@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{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{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} }