@article{wells_gilmore_nelson_mittelstet_bohlke_2021, title={Determination of vadose zone and saturated zone nitrate lag times using long-term groundwater monitoring data and statistical machine learning}, volume={25}, ISSN={["1607-7938"]}, url={https://doi.org/10.5194/hess-25-811-2021}, DOI={10.5194/hess-25-811-2021}, abstractNote={Abstract. In this study, we explored the use of statistical machine learning and long-term groundwater nitrate monitoring data to estimate vadose zone and saturated zone lag times in an irrigated alluvial agricultural setting. Unlike most previous statistical machine learning studies that sought to predict groundwater nitrate concentrations within aquifers, the focus of this study was to leverage available groundwater nitrate concentrations and other environmental variables to determine mean regional vertical velocities (transport rates) of water and solutes in the vadose zone and saturated zone (3.50 and 3.75 m yr−1, respectively). The statistical machine learning results are consistent with two primary recharge processes in this western Nebraska aquifer, namely (1) diffuse recharge from irrigation and precipitation across the landscape and (2) focused recharge from leaking irrigation conveyance canals. The vadose zone mean velocity yielded a mean recharge rate (0.46 m yr−1) consistent with previous estimates from groundwater age dating in shallow wells (0.38 m yr−1). The
saturated zone mean velocity yielded a recharge rate (1.31 m yr−1) that was more consistent with focused recharge from leaky irrigation canals, as indicated by previous results of groundwater age dating in intermediate-depth wells (1.22 m yr−1). Collectively, the statistical machine learning model results are consistent with previous observations of relatively high water fluxes and short transit times for water and nitrate in the primarily oxic aquifer. Partial dependence plots from the model indicate a sharp threshold in which high groundwater nitrate concentrations are mostly associated with total travel times of 7 years or less, possibly reflecting some combination of recent management practices and a tendency for nitrate concentrations to be higher in diffuse infiltration recharge than in canal leakage water. Limitations to the machine learning approach include the non-uniqueness of different transport rate combinations when comparing model performance and highlight the need to corroborate statistical model results with a robust conceptual model and complementary information such as groundwater age.
                    }, number={2}, journal={HYDROLOGY AND EARTH SYSTEM SCIENCES}, publisher={Copernicus GmbH}, author={Wells, Martin J. and Gilmore, Troy E. and Nelson, Natalie and Mittelstet, Aaron and Bohlke, John K.}, year={2021}, month={Feb}, pages={811–829} }
 @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{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={Abstract}, 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{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_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={Abstract}, 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{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={Abstract}, 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{gilmore_birgand_chapman_2013, title={Source and magnitude of error in an inexpensive image-based water level measurement system}, volume={496}, ISSN={0022-1694}, url={http://dx.doi.org/10.1016/j.jhydrol.2013.05.011}, DOI={10.1016/j.jhydrol.2013.05.011}, abstractNote={Recent technological advances have opened the possibility to use webcams and images as part of the environmental monitoring arsenal. The potential sources and magnitude of uncertainties inherent to an image-based water level measurement system are evaluated in an experimental design in the laboratory. Sources of error investigated include image resolution, lighting effects, perspective, lens distortion and water meniscus. Image resolution and meniscus were found to weigh the most in the overall uncertainty of this system. Image distortion, although largely taken into account by the software developed, may also significantly add to uncertainty. Results suggest that “flat” images with little distortion are preferable. After correction for the water meniscus, images captured with a camera (12 mm or 16 mm focal lengths) positioned 4–7 m from the water level edge have the potential to yield water level measurements within ±3 mm when using this technique.}, journal={Journal of Hydrology}, publisher={Elsevier BV}, author={Gilmore, Troy E. and Birgand, François and Chapman, Kenneth W.}, year={2013}, month={Jul}, pages={178–186} }