@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{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} }