reaches (170–890 m), arranged in three to six transects (three to eight points each) per reach across the channel. In each reach, the mean seepage flux from ASMs was compared to the seepage flux from bromide tracer dilution. Agreement between the two methods indicates the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the stream and can guide sampling for solutes and environmental tracers.}, number={6}, journal={WATER RESOURCES RESEARCH}, author={Humphrey, C. Eric and Solomon, D. Kip and Genereux, David P. and Gilmore, Troy E. and Mittelstet, Aaron R. and Zlotnik, Vitaly A. and Zeyrek, Caner and Jensen, Craig R. and MacNamara, Markus R.}, year={2022}, month={Jun} }
 @article{petre_genereux_koropeckyj-cox_knappe_duboscq_gilmore_hopkins_2021, title={Per- and Polyfluoroalkyl Substance (PFAS) Transport from Groundwater to Streams near a PFAS Manufacturing Facility in North Carolina, USA}, volume={55}, ISSN={["1520-5851"]}, url={https://doi.org/10.1021/acs.est.0c07978}, DOI={10.1021/acs.est.0c07978}, abstractNote={We quantified per- and polyfluoroalkyl substance (PFAS) transport from groundwater to five tributaries of the Cape Fear River near a PFAS manufacturing facility in North Carolina (USA). Hydrologic and PFAS data were coupled to quantify PFAS fluxes from groundwater to the tributaries. Up to 29 PFAS were analyzed, including perfluoroalkyl acids and recently identified fluoroethers. Total quantified PFAS (ΣPFAS) in groundwater was 20–4773 ng/L (mean = 1863 ng/L); the range for stream water was 426–3617 ng/L (mean = 1717 ng/L). Eight PFAS constituted 98% of ΣPFAS; perfluoro-2-(perfluoromethoxy)propanoic acid (PMPA) and hexafluoropropylene oxide dimer acid (GenX) accounted for 61%. For PFAS discharge from groundwater to one tributary, values estimated from stream water measurements (18 ± 4 kg/yr) were similar to those from groundwater measurements in streambeds (22–25 ± 5 kg/yr). At baseflow, 32 ± 7 kg/yr of PFAS discharged from groundwater to the five tributaries, eventually reaching the Cape Fear River. Given the PFAS emission timeline at the site, groundwater data suggest the abundant fluoroethers moved through the subsurface to streams in ≪50 yr. Discharge of contaminated groundwater may lead to long-term contamination of surface water and impacts on downstream drinking water supplies. This work addresses a gap in the PFAS literature: quantifying PFAS mass transfer between groundwater and surface water using field data.}, number={9}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, publisher={American Chemical Society (ACS)}, author={Petre, Marie-Amelie and Genereux, David P. and Koropeckyj-Cox, Lydia and Knappe, Detlef R. U. and Duboscq, Sandrine and Gilmore, Troy E. and Hopkins, Zachary R.}, year={2021}, month={May}, pages={5848–5856} }
 @article{solomon_humphrey_gilmore_genereux_zlotnik_2020, title={An Automated Seepage Meter for Streams and Lakes}, volume={56}, ISSN={["1944-7973"]}, DOI={10.1029/2019WR026983}, abstractNote={AbstractWe describe a new automatic seepage meter for use in soft bottom streams and lakes. The meter utilizes a thin‐walled tube that is inserted into the streambed or lakebed. A hole in the side of the tube is fitted with an electric valve. Prior to the test, the valve is open and the water level inside the tube is the same as the water level outside the tube. The test starts with closure of the valve, and the water level inside the tube changes as it moves toward the equilibrium hydraulic head that exists at the bottom of the tube. The time rate of change of the water level immediately after the valve closes is a direct measure of the seepage rate (q). The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ±0.1 mm. The meter can also provide an estimate of vertical hydraulic conductivity (K

v
) if data are collected for a characteristic time. The detection limit for q depends on the vertical hydraulic head gradient. For K

v
 = 1 m/day, q of about 2 mm/day can be measured. Results from a laboratory sand tank show excellent agreement between measured and true q, and results from a field site are similar to values from calculations based on independent measurements of K

v
 and vertical head gradients. The meter can provide rapid (30 min) q measurements for both gaining and losing systems and complements other methods for quantifying surface water groundwater interactions.}, number={4}, journal={WATER RESOURCES RESEARCH}, author={Solomon, D. Kip and Humphrey, Eric and Gilmore, Troy E. and Genereux, David P. and Zlotnik, Vitaly}, year={2020}, month={Apr} }
 @article{stewart-maddox_frisbee_andronicos_genereux_meyers_2018, title={Identifying the regional extent and geochemical evolution of interbasin groundwater flow using geochemical inverse modeling and Sr-87/Sr-86 ratios in a complex conglomeratic aquifer}, volume={500}, ISSN={["1878-5999"]}, DOI={10.1016/j.chemgeo.2018.07.026}, abstractNote={Interbasin groundwater flow (IGF) occurs when water that is recharged in one watershed or basin discharges into an adjacent watershed or basin. IGF can directly impact the water and solute mass discharged from a watershed and consequently, skew estimates of watershed-scale weathering rates. It is extremely difficult to determine the impact of IGF on weathering processes in adjacent watersheds because it is difficult to differentiate between solute mass originating within the watershed from solute mass transported into the watershed from sources outside the watershed. We quantify the regional extent of IGF in three watersheds (Canjilon, El Rito, and Vallecitos) draining the Tusas Mountains of northern New Mexico, USA (sites where IGF has been shown to occur) using spatial trends in 87Sr/86Sr in springs and streams and geochemical modeling using PHREEQC. 87Sr/86Sr ratios show a strong connection from the Canjilon watershed to the El Rito watershed and suggest that the IGF connection may extend to the Vallecitos watershed. Geochemical modeling constrained by XRD analysis of aquifer mineralogy indicates that IGF from the Canjilon watershed is the dominant control on perennial flow in springs and streams in the adjacent El Rito watershed. However, the geochemical signature of an IGF connection from El Rito to the Vallecitos watershed is weak to non-existent. The maximum possible IGF contribution from El Rito to Vallecitos is during snowmelt, contributing as much as 20% of the water and solute mass to Vallecitos. However, the maximum IGF contribution is <5% during summer and fall months. In the El Rito watershed, the solute load is controlled by weathering kinetics occurring from within and outside the drainage area of the watershed. This diverges from the traditional watershed weathering model which assumes that the solute mass comes from only within the drainage area of the watershed. In comparison, the Vallecitos watershed conforms more to the traditional watershed weathering model. Our methodology is unique because it accounts for the background geochemical evolution of IGF in addition to mixing with locally-recharged groundwater. The mineral-weathering models, when combined with 87Sr/86Sr ratios, appear to be a robust methodology in sorting out the geochemical kinetics of watersheds connected through IGF and may also offer unique capabilities to investigate the regional extent of IGF in other settings.}, journal={CHEMICAL GEOLOGY}, author={Stewart-Maddox, Noah S. and Frisbee, Marty D. and Andronicos, Christopher L. and Genereux, David P. and Meyers, Zachary P.}, year={2018}, month={Nov}, pages={20–29} }
 @article{osburn_oviedo-vargas_barnett_dierick_oberbauer_genereux_2018, title={Regional Groundwater and Storms Are Hydrologic Controls on the Quality and Export of Dissolved Organic Matter in Two Tropical Rainforest Streams, Costa Rica}, volume={123}, ISSN={2169-8953}, url={http://dx.doi.org/10.1002/2017JG003960}, DOI={10.1002/2017jg003960}, abstractNote={AbstractA paired‐watershed approach was used to compare the quality and fluxes of dissolved organic matter (DOM) during stormflow and baseflow in two lowland tropical rainforest streams located in northeastern Costa Rica. The Arboleda stream received regional groundwater (RGW) flow, whereas the Taconazo stream did not. DOM quality was assessed with absorbance and fluorescence and stable carbon isotope (δ13C‐DOC) values. RGW DOM lacked detectable fluorescence and had specific ultraviolet absorption (SUVA254) and absorbance slope ratio (SR) values consistent with low aromaticity and low molecular weight material, respectively. We attributed these properties to microbial degradation and sorption of humic DOM to mineral surfaces during transport through bedrock. SUVA254 values were lower and SR values were higher in the Arboleda stream during baseflow compared to the Taconazo stream, presumably due to dilution by RGW. However, no significant difference in SUVA254 or SR occurred between the streams during stormflow. SUVA254 was negatively correlated to δ13C‐DOC (r2 = 0.61, P < 0.001), demonstrating a strong linkage between stream DOM characteristics and the relative amounts of RGW flow and local watershed runoff containing soil and throughfall C sources. Mean DOC export from the Taconazo stream during the study period was 2.62 ± 0.39 g C m−2 year−1, consistent with other tropical streams, yet mean DOC export from the Arboleda stream was 13.79 ± 2.07 g C m−2 year−1, one of the highest exports reported and demonstrating a substantial impact of old RGW from outside the watershed boundary can have on surface water carbon cycling.}, number={3}, journal={Journal of Geophysical Research: Biogeosciences}, publisher={American Geophysical Union (AGU)}, author={Osburn, Christopher L. and Oviedo-Vargas, Diana and Barnett, Emily and Dierick, Diego and Oberbauer, Steven F. and Genereux, David P.}, year={2018}, month={Mar}, pages={850–866} }
 @article{koh_genereux_koh_ko_2017, title={Relationship of groundwater geochemistry and flow to volcanic stratigraphy in basaltic aquifers affected by magmatic CO2, Jeju Island, Korea}, volume={467}, ISSN={["1872-6836"]}, DOI={10.1016/j.chemgeo.2017.08.009}, abstractNote={Groundwater from pristine volcanic rock aquifers on dormant Jeju Island, Korea, was investigated to identify sources and geochemical processes of dissolved inorganic carbon (DIC) and related solutes along with characteristics of groundwater flow using a multi-isotope approach. δ18O, δ2H, and 3H of groundwater indicated the water is of meteoric origin, and is mainly derived from recent recharge, except Na-rich mineral water which has a significant contribution of 3H-free old water. 87Sr/86Sr of dissolved Sr suggests that solutes in groundwater were mainly derived from old alkali basalt in the lower part of lava flows. δ13CCO2(g) calculated using δ13CDIC and measured hydrogeochemical parameters indicated that (1) biogenic soil CO2 is a dominant source of DIC in low mineralized water, (2) CO2-rich water had mostly mantle-derived magmatic CO2, and (3) mineral water contained both biogenic and magmatic CO2. δ13CCO2(g) in mineral water was affected by precipitation of calcite, which complicated the estimation of the contribution of CO2 sources to DIC. Concentrations of mobile elements were mainly controlled by the amount of magmatic CO2 and groundwater residence time. δ13CCO2(g) and 3H content were used to distinguish four types of groundwater with different spatial patterns of occurrence in relation to the volcanic stratigraphy of Jeju Island. Groundwater types I and II are low mineralized water with lower levels of DIC from biogenic CO2, composed of either recent recharge (< 40 years, type I) or older water with negligible 3H (> 60 years, type II). Type III is Mg-rich mineral water with significant recent recharge while type IV is Na-rich mineral water with older residence time. Types III and IV had somewhat elevated levels of DIC (up to 7.6 mM) with mixed signatures of magmatic and biogenic CO2, while CO2-rich water (a subset of type III) had DIC up to 100 mM with signatures of mantle-derived magmatic CO2. Groundwater in the eastern area of Jeju Island is mostly type I, which can be attributed to a lack of both low-permeability rocks and recent volcanic rocks in that area. Types III and IV were found in the western area where the latest volcanic rocks are distributed. In the southwestern area, type III is associated with permeable basaltic aquifers with a contribution of magmatic CO2 and type IV is from the younger lava flows dissected by older massive trachytic rocks and shallower hydrovolcanic tuffs, which led to the higher degree of mineralization. A conceptual model is proposed to integrate the chemical and isotopic constraints of groundwater with volcanic stratigraphy and suggest the processes that generated groundwater types with different residence times and chemical compositions. This study may contribute to understanding the processes of groundwater flow and mineralization in volcanic aquifers affected by deep-source CO2 and variable residence time of groundwater in both dormant and active volcanic areas.}, journal={CHEMICAL GEOLOGY}, author={Koh, Dong-Chan and Genereux, David P. and Koh, Gi-Won and Ko, Kyung-Seok}, year={2017}, month={Sep}, pages={143–158} }
 @article{oviedo-vargas_dierick_genereux_oberbauer_2016, title={Chamber measurements of high CO2 emissions from a rainforest stream receiving old C-rich regional groundwater}, volume={130}, ISSN={["1573-515X"]}, DOI={10.1007/s10533-016-0243-3}, abstractNote={Carbon emissions from fluvial systems are a key component of local and regional carbon cycles. We used floating chambers to investigate the CO2 flux from stream water to air ( $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ ) in the Arboleda, a stream in the lowland rainforest of Costa Rica, fed partly by old regional groundwater high in dissolved inorganic carbon (DIC). Drifting and static chambers showed $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ averaging 35.5 and 72.7 μmol C m−2 s−1, respectively, bracketing the previously-published $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ value of 56 μmol C m−2 s−1 obtained using tracer methods in this stream. These values are much higher than most $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ data in the literature and reflect a large flux of deep crustal (non-biogenic) CO2 out of the Arboleda, a flux that does not represent a component of ecosystem respiration. Static chambers appeared to overestimate $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ by creating artificial turbulence, while drifting chambers may have underestimated $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ by under-sampling areas of potentially high gas exchange (e.g., riffles around coarse woody debris obstructions). Both static and drifting chambers revealed high spatial heterogeneity in $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ at the scale of 5–30 m reaches. Some observed temporal trends were localized, e.g., among three reaches with repeated measurements through the wet and dry seasons, (1) only the reach located between the other two showed significantly lower $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ during the dry season, and (2) the highest and lowest $${\text{f}}_{{{\text{CO}}_{ 2} }}$$ were consistently observed in the reaches farthest upstream and downstream, respectively. Streams like the Arboleda receiving significant inputs of high-DIC regional groundwater merit additional study as hotspots for C emissions from terrestrial ecosystems.}, number={1-2}, journal={BIOGEOCHEMISTRY}, author={Oviedo-Vargas, Diana and Dierick, Diego and Genereux, David P. and Oberbauer, Steven F.}, year={2016}, month={Oct}, pages={69–83} }
 @article{heilweil_solomon_darrah_gilmore_genereux_2016, title={Gas-Tracer Experiment for Evaluating the Fate of Methane in a Coastal Plain Stream: Degassing versus in-Stream Oxidation}, volume={50}, ISSN={["1520-5851"]}, DOI={10.1021/acs.est.6b02224}, abstractNote={Methane emissions from streams and rivers have recently been recognized as an important component of global greenhouse budgets. Stream methane is lost as evasion to the atmosphere or in-stream methane oxidation. Previous studies have quantified evasion and oxidation with point-scale measurements. In this study, dissolved gases (methane, krypton) were injected into a coastal plain stream in North Carolina to quantify stream CH4 losses at the watershed scale. Stream-reach modeling yielded gas transfer and oxidation rate constants of 3.2 ± 0.5 and 0.5 ± 1.5 d-1, respectively, indicating a ratio of about 6:1. The resulting evasion and oxidation rates of 2.9 mmol m-2 d-1 and 1,140 nmol L-1 d-1, respectively, lie within ranges of published values. Similarly, the gas transfer velocity (K600) of 2.1 m d-1 is consistent with other gas tracer studies. This study illustrates the utility of dissolved-gas tracers for evaluating stream methane fluxes. In contrast to point measurements, this approach provides a larger watershed-scale perspective. Further work is needed to quantify the magnitude of these fluxes under varying conditions (e.g., stream temperature, nutrient load, gradient, flow rate) at regional and global scales before reliable bottom-up estimates of methane evasion can be determined at global scales.}, number={19}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Heilweil, Victor M. and Solomon, D. Kip and Darrah, Thomas H. and Gilmore, Troy E. and Genereux, David P.}, year={2016}, month={Oct}, pages={10504–10511} }
 @article{gilmore_genereux_solomon_solder_2016, title={Groundwater transit time distribution and mean from streambed sampling in an agricultural coastal plain watershed, North Carolina, USA}, volume={52}, ISSN={["1944-7973"]}, DOI={10.1002/2015wr017600}, abstractNote={AbstractWe measured groundwater apparent age (τ) and seepage rate (v) in a sandy streambed using point‐scale sampling and seepage blankets (a novel seepage meter). We found very similar MTT estimates from streambed point sampling in a 58 m reach (29 years) and a 2.5 km reach (31 years). The TTD for groundwater discharging to the stream was best fit by a gamma distribution model and was very similar for streambed point sampling in both reaches. Between adjacent point‐scale and seepage blanket samples, water from the seepage blankets was generally younger, largely because blanket samples contained a fraction of “young” stream water. Correcting blanket data for the stream water fraction brought τ estimates for most blanket samples closer to those for adjacent point samples. The MTT estimates from corrected blanket data were in good agreement with those from sampling streambed points adjacent to the blankets. Collectively, agreement among age‐dating tracers, general accord between tracer data and piston‐flow model curves, and large groundwater age gradients in the streambed, suggested that the piston flow apparent ages were reasonable estimates of the groundwater transit times for most samples. Overall, our results from two field campaigns suggest that groundwater collected in the streambed can provide reasonable estimates of apparent age of groundwater discharge, and that MTT can be determined from different age‐dating tracers and by sampling with different groundwater collection devices. Coupled streambed point measurements of groundwater age and groundwater seepage rate represent a novel, reproducible, and effective approach to estimating aquifer TTD and MTT.}, number={3}, journal={WATER RESOURCES RESEARCH}, author={Gilmore, Troy E. and Genereux, David P. and Solomon, D. Kip and Solder, John E.}, year={2016}, month={Mar}, pages={2025–2044} }
 @article{burnette_genereux_birgand_2016, title={In-situ falling-head test for hydraulic conductivity: Evaluation in layered sediments of an analysis derived for homogenous sediments}, volume={539}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/j.jhydrol.2016.05.030}, DOI={10.1016/j.jhydrol.2016.05.030}, abstractNote={The hydraulic conductivity (K) of streambeds is a critical variable controlling interaction of groundwater and surface water. The Hvorslev analysis for estimating K from falling-head test data has been widely used since the 1950s, but its performance in layered sandy sediments common in streams and lakes has not previously been examined. Our numerical simulations and laboratory experiments show that the Hvorslev analysis yields accurate K values in both homogenous sediment (for which the analysis was originally derived) and layered deposits with low-K sand over high-K sand. K from the Hvorslev analysis deviated significantly from true K only when two conditions were present together: (1) high-K sand was present over low-K sand, and (2) the bottom of the permeameter in which K was measured was at or very near the interface between high-K and low-K. When this combination of conditions exists, simulation and laboratory sand tank results show that in-situ Hvorslev K underestimates the true K of the sediment within a permeameter, because the falling-head test is affected by low-K sediment outside of (below the bottom of) the permeameter. In simulation results, the maximum underestimation (occurring when the bottom of the permeameter was at the interface of high K over low K) was by a factor of 0.91, 0.59, and 0.12 when the high-K to low-K ratio was 2, 10, and 100, respectively. In laboratory sand tank experiments, the underestimation was by a factor of about 0.83 when the high-K to low-K ratio was 2.3. Also, this underestimation of K by the Hvorslev analysis was about the same whether the underlying low-K layer was 2 cm or 174 cm thick (1% or 87% of the domain thickness). Numerical model simulations were useful in the interpretation of in-situ field K profiles at streambed sites with layering; specifically, scaling the model results to the maximum measured K at the top of the field K profiles helped constrain the likely ratio of high K to low K at field locations with layered heterogeneity. Vertical K values are important in field studies of groundwater–surface water interaction, and the Hvorslev analysis can be a useful tool, even in layered media, when applied carefully.}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Burnette, Matthew C. and Genereux, David P. and Birgand, François}, year={2016}, month={Aug}, pages={319–329} }
 @article{gilmore_genereux_solomon_farrell_mitasova_2016, title={Quantifying an aquifer nitrate budget and future nitrate discharge using field data from streambeds and well nests}, volume={52}, ISSN={["1944-7973"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85005896258&partnerID=MN8TOARS}, DOI={10.1002/2016wr018976}, abstractNote={AbstractNovel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage, 
/dt, in a contaminated unconfined aquifer, and (2) future [
]FWM (the flow‐weighted mean nitrate concentration in groundwater discharge) and 
 (the nitrate flux from aquifer to stream). Estimates of 
/dt suggested that at the time of sampling (2013) the nitrate storage in the aquifer was decreasing at an annual rate (mean = −9 mmol/m2yr) equal to about one‐tenth the rate of nitrate input by recharge. This is consistent with data showing a slow decrease in the [
] of groundwater recharge in recent years. Regarding future [
]FWM and 
, predictions based on well data show an immediate decrease that becomes more rapid after ∼5 years before leveling out in the early 2040s. Predictions based on streambed data generally show an increase in future [
]FWM and 
 until the late 2020s, followed by a decrease before leveling out in the 2040s. Differences show the potential value of using information directly from the groundwater—surface water interface to quantify the future impact of groundwater nitrate on surface water quality. The choice of denitrification kinetics was similarly important; compared to zero‐order kinetics, a first‐order rate law levels out estimates of future [
]FWM and 
 (lower peak, higher minimum) as legacy nitrate is flushed from the aquifer. Major fundamental questions about nonpoint‐source aquifer contamination can be answered without a complex numerical model or long‐term monitoring program.}, number={11}, journal={WATER RESOURCES RESEARCH}, author={Gilmore, Troy E. and Genereux, David P. and Solomon, D. Kip and Farrell, Kathleen M. and Mitasova, Helena}, year={2016}, month={Nov}, pages={9046–9065} }
 @article{gilmore_genereux_solomon_solder_kimball_mitasova_birgand_2016, title={Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream-based observations at three measurement scales}, volume={52}, ISSN={0043-1397}, url={http://dx.doi.org/10.1002/2015WR017599}, DOI={10.1002/2015wr017599}, abstractNote={AbstractWe compared three stream‐based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage‐meter design), and reach mass‐balance. The methods gave similar mean groundwater seepage rates into the stream (0.3–0.6 m/d) during two 3–4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flow‐weighted mean nitrate concentrations in groundwater discharge ([
]FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [
]FWM was 654, 561, and 451 µM for point, blanket, and reach mass‐balance sampling, respectively. At high flow the trend was reversed, likely because reach mass‐balance captured inputs from shallow transient high‐nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass‐balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel‐based nitrate removal). Modeling dissolved N2 from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700–1300 µM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.}, number={3}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Gilmore, Troy E. and Genereux, David P. and Solomon, D. Kip and Solder, John E. and Kimball, Briant A. and Mitasova, Helena and Birgand, François}, year={2016}, month={Mar}, pages={1961–1983} }
 @article{solder_gilmore_genereux_solomon_2016, title={A Tube Seepage Meter for In Situ Measurement of Seepage Rate and Groundwater Sampling}, volume={54}, ISSN={["1745-6584"]}, DOI={10.1111/gwat.12388}, abstractNote={AbstractWe designed and evaluated a “tube seepage meter” for point measurements of vertical seepage rates (q), collecting groundwater samples, and estimating vertical hydraulic conductivity (K) in streambeds. Laboratory testing in artificial streambeds show that seepage rates from the tube seepage meter agreed well with expected values. Results of field testing of the tube seepage meter in a sandy‐bottom stream with a mean seepage rate of about 0.5 m/day agreed well with Darcian estimates (vertical hydraulic conductivity times head gradient) when averaged over multiple measurements. The uncertainties in q and K were evaluated with a Monte Carlo method and are typically 20% and 60%, respectively, for field data, and depend on the magnitude of the hydraulic gradient and the uncertainty in head measurements. The primary advantages of the tube seepage meter are its small footprint, concurrent and colocated assessments of q and K, and that it can also be configured as a self‐purging groundwater‐sampling device.}, number={4}, journal={GROUNDWATER}, author={Solder, John E. and Gilmore, Troy E. and Genereux, David P. and Solomon, D. Kip}, year={2016}, pages={588–595} }
 @article{solomon_gilmore_solder_kimball_genereux_2015, title={Evaluating an unconfined aquifer by analysis of age-dating tracers in stream water}, volume={51}, ISSN={["1944-7973"]}, DOI={10.1002/2015wr017602}, abstractNote={AbstractThe mean transit time (MTT) is a fundamental property of a groundwater flow system that is strongly related to the ratio of recharge rate to storage volume. However, obtaining samples for estimating the MTT using environmental tracers is problematic as flow‐weighted samples over the full spectrum of transit times are needed. Samples collected from the base flow of a gaining stream in the North Carolina Coastal Plain (West Bear Creek) that were corrected for exchange with the atmosphere yielded environmental tracer concentrations (SF6 and CFC‐11) very similar to flow‐weighted values from nine or ten streambed piezometers that directly sampled groundwater during low streamflow. At higher streamflow on the falling limb of the hydrograph, stream tracer concentrations (after correction for gas exchange) were significantly higher than the flow‐weighted mean from piezometers, consistent with dominance of the stream tracer signal by transient influx of surface water and/or younger subsurface water. The apparent MTT derived from SF6 in low flow stream water samples was 26 years, suggesting a groundwater recharge rate of about 210 mm/yr, that is consistent with vertical profiles obtained by sampling nested piezometers in the aquifer. When sampled under low flow conditions when streamflow consists of a high component of groundwater discharge, West Bear Creek appears to act as a flow‐weighted integrator of transit times and, streamflow samples can provide fundamental information regarding groundwater recharge rate and MTT. Our study suggests that watershed‐scale evaluation of some groundwater flow systems is possible without utilizing monitoring wells.}, number={11}, journal={WATER RESOURCES RESEARCH}, author={Solomon, D. K. and Gilmore, T. E. and Solder, J. E. and Kimball, B. and Genereux, D. P.}, year={2015}, month={Nov}, pages={8883–8899} }
 @article{ganong_small_ardón_mcdowell_genereux_duff_pringle_2015, title={Interbasin flow of geothermally modified ground water stabilizes stream exports of biologically important solutes against variation in precipitation}, volume={34}, ISSN={2161-9549 2161-9565}, url={http://dx.doi.org/10.1086/679739}, DOI={10.1086/679739}, abstractNote={Geothermally modified ground water (GMG) in tectonically active areas can be an important source of stream nutrients, and the relative importance of GMG inflows is likely to change with shifts in precipitation that are predicted to occur in response to climate change. However, few studies have quantified the influence of GMG inflows on export of biologically important solutes from watersheds across years differing in precipitation. We quantified N, soluble reactive P (SRP), and dissolved organic C (DOC) export during a year with high precipitation (6550 mm rain) and a year with average precipitation (4033 mm rain) in 2 gauged tropical streams at La Selva Biological Station in lowland Costa Rica. One stream receives extensive inputs of regional GMG, whereas the other is fed entirely by local runoff. In the stream fed only by local runoff, a 62% increase in precipitation from the dry year to the wet year led to a 68% increase in stream discharge, a 67% increase in export of SRP, DOC, dissolved organic N (DON), and NH4+, and a 91% increase in NO3– export. In contrast, in an adjacent stream where >⅓ of discharge consists of GMG, the same increase in precipitation from dry year to wet year led to a 14% increase in discharge, a 14 to 31% increase in export of NO3–, NH4+, DON, and DOC, and only a 2% increase in SRP export. We are unaware of an SRP export rate from a natural system that is higher than the export from the stream receiving interbasin flow of GMG (19 kg ha–1 y–1). Our results illustrate that regional ground water, geothermally modified or not, can stabilize stream export of biologically relevant solutes and water across a varying precipitation regime.}, number={1}, journal={Freshwater Science}, publisher={University of Chicago Press}, author={Ganong, Carissa N. and Small, Gaston E. and Ardón, Marcelo and McDowell, William H. and Genereux, David P. and Duff, John H. and Pringle, Catherine M.}, year={2015}, month={Mar}, pages={276–286} }
 @article{oviedo‐vargas_genereux_dierick_oberbauer_2015, title={The effect of regional groundwater on carbon dioxide and methane emissions from a lowland rainforest stream in Costa Rica}, volume={120}, ISSN={2169-8953 2169-8961}, url={http://dx.doi.org/10.1002/2015JG003009}, DOI={10.1002/2015JG003009}, abstractNote={AbstractIn the tropical rainforest at La Selva Biological Station in Costa Rica, regional bedrock groundwater high in dissolved carbon discharges into some streams and wetlands, with the potential for multiple cascading effects on ecosystem carbon pools and fluxes. We investigated carbon dioxide (CO2) and methane (CH4) degassing from two streams at La Selva: the Arboleda, where approximately one third of the streamflow is from regional groundwater, and the Taconazo, fed exclusively by local groundwater recharged within the catchment. The regional groundwater inflow to the Arboleda had no measurable effect on stream gas exchange velocity, dissolved CH4 concentration, or CH4 emissions but significantly increased stream CO2 concentration and degassing. CO2 evasion from the reach of the Arboleda receiving regional groundwater (lower Arboleda) averaged 5.5 mol C m−2 d−1, ~7.5 times higher than the average (0.7 mol C m−2 d−1) from the stream reaches with no regional groundwater inflow (the Taconazo and upper Arboleda). Carbon emissions from both streams were dominated by CO2; CH4 accounted for only 0.06–1.70% of the total (average of both streams: 5 × 10−3 mol C m−2 d−1). Annual stream degassing fluxes normalized by watershed area were 48 and 299 g C m−2 for the Taconazo and Arboleda, respectively. CO2 degassing from the Arboleda is a significant carbon flux, similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of catchment connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance our understanding of carbon source/sink status and overall terrestrial ecosystem carbon budgets.}, number={12}, journal={Journal of Geophysical Research: Biogeosciences}, publisher={American Geophysical Union (AGU)}, author={Oviedo‐Vargas, Diana and Genereux, David P. and Dierick, Diego and Oberbauer, Steven F.}, year={2015}, month={Dec}, pages={2579–2595} }
 @article{genereux_nagy_osburn_oberbauer_2013, title={A connection to deep groundwater alters ecosystem carbon fluxes and budgets: Example from a Costa Rican rainforest}, volume={40}, ISSN={0094-8276}, url={http://dx.doi.org/10.1002/grl.50423}, DOI={10.1002/grl.50423}, abstractNote={AbstractField studies of watershed carbon fluxes and budgets are critical for understanding the carbon cycle, but the role of deep regional groundwater is poorly known and field examples are lacking. Here we show that discharge of regional groundwater into a lowland Costa Rican rainforest has a major influence on ecosystem carbon fluxes. This influence is observable through chemical, isotopic, and flux signals in groundwater, surface water, and air. Not addressing the influence of regional groundwater in the field measurement program and data analysis would give a misleading impression of the overall carbon source or sink status of the rainforest. In quantifying a carbon budget with the traditional “small watershed” mass balance approach, it would be critical at this site and likely many others to consider watershed inputs or losses associated with exchange between the ecosystem and the deeper hydrogeological system on which it sits.}, number={10}, journal={Geophysical Research Letters}, publisher={American Geophysical Union (AGU)}, author={Genereux, David P. and Nagy, Laura A. and Osburn, Christopher L. and Oberbauer, Steven F.}, year={2013}, month={May}, pages={2066–2070} }
 @article{zanon_genereux_oberbauer_2014, title={Use of a watershed hydrologic model to estimate interbasin groundwater flow in a Costa Rican rainforest}, volume={28}, ISSN={["1099-1085"]}, DOI={10.1002/hyp.9917}, abstractNote={AbstractThe watershed hydrologic model TOPMODEL was used to estimate interbasin groundwater flow (IGF) into a small lowland rainforest watershed in Costa Rica. IGF is a common hydrological process but often difficult to quantify. Four‐year simulations (2006–2009) using three different model approaches gave estimates of IGF that were very similar to each other (10.1, 10.2, and 9.8 m/year) and to an earlier estimate (10.0 m/year) based on 1998–2002 data from a budget study that did not use a hydrologic simulation model, providing confidence in the new estimates and suggesting each of the three model approaches is viable. Results show no significant temporal variation in IGF during 2006–2009 (or between this period and the earlier study from 1998–2002). Simulations of the 16 consecutive 3‐month periods in 2006–2009 gave 16 values of IGF rate with a mean (10.1 m/year, standard deviation = 0.6 m/year) very similar to the estimates above from the 4‐year simulations. This suggests the modified version of TOPMODEL can be used to model stream discharge and estimate IGF for sub‐annual time periods during which change in water storage is not necessarily equal to zero. Thus, simple watershed models may be used to estimate IGF based on even relatively short calibration periods, making such models useful tools in the study of this widespread hydrological process that affects water and chemical fluxes and budgets but is often difficult and costly to quantify. Copyright © 2013 John Wiley & Sons, Ltd.}, number={10}, journal={HYDROLOGICAL PROCESSES}, author={Zanon, Carlo and Genereux, David P. and Oberbauer, Steven F.}, year={2014}, month={May}, pages={3670–3680} }
 @article{kennedy_murdoch_genereux_corbett_stone_pham_mitasova_2010, title={Comparison of Darcian flux calculations and seepage meter measurements in a sandy streambed in North Carolina, United States}, volume={46}, ISSN={["0043-1397"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77956498672&partnerID=MN8TOARS}, DOI={10.1029/2009wr008342}, abstractNote={We compared two methods for estimating groundwater flux into a stream reach: seepage meters and Darcian flux calculations. Both methods were applied at the same 53 points on the streambed of a 62.5‐m‐long reach of West Bear Creek in the coastal plain of North Carolina. At each point, a seepage meter was used to measure groundwater flux, vmeter, and several minutes later the streambed hydraulic conductivity (K) and hydraulic head gradient (J) were measured. We compared the 53 paired point values of groundwater flux from each method (vmeter and vdarcy = KJ), the integrated volumetric groundwater flux to the stream reach from each method (Qdarcy and Qmeter), and the spatial distributions of vdarcy and vmeter on the streambed. Values of Qmeter (268 m3/d) and mean vmeter (0.66 m/d) were each about 0.70 × the corresponding Darcian flux quantities (Qdarcy = 375 m3/d and mean vdarcy = 0.92 m/d). Despite their differences, the two methods gave the same direction of groundwater flow (into the stream) at all 53 points, a result not found in all previous comparison studies and thus not to be taken for granted. Also, vdarcy and vmeter had similar spatial distributions on the streambed. The ratio vmeter/vdarcy = 0.70 is within the range of 0.3–7 from previous studies and is closer to 1 than in previous studies. The differences between the two methods are probably in part due to random measurement error and to the spatial scales for the Darcy and seepage meter measurements being of a slightly different size and offset several centimeters from each other on the heterogeneous streambed, but these effects are unlikely to produce the observed consistent bias. The mean bias between the methods (ratio of 0.70) may be related to gas bubbles in the streambed pore spaces (e.g., collection of gas bubbles in the seepage meters lowering vmeter and loss of gas from the streambed upon seepage meter removal or permeameter insertion increasing the subsequent vdarcy measurement).}, number={9}, journal={WATER RESOURCES RESEARCH}, author={Kennedy, Casey D. and Murdoch, Lawrence C. and Genereux, David P. and Corbett, D. Reide and Stone, Katie and Pham, Phung and Mitasova, Helena}, year={2010}, month={Sep} }
 @article{solomon_genereux_plummer_busenberg_2010, title={Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers}, volume={46}, ISSN={["0043-1397"]}, DOI={10.1029/2009wr008341}, abstractNote={We tested three models of mixing between old interbasin groundwater flow (IGF) and young, locally derived groundwater in a lowland rain forest in Costa Rica using a large suite of environmental tracers. We focus on the young fraction of water using the transient tracers CFC‐11, CFC‐12, CFC‐113, SF6, 3H, and bomb 14C. We measured 3He, but 3H/3He dating is generally problematic due to the presence of mantle 3He. Because of their unique concentration histories in the atmosphere, combinations of transient tracers are sensitive not only to subsurface travel times but also to mixing between waters having different travel times. Samples fall into three distinct categories: (1) young waters that plot along a piston flow line, (2) old samples that have near‐zero concentrations of the transient tracers, and (3) mixtures of 1 and 2. We have modeled the concentrations of the transient tracers using (1) a binary mixing model (BMM) of old and young water with the young fraction transported via piston flow, (2) an exponential mixing model (EMM) with a distribution of groundwater travel times characterized by a mean value, and (3) an exponential mixing model for the young fraction followed by binary mixing with an old fraction (EMM/BMM). In spite of the mathematical differences in the mixing models, they all lead to a similar conceptual model of young (0 to 10 year) groundwater that is locally derived mixing with old (>1000 years) groundwater that is recharged beyond the surface water boundary of the system.}, journal={WATER RESOURCES RESEARCH}, author={Solomon, D. Kip and Genereux, David P. and Plummer, L. Niel and Busenberg, Eurybiades}, year={2010}, month={Apr} }
 @article{genereux_webb_solomon_2009, title={Chemical and isotopic signature of old groundwater and magmatic solutes in a Costa Rican rain forest: Evidence from carbon, helium, and chlorine}, volume={45}, ISSN={["1944-7973"]}, DOI={10.1029/2008wr007630}, abstractNote={C, He, and Cl concentrations and isotopes in groundwater and surface water in a lowland Costa Rican rain forest are consistent with the mixing of two distinct groundwaters: (1) high‐solute bedrock groundwater representing interbasin groundwater flow (IGF) into the rain forest and (2) low‐solute local groundwater recharged in the lowlands. In bedrock groundwater, high δ13C (−4.89‰), low 14C (7.98 pM), high R/RA for He (6.88), and low 36Cl/Cl (17 × 10−15) suggest that elevated tracer concentrations are derived from magmatic outgassing and/or weathering of volcanic rock beneath nearby Volcan Barva. In local groundwater, the magmatic signature is absent, and data suggest atmospheric sources for He and Cl and a biogenic soil gas CO2 source for dissolved inorganic carbon. Dating of 14C suggests that the age of bedrock groundwater is 2400–4000 years (most likely at the lower end of the range). Local groundwater has 14C > 100 pM, indicating the presence of “bomb carbon” and thus ages less than ∼55 years. Overall, data are consistent with a conceptual hydrologic model originally proposed on the basis of water budget and major ion data: (1) large variation in solute concentrations can be explained by mixing of the two distinct groundwaters, (2) bedrock groundwater is much older than local groundwater, (3) elevated solute concentrations in bedrock groundwater are derived from volcanic fluids and/or rock, and (4) local groundwater has not interacted with volcanic rock. Tracers with different capabilities converge on the same hydrologic interpretation. Also, transport of magmatic CO2 into the lowland rain forest via IGF seems to be significant relative to other large ecosystem‐level carbon fluxes.}, journal={WATER RESOURCES RESEARCH}, author={Genereux, David P. and Webb, Mathew and Solomon, D. Kip}, year={2009}, month={Aug} }
 @article{kennedy_genereux_corbett_mitasova_2009, title={Relationships among groundwater age, denitrification, and the coupled groundwater and nitrogen fluxes through a streambed}, volume={45}, ISSN={["1944-7973"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-72149099351&partnerID=MN8TOARS}, DOI={10.1029/2008wr007400}, abstractNote={The relationships among coupled groundwater and nitrogen (N) fluxes, groundwater age, and denitrification were examined for a section of West Bear Creek, an agricultural stream in the coastal plain of North Carolina, United States. Simultaneous streambed measurements of hydraulic conductivity (K) and hydraulic head gradient (J) and the concentrations of NO3− ([NO3−]), dissolved gases, and chlorofluorocarbons in groundwater were interpolated, mapped, and (for water flux v = KJ and nitrate flux fNO3 = v[NO3−]) integrated over the streambed area. Nitrate and dissolved organic N accounted for 92 and 8% of N flux through the streambed, respectively. Streambed maps show a band of greater groundwater age, and lower [NO3−] and fNO3, running through the center of most of the study reach. Nitrate flux (fNO3) exhibits this “center‐low” pattern even though one of its controlling factors, groundwater flux (v), has on average the opposite “center‐high” pattern. An inverse relationship between [NO3−] and age is indicative of fertilizer as the primary source of groundwater NO3−. Denitrification reduced mean fNO3 by ∼50%, from 370 mmol m−2 d−1 (what it would have been in the absence of denitrification) to 173 mmol m−2 d−1 (what it actually was). Measurement of both groundwater age and v made possible a new method for estimating flow‐weighted mean groundwater age (τFWM), an important aquifer hydraulic characteristic related to groundwater storage and recharge rate. This method gives τFWM = 30 years, which, along with the overall distribution of groundwater ages, suggests the possibility of a significant time lag between changes in N fertilizer application rates and NO3− flux from groundwater to West Bear Creek. Differences in streambed groundwater chemistry between the left and right sides of the streambed suggest differences in agricultural practices on opposite sides of the stream.}, number={9}, journal={WATER RESOURCES RESEARCH}, author={Kennedy, Casey D. and Genereux, David P. and Corbett, D. Reide and Mitasova, Helena}, year={2009}, month={Sep} }
 @article{kennedy_genereux_corbett_mitasova_2009, title={Spatial and temporal dynamics of coupled groundwater and nitrogen fluxes through a streambed in an agricultural watershed}, volume={45}, ISSN={["0043-1397"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-72149089873&partnerID=MN8TOARS}, DOI={10.1029/2008wr007397}, abstractNote={This paper presents results on the spatiotemporal dynamics of the coupled water flux (v) and nitrogen fluxes (fN = v[N], where [N] is the concentration of a dissolved N species) through a streambed in an agricultural watershed in North Carolina. Physical and chemical variables were measured at numerous points in the streambed of a 0.26‐km reach: hydraulic conductivity (K) and head gradient (J) and the concentrations of NO3− and other N species in streambed groundwater, from which water flux (v = KJ) and N fluxes (e.g., fNO3 = v[NO3−]) through the streambed were computed, mapped, and integrated in space. The result was a novel set of streambed maps of the linked variables (K, J, v, and N concentrations and fluxes), showing their spatial variability and how it changed over a year (on the basis of seven bimonthly sets of maps). Mean fNO3 during the study year was 154 mmol m−2 d−1; this NO3− flux, together with that of dissolved organic nitrogen (fDON = 17 mmol m−2 d−1), accounted for >99% of the total dissolved N flux through the streambed. Repeat measurements at the same locations on the streambed show significant temporal variability in fNO3, controlled largely by changes in v rather than changes in [NO3−]. One of the clearest and most persistent aspects of spatial variability was lateral variability across the channel from bank to bank. K and v values were greater in the center of the channel; this distribution of K (ultimately a reflection of sediment dynamics in the channel) apparently focuses groundwater discharge toward the center of the channel. The opposite pattern (low values in the center) was found for J, [NO3−], and (to a lesser extent) fNO3. Overall, fNO3 was characterized by localized zones of high and low values that changed in size and shape over time but remained in basically the same locations (the same was true of K, J, and [NO3−], though less so for v), with 70% of NO3− flux occurring through about 38% of the streambed area. Lateral distributions of the physical hydrologic attributes (K, J, and v) were highly symmetrical across the channel, while those of [NO3−] and fNO3 showed higher values on the left than on the right, likely a reflection of different N use on opposite sides of the stream. The streambed‐based approach taken here offers insights concerning the spatial and temporal dynamics of linked water and N fluxes through a streambed and their controls.}, number={9}, journal={WATER RESOURCES RESEARCH}, author={Kennedy, Casey D. and Genereux, David P. and Corbett, D. Reide and Mitasova, Helena}, year={2009}, month={Sep} }
 @article{kennedy_genereux_mitasova_corbett_leahy_2008, title={Effect of sampling density and design on estimation of streambed attributes}, volume={355}, ISSN={["1879-2707"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-43949128733&partnerID=MN8TOARS}, DOI={10.1016/j.jhydrol.2008.03.018}, abstractNote={Underlying questions that are central to field studies of the groundwater–surface water interaction are the extent of error in means and spatial distributions of streambed attributes such as groundwater seepage rate or solute flux, and the relation of this error to the number and location of point measurements. To investigate these questions, spatially intensive point measurements of five streambed attributes were made in two 63-m long stream reaches in the North Carolina Coastal Plain: hydraulic conductivity (K), hydraulic head gradient between groundwater and stream water (J), nitrate concentration in streambed groundwater (C), and groundwater seepage (v = KJ) and nitrate flux (f = vC) through the streambed. In all, 10 datasets (2 reaches, 5 attributes), each with 54 closely-spaced point values, were created (540-point values in all). For each dataset, subsets of 8- to 40-point values were selected from the 54 available to evaluate the effects of sampling density (the number of point values per reach, or per m2 of streambed) and sampling design (the relative number of point values from the right side, left side, and center of the channel) on the mean and the spatial field of the streambed attribute. Specifically, we evaluated the following as a function of sampling density and sampling design: (1) the likelihood of error in the reach-average value of each streambed attribute, (2) the average magnitude of error and distribution of error in the reach-average value of each attribute, (3) the magnitude of error in the prediction of point values of each attribute, and (4) the geometry of interpolated surfaces of two attributes (K and f). In all cases, "error" in a value or interpolated surface based on a subset of points was taken as a deviation from the corresponding result based on the full dataset of 54 points. The probability (p) that error did not occur increased with sampling density for each sampling design and attribute in both reaches, though the effect of "diminishing returns" was evident for sampling densities greater than ∼24 points per reach (roughly 0.05–0.06 points per m2 of streambed). Relative to sampling density, sampling design had little effect on values of p. Average error in streambed attributes was generally small (⩽10%) and less than the 95% confidence limits about the reach-average values of the attributes. The ability to estimate unknown point values by interpolation among other point values was poor using both 12- and 36-point subsets, though results suggest the 24 additional known point values (in going from 12 to 36) were helpful in one case in which there was some degree of autocorrelation between the additional known values and the values to be predicted in the interpolation. Visual inspection of 130 contour maps showed that those based on 36-point values were far more realistic in appearance than those based on 12-point values (where the standard for "realistic" appearance was the contour maps based on the full datasets of 54-point values).}, number={1-4}, journal={JOURNAL OF HYDROLOGY}, author={Kennedy, Casey D. and Genereux, David P. and Mitasova, Helena and Corbett, D. Reide and Leahy, Scott}, year={2008}, month={Jun}, pages={164–180} }
 @article{genereux_leahy_mitasova_kennedy_corbett_2008, title={Spatial and temporal variability of streambed hydraulic conductivity in West Bear Creek, North Carolina, USA}, volume={358}, ISSN={["1879-2707"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-48849088675&partnerID=MN8TOARS}, DOI={10.1016/j.jhydrol.2008.06.017}, abstractNote={The hydraulic conductivity (K) of the streambed is an important variable influencing water and solute exchange between streams and surrounding groundwater systems. However, there are few detailed data on spatial variability in streambed K and almost none on temporal variability. The spatial and temporal variability of streambed K in a North Carolina stream were investigated with 487 field measurements of K over a 1-year period. Measurements were made bimonthly from December 2005 to December 2006 at 46 measurement locations in a 262.5 m reach (the "large reach"). To give a more detailed picture of spatial variability, closely-spaced one-time measurements were made in two 62.5 m reaches (the "small reaches", one investigated in July 2006 and the other in August 2006) that were part of the large reach. Arithmetic mean K for the large reach was ∼16 m/day (range was 0.01 to 66 m/day). Neither K nor lnK was normally distributed, and K distributions appeared somewhat bimodal. There was significant spatial variability over horizontal length scales of a few m. Perhaps the clearest feature within this variability was the generally higher K in the center of the channel. This feature may be an important control on water and chemical fluxes through the streambed (e.g., other measurements show generally higher water seepage velocity, but lower porewater nitrate concentration, in the center of the streambed). Grain size analysis of streambed cores showed that layers of elevated fines (silt + clay) content were less common in the center of the channel (overall, the streambed was about 94% sand). Results also suggest a modest but discernable difference in average streambed K upstream and downstream of a small beaver dam: K was about 23% lower upstream, when the dam was present during the first few months of the study. This upstream/downstream difference in K disappeared after the dam collapsed, perhaps in response to re-mobilization of fine sediments or leaf matter that had accumulated in quiet waters ponded on the upstream side of the dam. Temporal variability was significant and followed a variety of different patterns at the 46 measurement locations in the large reach. Temperature data show that variation in streambed and groundwater temperature was not an important cause of the observed temporal variability in K. Measurements of changes in the elevation of the streambed surface suggest erosion and deposition played an important role in causing the observed temporal variability in streambed K (of which the change described above following collapse of the beaver dam was a special case), though other potentially time-varying factors (e.g., gas content, bioturbation, or biofilms in the streambed) were not explicitly addressed and cannot be ruled out as contributors to the temporal variability in streambed K. Temporal variability in streambed K merits additional study as a potentially important control on temporal variability in the magnitudes and spatial patterns of water and solute fluxes between groundwater and surface water. From the data available it seems appropriate to view streambed K as a dynamic attribute, variable in both space and time.}, number={3-4}, journal={JOURNAL OF HYDROLOGY}, author={Genereux, David P. and Leahy, Scott and Mitasova, Helena and Kennedy, Casey D. and Corbett, D. Reide}, year={2008}, month={Sep}, pages={332–353} }
 @article{kennedy_genereux_2007, title={C-14 groundwater age and the importance of chemical fluxes across aquifer boundaries in confined cretaceous aquifers of North Carolina, USA}, volume={49}, ISSN={["1945-5755"]}, DOI={10.1017/s0033822200043101}, abstractNote={Radiocarbon activity, He concentrations, and other geochemical parameters were measured in groundwater from the confined Black Creek (BC) and Upper Cape Fear (UCF) aquifers in the Coastal Plain of North Carolina.14C ages adjusted for geochemical and diffusion effects ranged from 400 to 21,900 BP in the BC, and 13,400 to 26,000 BP in the underlying UCF; ages increased coastward in both aquifers. Long-term average linear groundwater velocity is about 2.5 m/yr for the BC, and somewhat larger for the UCF. Aquifer-aquitard exchange is an important influence on the DIC concentration,14C activity, and estimated age of aquifer groundwater. Accounting for this exchange in14C age calculations places the groundwater samples with the lowest estimated recharge temperatures nearest in time to the last glacial maximum. Traditional geochemical correction models that do not account for aquifer-aquitard exchange significantly overestimate groundwater age. He concentration in groundwater varies with both age and stratigraphic position. Dissolved He data provide strong evidence of upward vertical He transport through the study aquifers; data from the UCF are broadly consistent with the pattern expected for a confined aquifer receiving a concentrated, localized He flux from below (based on a previously published model for this situation), in this case most likely from crystalline bedrock. He has potential as an indicator of groundwater age in the study aquifers, if interpreted within an appropriate analytical framework that includes the observed strong vertical transport. δ18O in the oldest groundwater is enriched (relative to modern groundwater) by 1 to 1.2‰, the opposite of the δ18O depletion found in many old groundwaters but consistent with the enrichment found in groundwater in this age range in Georgia and Florida.}, number={3}, journal={RADIOCARBON}, author={Kennedy, Casey D. and Genereux, David P.}, year={2007}, pages={1181–1203} }
 @article{kennedy_genereux_corbett_mitasova_2007, title={Design of a light-oil piezomanometer for measurement of hydraulic head differences and collection of groundwater samples}, volume={43}, ISSN={["1944-7973"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-35848960791&partnerID=MN8TOARS}, DOI={10.1029/2007wr005904}, abstractNote={This paper describes a device (“piezomanometer”) that combines three components (an oil‐water manometer, a pushable screened PVC probe, and a system for groundwater sample collection) into a single inexpensive ($130), easily built, reliable tool for rapid collection of shallow groundwater from a streambed or lake bed and accurate measurement of even very small head differences between this groundwater and overlying surface water. The piezomanometer has been tested with excellent results both in the lab and in a stream shallow enough for wading; in principle, it could be adapted for use in deeper water where work is done from a dock, boat, or other platform. The problem of gas bubbles collecting in the groundwater line (a common drawback of field manometers) was nearly eliminated by use of a three‐way valve at a local elevation maximum in the groundwater line (gas bubbles in the groundwater line can be purged through this valve). Field application is illustrated here with data from a 2‐day study using four piezomanometers in a North Carolina stream.}, number={9}, journal={WATER RESOURCES RESEARCH}, author={Kennedy, Casey D. and Genereux, David P. and Corbett, D. Reide and Mitasova, Helena}, year={2007}, month={Sep} }
 @article{genereux_jordan_carbonell_2005, title={A paired-watershed budget study to quantify interbasin groundwater flow in a lowland rain forest, Costa Rica}, volume={41}, ISSN={["0043-1397"]}, DOI={10.1029/2004wr003635}, abstractNote={A paired‐watershed budget study was used to quantify the annual water and major ion (sodium, potassium, magnesium, calcium, chloride, and sulfate) budgets of two adjacent lowland rain forest watersheds in Costa Rica. Interbasin groundwater flow (IGF) accounted for about two thirds of the water input and about 97% of the solute input (an average over the six major ions) to one watershed but little or none of the inputs to the adjacent watershed in which IGF was at most marginally distinguishable from zero. Results underscore the significance of IGF as a potential control on the hydrology and water quality of lowland watersheds, the spatial complexity of its occurrence in lowlands (where its influence may range from dominating to negligible on adjacent watersheds), and the importance of accounting for IGF in the design and execution of watershed studies and in water management.}, number={4}, journal={WATER RESOURCES RESEARCH}, author={Genereux, DP and Jordan, MT and Carbonell, D}, year={2005}, month={Apr} }
 @article{genereux_jordan_2006, title={Interbasin groundwater flow and groundwater interaction with surface water in a lowland rainforest, Costa Rica: A review}, volume={320}, ISSN={["1879-2707"]}, DOI={10.1016/j.jhydrol.2005.07.023}, abstractNote={Abstract This paper reviews work related to interbasin groundwater flow (naturally occurring groundwater flow beneath watershed topographic divides) into lowland rainforest watersheds at La Selva Biological Station in Costa Rica. Chemical mixing calculations (based on dissolved chloride) have shown that up to half the water in some streams and up to 84% of the water in some riparian seeps and wells is due to high-solute interbasin groundwater flow (IGF). The contribution is even greater for major ions; IGF accounts for well over 90% of the major ions at these sites. Proportions are highly variable both among watersheds and with elevation within the same watershed (there is greater influence of IGF at lower elevations). The large proportion of IGF found in water in some riparian wetlands suggests that IGF is largely responsible for maintaining these wetlands. δ 18 O data support the conclusions from the major ion data. Annual water and major ion budgets for two adjacent watersheds, one affected by IGF and the other not, showed that IGF accounted for two-thirds of the water input and 92–99% of the major ion input (depending on the major ion in question) to the former watershed. The large (in some cases, dominating) influence of IGF on watershed surface water quantity and quality has important implications for stream ecology and watershed management in this lowland rainforest. Because of its high phosphorus content, IGF increases a variety of ecological variables (algal growth rates, leaf decay rate, fungal biomass, invertebrate biomass, microbial respiration rates on leaves) in streams at La Selva. The significant rates of IGF at La Selva also suggest the importance of regional (as opposed to small-scale local) water resource planning that links lowland watersheds with regional groundwater. IGF is a relatively unexplored and potentially critical factor in the conservation of lowland rainforest.}, number={3-4}, journal={JOURNAL OF HYDROLOGY}, author={Genereux, DP and Jordan, M}, year={2006}, month={Apr}, pages={385–399} }
 @article{genereux_2004, title={Comparison of naturally-occurring chloride and oxygen-18 as tracers of interbasin groundwater transfer in lowland rainforest, Costa Rica}, volume={295}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/j.jhydrol.2004.02.020}, DOI={10.1016/j.jhydrol.2004.02.020}, abstractNote={δ18O data were used to test a two-end-member hydrologic mixing model previously published for an area of lowland rainforest in Costa Rica. The model distinguishes interbasin groundwater transfer from locally-recharged groundwater. The model was originally based on data for chloride (Cl) and other major ions and is fully consistent with new Cl data presented here. The mixing model was used to generate five predictions concerning δ18O at 13 groundwater and surface water sampling sites, on the premise that the model would be supported if δ18O data agreed with the predictions. Overall, the δ18O data are consistent with the predictions, giving reasonably clear support for 3 of the 5, and neither clearly supporting nor ruling out the other 2. The large intra-site temporal variability in δ18O precludes conclusive confirmation of the latter two predictions. This variability also affects the results of mixing calculations to quantify the proportions of each end member in water samples representing mixtures of the end members. Mixing calculations based on δ18O give different results and much higher uncertainties than those based on Cl. For δ18O data, intra-site variability is large relative to the difference between the two end member waters, leading to large uncertainty in δ18O-based mixing calculations. In this case, Cl appears to be the superior tracer for interbasin groundwater transfer, mainly because intra-site variability in Cl is small relative to the large Cl difference between end members.}, number={04-Jan}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, D.}, year={2004}, pages={17–27} }
 @article{saiers_genereux_bolster_2004, title={Influence of calibration methodology on ground water flow predictions}, volume={42}, ISSN={["0017-467X"]}, DOI={10.1111/j.1745-6584.2004.tb02448.x}, abstractNote={AbstractWe constructed a numerical model of transient ground water flow and solute transport for a portion of the Biscayne Aquifer in Florida, and calibrated the model with three different combinations of data from a 193‐day period: head (h) data alone, data on h and ground water discharge to a canal (q), and data on h, q, and ground water chloride concentration (C). We used each of the three calibrated models to predict h and q during a 182‐day test period separate from the calibration period. All three calibrated models predicted h equally well during the test period (r= 0.95, where r= 1 indicates perfect agreement between measured and simulated values), though the model calibrated on h alone had significantly different parameter values than the other two models. Predictions of q during the test period depended on calibration methodology; models calibrated with multiple targets simulated q more accurately than the model calibrated on h alone (r = 0.79 compared to r = 0.49). Based on the results of these simulations, we conclude: (1) Post‐calibration prediction is important in assessing the value of different data types in automated calibration; (2) inverse‐solution uniqueness is not a requirement for accurate h predictions; (3) relatively simple models can predict with reasonable accuracy transient ground water flow in a complex aquifer, and parameters governing this prediction can be estimated by nonlinear regression methods that incorporate both h and q data; (4) addition of C data to the calibration did not improve model predictive capacity because the information in the C data was similar to that in the q data, from the perspective of model calibration (the subsurface chemical signal in question was controlled mainly by seepage of high‐chloride canal water into the low‐chloride ground water system).}, number={1}, journal={GROUND WATER}, author={Saiers, JE and Genereux, DP and Bolster, CH}, year={2004}, pages={32–44} }
 @article{genereux_2003, title={Comparison of methods for estimation of 50-year peak discharge from a small, rural watershed in North Carolina}, volume={44}, ISSN={["0943-0105"]}, DOI={10.1007/s00254-002-0734-5}, number={1}, journal={ENVIRONMENTAL GEOLOGY}, author={Genereux, DP}, year={2003}, month={May}, pages={53–58} }
 @article{genereux_wood_pringle_2002, title={Chemical tracing of interbasin groundwater transfer in the lowland rainforest of Costa Rica}, volume={258}, ISSN={["1879-2707"]}, DOI={10.1016/S0022-1694(01)00568-6}, abstractNote={Chemical data from several hundred surface water and groundwater samples collected mainly during baseflow over 4.5 years were used to detect and quantify the natural interbasin transfer of deep groundwater into watersheds at La Selva Biological Station, a research site in the lowland rainforest of Costa Rica. Most of the variability in major ion concentrations at La Selva can be explained by mixing of two chemically and hydrologically distinct waters: high-solute bedrock groundwater, and low-solute local water draining from hillslope soils within the study watersheds. Several lines of evidence indicate that high-solute bedrock groundwater represents subsurface interbasin transfer into the study site. The fraction of water due to interbasin transfer (fwater) ranged from zero to about 0.49 for major streams at La Selva; fwater values were even higher (up to 0.84) for small riparian seeps and shallow groundwater near the Salto stream. The relative contribution of major ions by interbasin transfer was even more significant than of water itself. fwater values of 0.49 and 0.84 correspond to fCl values of 0.92 and 0.99, respectively (fCl, the fraction of dissolved chloride in a water sample that is due to interbasin transfer, is approximately equal to the fraction of all major ions contributed to the sample by interbasin transfer, given the observed linear correlation between Cl and other major ions). fwater and fCl of streams and riparian seeps varied on both long (monthly/seasonal) and short (storm event) time scales, in each case decreasing as conditions at La Selva became wetter. The high fwater values found in riparian groundwater and seeps indicate that local water and bedrock groundwater derived from interbasin transfer mix in the shallow subsurface at La Selva, not just in stream channels. With fwater values up to 0.84, it appears that some areas of riparian wetland may be maintained largely by interbasin transfer. This large interbasin transfer significantly affects both terrestrial (e.g. wetland) and aquatic ecosystems. Results suggest the importance of a regional approach to land use planning in this and similar environments. Complete protection of lowland streams, wetlands, and ecosystems in this hydrogeologic setting requires protection of a deep interbasin groundwater system whose precise volume, boundaries, and recharge areas are presently unknown.}, number={1-4}, journal={JOURNAL OF HYDROLOGY}, author={Genereux, DP and Wood, SJ and Pringle, CM}, year={2002}, month={Feb}, pages={163–178} }
 @article{genereux_bandopadhyay_2002, title={Reply to comment on "numerical investigation of lake bed seepage patterns: effects of porous medium and lake properties"' by Genereux, D., and Bandopadhyay, I., 2001. Journal of Hydrology 241, 286-303}, volume={258}, ISSN={["0022-1694"]}, DOI={10.1016/S0022-1694(01)00572-8}, abstractNote={Seawater intrusion (SI) is an increased threat to the groundwater resource in coastal regions. The cut-off wall is widely used to arrest the advance of a seawater wedge, but it is likely to cause groundwater contamination in landward aquifers. In order to alleviate the contamination problem, we proposed a new variable-permeability cut-off wall (VPFW) with a semi-permeable section at the bottom of the physical barrier. This study is the first to investigate the spatial distributions and transient behaviors of SI and nitrate pollution at field scale under the joint effects of physical barriers and denitrification. Results show the dissolved oxygen (DO) decreases almost synchronously with the intrusion of SW near the sea boundary, and the denitrification of NO3− and retreat of the dissolved organic carbon (DOC) wedge occurred in the inland aquifer. Notably, the freshwater discharge flux and nitrate discharge flux for the developed VPFW increase by 35% and 20%, respectively, while the enrichment ratio of specific volume of nitrate contamination zone (SVNR) went down by 15% on average compared with the traditional cut-off wall. Four indices were proposed to quantify the impervious barrier: heights (H), relative hydraulic conductivities (K*), and control effectiveness of SI and nitrate pollution. Moreover, we found the control effectiveness for SI and nitrate pollution was continually enhanced with the reduction of K* and increase of H; however, excessive reduction and increase may result in the opposite impacts. The optimal K* and H for the VPFW can be determined with the required value of the reduction rate of saltwater wedge length (SWR) and SVNR; but all cut-off wall cases fail to meet the demand. The results show that the VPFW is much more effective in controlling SI and nitrate pollution in comparison with the traditional cut-off wall.}, number={1-4}, journal={JOURNAL OF HYDROLOGY}, author={Genereux, D and Bandopadhyay, I}, year={2002}, month={Feb}, pages={265–266} }
 @article{genereux_guardiario_2001, title={A borehole flowmeter investigation of small-scale hydraulic conductivity variation in the Biscayne Aquifer, Florida}, volume={37}, ISSN={["1944-7973"]}, DOI={10.1029/2001WR900023}, abstractNote={Geostatistical analysis of closely spaced borehole flowmeter measurements was used to estimate the variance (2.53) and vertical and horizontal correlation lengths of ln K(0.57 and 7.3 m) in the Biscayne Aquifer, a limestone aquifer critical for Florida's water supply and for Everglades restoration efforts. The variance and correlation lengths of the Biscayne Aquifer are similar to some of the values for unconsolidated siliciclastic sediments (especially those at Columbus, Mississippi∥. The larger λh, for the Biscayne Aquifer (7.3 m) is thought to be due at least in part to the lower lateral variability of the carbonate platform depositional environment, compared to the fluvial environments in which the siliciclastic sediments were deposited. An improved down hole packer would allow for data with finer vertical resolution; the current system is adequate for work inside well screens but cannot adequately seal many spots in open, irregular rock boreholes. Research in rock presents additional logistical difficulties but is important for addressing fundamental questions about solute transport in a wider range of geological media, beyond unconsolidated siliciclastic deposits.}, number={5}, journal={WATER RESOURCES RESEARCH}, author={Genereux, D and Guardiario, J}, year={2001}, month={May}, pages={1511–1517} }
 @article{bolster_genereux_saiers_2001, title={Determination of specific yield for the Biscayne Aquifer with a canal-drawdown test}, volume={39}, ISSN={["0017-467X"]}, DOI={10.1111/j.1745-6584.2001.tb02368.x}, abstractNote={AbstractData from a large‐scale canal‐drawdown test were used to estimate the specific yield (s) of the Biscayne Aquifer, an uncon‐fined limestone aquifer in southeast Florida. The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in the surrounding aquifer. Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream‐aquifer interaction model (Zlotnik and Huang 1999). Specific yield values computed from drawdown at individual piezometers ranged from 0.050 to 0.57, most likely indicating heterogeneity of specific yield within the aquifer (small‐scale variation in hydraulic conductivity may also have contributed to the differences in sy among piezometers). A value of 0.15 (our best estimate) was computed based on all drawdown data from all piezometers.We incorporated our best estimate of specific yield into a large‐scale two‐dimensional numerical MODFLOW‐based ground water flow model and made predictions of head during a 183–day period at four wells located 337 to 2546 m from the canal. We found good agreement between observed and predicted heads, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. This work represents a practical and novel approach to the determination of a key hydrogeological parameter (the storage parameter needed for simulation and calculation of transient unconfined ground water flow), at a large spatial scale (a common scale for water resource modeling), for a highly transmissive limestone aquifer (in which execution of a traditional pump test would be impractical and would likely yield ambiguous results). Accurate estimates of specific yield and other hydrogeological parameters are critical for management of water supply, Everglades environmental restoration, flood control, and other issues related to the ground water hydrology of the Biscayne Aquifer.}, number={5}, journal={GROUND WATER}, author={Bolster, CH and Genereux, DP and Saiers, JE}, year={2001}, pages={768–777} }
 @article{genereux_bandopadhyay_2001, title={Numerical investigation of lake bed seepage patterns: effects of porous medium and lake properties}, volume={241}, ISSN={["0022-1694"]}, DOI={10.1016/s0022-1694(00)00380-2}, abstractNote={Three-dimensional steady-state numerical models were used to investigate the relative significance of several factors controlling lake bed seepage patterns: lake depth, lake bed slope, orientation of an asymmetric lake with respect to a regional hydraulic gradient, lake bed sediments, and heterogeneity and anisotropy of the porous medium. We considered both inflow and flow-through lakes, and our focus was on the details of seepage flux at the lake bed (not on the surrounding porous medium). While porous medium factors (anisotropy and heterogeneity) are important, we found several conditions where lake bed factors were nearly as important in controlling the distribution of seepage. Varying lake bed slope from 0.013 to 0.04 in different simulations had a significant effect on shoreline seepage (rates were 10–40% higher for lakes of low slope than lakes of steep slope). Also, significantly elevated seepage (in some cases, a local maximum) was observed offshore at the break in bed slope between the sloping side and flat central portions of the lake bed, whenever the surrounding porous medium had a high anisotropy (1000 or 100). For flow-through lakes in media of high anisotropy, the annual volume of groundwater inseepage was significantly higher (about 20%) in lakes with steep bed slope compared to those with low slope; this effect of slope was smaller at lower anisotropy. For an asymmetric flow-through lake (a lake with a steep bed slope on one side, moderate slope on the other) the percentage of lake bed experiencing inseepage was greatest when the steep side was downgradient, and the effect was larger at higher anisotropy. These effects illustrate the complex interaction between lake bed slope and the anisotropy of the surrounding porous medium in controlling lake:groundwater exchange. Adding low-conductivity lake sediments, and decreasing their conductivity, shifted groundwater seepage further offshore in inflow lakes; increasing the anisotropy of the surrounding porous medium had the same effect. Adding sediments and increasing anisotropy also decreased nearshore seepage in flow-through lakes, but without increasing offshore seepage; in this case, the net effect was a smaller annual volume of lake:groundwater exchange. At the same time, the percentage of the lake bed experiencing inseepage increased in flow-through lakes, even as the annual volume of inseepage was decreasing. Thus, for flow-through lakes, a larger area of inseepage may not be a good indicator of a greater volume of inseepage. Lake depth did not have a significant effect on the quantity or distribution of seepage to inflow or flow-through lakes. Many of the physical factors investigated here influence the amount of lake:groundwater exchange and the proportions of nearshore and offshore seepage; therefore, they are potentially significant to lake water quality and ecology in addition to hydrology.}, number={3-4}, journal={JOURNAL OF HYDROLOGY}, author={Genereux, D and Bandopadhyay, I}, year={2001}, month={Jan}, pages={286–303} }
 @inbook{zechner_genereux_saiers_2000, series={IAHS Publication}, title={The benefit of using data on canal seepage and tracer concentration in aquifer parameter estimation}, booktitle={Calibration and Reliability in Groundwater Modelling: Coping with Uncertainty}, author={Zechner, E. and Genereux, D. and Saiers, J.}, editor={Stauffer, F. and Kinzelbach, W. and Kovar, K. and Hoehn, E.Editors}, year={2000}, pages={256–264}, collection={IAHS Publication} }
 @article{genereux_slater_1999, title={Water exchange between canals and surrounding aquifer and wetlands in the Southern Everglades, USA}, volume={219}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/S0022-1694(99)00060-8}, DOI={10.1016/S0022-1694(99)00060-8}, abstractNote={Continuous volumetric discharge data was collected at six acoustic velocity meters (AVMs) installed in two canals along the eastern boundary of Everglades National Park (ENP) in Florida, USA. Discharge data from these six sites, and from two others monitored by state and federal government agencies, were used to compute the rate of water exchange (Qg) between six canal reaches and their surroundings (the unconfined Biscayne Aquifer and its overlying wetlands). Net water exchange (Qg) for the two northernmost canal reaches corresponded to groundwater, but further south in the study area Qg included a surface water component. While canal L-31W was built to supply water to ENP, data show that its primary (though unintended) function during the study period was redistribution of water from north to south within ENP. This “hydrologic short-circuiting” (where water flowed south in L-31W rather than in wetlands) is problematic for ecosystem restoration efforts. A similar short-circuiting operates on a larger spatial scale over the entire basin of the larger C-111 canal, the main flood control canal in the area. While this hydrologic regime does offer flood control benefits, changes in water management infrastructure and operations are needed to restore more natural spatial and temporal patterns of flow through wetlands in the area. Four of the AVM stations were located close to large water control structures on the canals. Agreement was generally good though slightly non-linear between daily average discharge (Q) values based on AVM data and those determined using hydraulic rating equations at nearby flow control structures. The AVMs used in this study required significant maintenance but provided sound data on discharge. They provided a good check on hydraulic rating equations at flow control structures, quantified rates of water leakage around and/or through these structures, allowed discharge measurements where there are no structures, and allowed calculation of rates of water exchange with canals.}, number={3-4}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, David and Slater, Eric}, year={1999}, month={Jul}, pages={153–168} }
 @article{genereux_guardiario_1998, title={A Canal Drawdown Experiment for Determination of Aquifer Parameters}, volume={3}, ISSN={1084-0699 1943-5584}, url={http://dx.doi.org/10.1061/(ASCE)1084-0699(1998)3:4(294)}, DOI={10.1061/(ASCE)1084-0699(1998)3:4(294)}, abstractNote={A canal drawdown experiment was used in conjunction with borehole flowmeter measurements to determine the following hydrologic parameters: (1) the transmissivity (\iT) and hydraulic conductivity (\iK) of the Biscayne Aquifer in southeast Florida; (2) the separate hydraulic conductivities of the Miami Limestone (\iK\i\dM) and the Fort Thompson Formation (\iK\i\dF)—the two geologic formations making up the Biscayne Aquifer; and (3) the conductance (\iC) controlling water seepage between the Biscayane Aquifer and the L-31W borrow canal (the canal in which the drawdown experiment was carried out). While the canal drawdown experiment alone would have yielded \iT and \iK, also making borehole flowmeter and aquifer thickness measurements allowed determination of \IK\dM and \IK\dF. Two methods of data analysis were used: one based on the Dupuit equation, the other on a previously published set of equations developed specifically for computing the \iC value required for a regional numerical ground-water flow model. The two methods were in good agreement (within 20% of each other), and averaging results from them gave \iT=1.0×10\u5 m²/day, \iK=7,590 m/day, \IK\dM\N=14,900 m/day, and \IK\dF\N=4,050 m/day. Estimates of \iC from the two methods were also similar, although the exact value derived from the second method would necessarily depend on the size of the model grid cell for which the value was calculated. The methods applied here illustrate an effective means for measurement of field-scale hydrologic parameters and confirm the importance of canal sediments in controlling ground-water exchange with the L-31W canal on the eastern boundary of Everglades National Park.}, number={4}, journal={Journal of Hydrologic Engineering}, publisher={American Society of Civil Engineers (ASCE)}, author={Genereux, David and Guardiario, Jose}, year={1998}, month={Oct}, pages={294–302} }
 @inbook{kraemer_genereux_1998, title={Applications of Uranium- and Thorium-Series Radionuclides in Catchment Hydrology Studies}, url={http://dx.doi.org/10.1016/B978-0-444-81546-0.50027-6}, DOI={10.1016/B978-0-444-81546-0.50027-6}, abstractNote={This chapter discusses applications of uranium- and thorium-series radionuclides in catchment hydrology studies. It also reviews the environmental behavior of the various radionuclides in order to understand and predict their presence and transport in the environment as a result of mining, milling, and other technological redistributions of ores and other materials naturally high in radionuclides. Uranium and thorium decay products are used to determine the age of speleothems and archeological remains, sedimentation rates of marine and lake sediments, rates of water-rock re-equilibration in hydrothermal systems, and calculation of erosion rates on a continental scale. The approaches and models used in interpreting uranium- and thorium-series radionuclide data are simpler and less sophisticated than those that have been developed for stable isotope. Uranium and thorium are long-lived radioactive elements found in all earth material. Uranium can also be removed from solution, even oxidizing ones, by adsorption onto solid substrates due to changing pH. Adsorption of uranium onto ferric oxyhydroxides, clay minerals, and even micaceous minerals at pH values common in natural waters is well known. The application of uranium- and thorium-series radionuclides to quantitatively solve problems in small catchment hydrologic studies has been largely neglected.}, booktitle={Isotope Tracers in Catchment Hydrology}, publisher={Elsevier}, author={Kraemer, Thomas F. and Genereux, David P.}, year={1998}, pages={679–722} }
 @inbook{genereux_hooper_1998, title={Oxygen and Hydrogen Isotopes in Rainfall-Runoff Studies}, url={http://dx.doi.org/10.1016/B978-0-444-81546-0.50017-3}, DOI={10.1016/B978-0-444-81546-0.50017-3}, abstractNote={This chapter provides an overview of traditional hydrograph separation applications on storm event time-scales, and discusses the requirements for successful separation and the results of several isotopic hydrograph separation studies. It also presents other frameworks for application of isotope data on storm-event and longer time-scales, and the findings from a number of studies in which oxygen and/or hydrogen isotopes were used to separate storm streamflow into pre-event and event water. Stable isotopes of oxygen and hydrogen have proven to be useful tools in determining the contribution of rainfall to stormflow, the residence time of water on catchments, and other aspects of watershed hydrology. The separation of contributions from event and pre-event waters to stormflow adds a constraint on streamflow generation that physical (hydrometric) measurements never could. The large contribution of pre-event water to storm streamflow is entirely consistent with the importance of subsurface stormflow as documented by hydrometric studies on forested catchments before isotopes became widely used. Pre-event water usually makes up the bulk of stormflow through some combination of mixing and displacement, even discharging to streams through macro-pores and soil pipes. The isotopes move as the water moves and do not fractionate between the time the water reaches the ground and the time the water exits the catchment as streamflow.}, booktitle={Isotope Tracers in Catchment Hydrology}, publisher={Elsevier}, author={Genereux, David P. and Hooper, Richard P.}, year={1998}, pages={319–346} }
 @article{genereux_1998, title={Quantifying uncertainty in tracer‐based hydrograph separations}, volume={34}, ISSN={0043-1397 1944-7973}, url={http://dx.doi.org/10.1029/98WR00010}, DOI={10.1029/98WR00010}, abstractNote={A method is presented for quantifying the uncertainty in two‐ and three‐component tracer‐based hydrograph separations. The method relates the uncertainty in computed mixing fractions to both the tracer concentrations used to perform the hydrograph separation and the uncertainties in those concentrations. A two‐component example and a three‐component example illustrate the application of the method. The three‐component example yields uncertainty results very similar to those from a previously published Monte Carlo analysis and requires less computation.}, number={4}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Genereux, David}, year={1998}, month={Apr}, pages={915–919} }
 @article{genereux_pringle_1997, title={Chemical mixing model of streamflow generation at La Selva Biological Station, Costa Rica}, volume={199}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/S0022-1694(96)03333-1}, DOI={10.1016/S0022-1694(96)03333-1}, abstractNote={La Selva Biological Station occupies an area of lowland tropical rainforest in central Costa Rica. Sodium and chloride data were used to quantify the mixing proportions of local runoff and geothermal groundwater at several sites throughout the stream channel system in late April, at the end of the dry season. The fraction of streamflow accounted for by geothermal groundwater varied spatially between 0 and 0.85, indicating a significant contribution to streamflow and to stream solute loads from geothermal groundwater at some sites. In general, higher inputs of geothermal groundwater were found at lower elevations. Over half the flow from one basin (the Salto) was due to geothermal groundwater, suggesting a minimum annual runoff of about 0.7 m of geothermal groundwater from this basin. A plot of NaCl vs. fraction of geothermal groundwater revealed watershed-scale chemical differences between the two major drainage systems (the Sura and the Salto), differences that were not apparent from a traditional two-solute plot of Cl vs. Na concentration. A small (21 mm) storm produced relatively little change in mixing proportions, as most throughfall was apparently retained in the relatively dry soils.}, number={3-4}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, David and Pringle, Catherine}, year={1997}, month={Dec}, pages={319–330} }
 @article{genereux_hemond_1994, title={Reply [to “Comment on ‘Determination of gas exchange rate constants for a small stream on Walker Branch watershed, Tennessee’ by D. P. Genereux and H. F. Hemond”]}, volume={30}, ISSN={0043-1397 1944-7973}, url={http://dx.doi.org/10.1029/94WR00160}, DOI={10.1029/94WR00160}, abstractNote={Water Resources ResearchVolume 30, Issue 5 p. 1633-1634 Regular Section Reply [to “Comment on ‘Determination of gas exchange rate constants for a small stream on Walker Branch watershed, Tennessee’ by D. P. Genereux and H. F. Hemond”] David P. Genereux, David P. GenereuxSearch for more papers by this authorHarold F. Hemond, Harold F. HemondSearch for more papers by this author David P. Genereux, David P. GenereuxSearch for more papers by this authorHarold F. Hemond, Harold F. HemondSearch for more papers by this author First published: May 1994 https://doi.org/10.1029/94WR00160AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Broecker, H. C., W. Siems, The role of bubbles for gas transfer from water to air at higher windspeeds: Experiments in the wind-wave facility in Hamburg, Gas Transfer at Water Surfaces W. Brutsaert, G. H. Jirka, 229–236, D. Reidel, Norwell, Mass., 1984. Genereux, D. P., H. F. Hemond, Determination of gas exchange rate constants for a small stream on Walker Branch watershed, Tennessee, Water Resour. Res., 28, 2365–2374, 1992. Gulliver, J. S., Comment on “Determination of gas exchange rate constants for a small stream on Walker Branch watershed, Tennessee” by D. P. Genereux and H. F. Hemond,Water Resour. Res., 5. Jähne, B., T. Wais, M. Barabas, A new optical bubble measuring device: A simple model for bubble contribution to gas exchange, Gas Transfer at Water Surfaces W. Brutsaert, G. H. Jirka, 237–246, D. Reidel, Norwell, Mass., 1984. Kline, S. J., The purposes of uncertainty analysis, J. Fluids Eng., 107, 153–160, 1985. Woolf, D. K., S. A. Thorpe, Bubbles and the air: sea exchange of gases in near-saturation conditions, J. Mar. Res., 49, 435–466, 1991. Volume30, Issue5May 1994Pages 1633-1634 ReferencesRelatedInformation}, number={5}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Genereux, David P. and Hemond, Harold F.}, year={1994}, month={May}, pages={1633–1634} }
 @article{genereux_hemond_mulholland_1993, title={Spatial and temporal variability in streamflow generation on the West Fork of Walker Branch Watershed}, volume={142}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/0022-1694(93)90009-X}, DOI={10.1016/0022-1694(93)90009-X}, abstractNote={Spatially intensive measurements of streamflow were used to document the spatial and temporal variability in streamflow generation on the West Fork of Walker Branch Watershed, a 38.4 ha forested catchment in Oak Ridge, Tennessee. The study focused on a 300 m section of a small stream, and covered a wide range of flow conditions (Qweir, streamflow at the basin outlet, varied from about 350 to 3500 l minȡ1). There was enormous spatial variability in the stream inflow, down to the finest scale investigated (reaches 20 m in length). Lateral inflow to longer reaches (60ȁ130 m) was linearly correlated with Qweir over the full range of flows studied, making it possible to estimate the spatial pattern of stream inflow from measurement of Qweir alone. The heterogeneous nature of the karstic dolomite bedrock was the dominant control on the observed spatial variability in streamflow generation. This thesis is consistent with the results of field investigations using natural tracers, reported in a companion paper. Bedrock structure and lithology may affect streamflow generation directly (via water movement through fractured rock), and indirectly (by influencing the slope and thickness of the overlying soil). While the West Fork contains all the topographic and surface hydrologic features of larger basins (ridge tops, valleys, hollows, spurs, ephemeral and perennial stream channels), it covers an area which is relatively small with respect to the bedrock heterogeneity. Therefore, while the hydrologic processes observed on the West Fork are no doubt typical of those occurring elsewhere in karst terrain, the particular patterns of spatial and temporal variability observed are somewhat specific to the study site.}, number={1-4}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, David P. and Hemond, Harold F. and Mulholland, Patrick J.}, year={1993}, month={Feb}, pages={137–166} }
 @article{genereux_hemond_mulholland_1993, title={Use of radon-222 and calcium as tracers in a three-end-member mixing model for streamflow generation on the West Fork of Walker Branch Watershed}, volume={142}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/0022-1694(93)90010-7}, DOI={10.1016/0022-1694(93)90010-7}, abstractNote={Measurements of naturally occurring radon-222 (222Rn) and calcium (Ca) in surface and subsurface waters on the West Fork of Walker Branch suggest that a simple three-end-member mixing model provides a realistic and useful framework for streamflow generation over a wide range of flow conditions. The three end members are vadose zone water, soil groundwater, and bedrock groundwater. Bedrock groundwater was distinguished from the soil end members on the basis of its high Ca content; 222Rn concentration was the basis for the distinction between vadose zone water (low 222Rn) and soil groundwater (high 222Rn). The behavior of the end members with changing flow was consistent with a wide variety of environmental observations, including temperature and flow variations at springs, water table responses, the general lack of saturated zones on hillslopes and even near the stream in some places, and the importance of water movement through bedrock. Variability in the chemistry of the end members precluded using other solutes (Na, K, and SO4) to test the mixing fractions derived from 222Rn and Ca data; during those times of year when the soil temperature is most different from that of the underlying bedrock (late summer and late winter), temperature may be a useful tracer for distinguishing between water from the soil end members and that from bedrock. The mixing model provides a simple framework for analyzing the essential features of streamflow generation in this highly heterogeneous terrain.}, number={1-4}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, David P. and Hemond, Harold F. and Mulholland, Patrick J.}, year={1993}, month={Feb}, pages={167–211} }
 @article{genereux_hemond_1992, title={Determination of gas exchange rate constants for a small stream on Walker Branch Watershed, Tennessee}, volume={28}, ISSN={0043-1397 1944-7973}, url={http://dx.doi.org/10.1029/92WR01083}, DOI={10.1029/92WR01083}, abstractNote={The steady state tracer gas method was used to determine gas exchange rate constants (k) for a small stream (annual average flow ≤1 m3/min) draining the West Fork of Walker Branch watershed in eastern Tennessee. Chloride was used as a conservative tracer to account for dilution by lateral inflow, and propane and ethane were used as volatile tracers. Gas exchange rate constants for propane (kp) were about 100 d−1 over a wide range of flow conditions, while those for ethane (ke) were about 117 d1; an equivalent rate constant for O2 (KO2) would be about 118–139 d−1, depending on the method used for its calculation. These rate constants are much larger than those typically found in rivers and large streams. Much lower kp values (about 50d−1) were found during one experiment conducted at low flow with much of the stream surface covered with floating leaves. Nineteen previously published empirical equations were used to predict kO2 values for one 72m stream reach; agreement between the predicted and measured values was generally very poor, underscoring the importance of field‐measured gas exchange rates in studies of the transport and fate of volatile compounds. Because ethane and propane have similar gas exchange rates and similar aqueous diffusion coefficients (ke/kp and De/Dp are both close to 1, where D is the compound's diffusion coefficient), accurate determination of the exponent n in the relationship ke/Kp− (De/Dp)n was not possible. The ratio ke/kp (1.17) is much closer to De/Dp (1.24) than to He/Hp (0.82, where H is the compound's Henry's law constant), suggesting that stripping of dissolved volatiles by air bubbles was not a dominant mode of gas exchange for the study stream.}, number={9}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Genereux, David P. and Hemond, Harold F.}, year={1992}, month={Sep}, pages={2365–2374} }
 @article{genereux_hemond_1991, title={Measurement of the radon-222 content of soil gas by liquid scintillation counting}, volume={87}, ISSN={0168-9622}, url={http://dx.doi.org/10.1016/0168-9622(91)90025-R}, DOI={10.1016/0168-9622(91)90025-R}, abstractNote={A method is described for measuring the 222Rn content of soil gas using a conventional liquid scintillation counter. Gas samples, collected in wetted ground-glass syringes, are equilibrated with a scintillation cocktail which is then expelled into a scintillation vial and counted. The method is straightforward and relatively fast (5–6 min. of operator's time per sample), and yields results having 95% confidence limits of ∼ ± 10% for samples containing ≥ 2·104 Bq m−3 of 222Rn. The method is applied to a watershed near Bickford Reservoir in Massachusetts, U.S.A., where soil-gas 22Rn content is found to be reasonably constant horizontally, but strongly and systematically increasing with depth, to > 5·104 Bq m−3 at soil depths of ∼ 1 m.}, number={3-4}, journal={Chemical Geology: Isotope Geoscience section}, publisher={Elsevier BV}, author={Genereux, David P. and Hemond, Harold F.}, year={1991}, month={Oct}, pages={265–275} }
 @article{genereux_hemond_1990, title={Naturally Occurring Radon 222 as a Tracer for Streamflow Generation: Steady State Methodology and Field Example}, volume={26}, ISSN={0043-1397 1944-7973}, url={http://dx.doi.org/10.1029/WR026i012p03065}, DOI={10.1029/WR026i012p03065}, abstractNote={This paper presents a quantitative framework for the use of naturally occurring 222Rn as a hydrologic flow path tracer under conditions of steady streamflow. The methodology consists of two distinct parts, the first of which is the determination of 
, the average 222Rn content of the water feeding a given stream reach. 
 is determined by measuring the concentrations of 222Rn and two injected tracers (one conservative, the other volatile) in the stream water and solving a mass balance equation for 222Rn around the reach of interest. The second part of the methodology involves using 
 values to determine the sources of stream inflow (and, implicitly, the flow paths important in streamflow generation). One means of accomplishing this, simple “geographic source” separations, is presented here. Both parts of the methodology were illustrated with a field experiment at the Bickford watershed in central Massachusetts. The injected tracers used were NaCl (conservative) and propane (volatile). The value of 
 (700 disintegrations per minute (dpm)/L) was found to be closer to the 222Rn content of vadose zone water (500 dpm/L) than to that of saturated zone water (2000 dpm/L), suggesting that lateral unsaturated flow was important in supplying base flow at the study site.}, number={12}, journal={Water Resources Research}, publisher={American Geophysical Union (AGU)}, author={Genereux, David P. and Hemond, Harold F.}, year={1990}, month={Dec}, pages={3065–3075} }
 @article{genereux_hemond_1990, title={Three-component tracer model for stormflow on a small appalachian forested catchment — Comment}, volume={117}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/0022-1694(90)90103-5}, DOI={10.1016/0022-1694(90)90103-5}, number={1-4}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Genereux, David P. and Hemond, Harold F.}, year={1990}, month={Sep}, pages={377–380} }