@article {2989,
	title = {The triple argon isotope composition of groundwater on ten-thousand-year timescales},
	journal = {Chemical Geology},
	volume = {583},
	year = {2021},
	month = {11/2021},
	pages = {120458},
	abstract = {Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100~ka). Here we present the first high-precision (<1{\textperthousand}) measurements of triple Ar isotopes (40Ar, 38Ar, 36Ar) in groundwater. We introduce a model that distinguishes radiogenic 40Ar from atmospheric 40Ar by using the non-radiogenic Ar isotopes (36Ar, 38Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40Ar excess (Δ40Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ40Ar ranges from ~0{\textperthousand} (the expected minimum value) to +4.2{\textperthousand} across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ40Ar on aquifer lithology. We suggest that Δ40Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14C- and 4He-based constraints on groundwater age and flow.},
	keywords = {groundwater, Groundwater dating, Isotope geochemistry, Noble gas},
	isbn = {0009-2541},
	doi = {10.1016/j.chemgeo.2021.120458},
	url = {https://www.sciencedirect.com/science/article/pii/S0009254121004010},
	author = {Seltzer, Alan M. and Krantz, John A. and Ng, Jessica and Danskin, Wesley R. and Bekaert, David V. and Barry, Peter H. and Kimbrough, David L. and Kulongoski, Justin T. and Severinghaus, Jeffrey P.}
}
@article {2919,
	title = {Aquifer-Scale Observations of Iron Redox Transformations in Arsenic-Impacted Environments to Predict Future Contamination},
	journal = {Environmental Science \& Technology Letters},
	volume = {7},
	year = {2020},
	month = {Aug-12-2020},
	pages = {916 - 922},
	abstract = {Iron oxides control the mobility of a host of contaminants in aquifer systems, and the microbial reduction of iron oxides in the subsurface is linked to high levels of arsenic in groundwater that affects greater than 150 million people globally. Paired observations of groundwater and solid-phase aquifer composition are critical to understand spatial and temporal trends in contamination and effectively manage changing water resources, yet field-representative mineralogical data are sparse across redox gradients relevant to arsenic contamination. We characterize iron mineralogy using X-ray absorption spectroscopy across a natural gradient of groundwater arsenic contamination in Vietnam. Hierarchical cluster analysis classifies sediments into meaningful groups delineating weathering and redox changes, diagnostic of depositional history, in this first direct characterization of redox transformations in the field. Notably, these groupings reveal a signature of iron minerals undergoing active reduction before the onset of arsenic contamination in groundwater. Pleistocene sediments undergoing postdepositional reduction may be more extensive than previously recognized due to previous misclassification. By upscaling to similar environments in South and Southeast Asia via multinomial logistic regression modeling, we show that active iron reduction, and therefore susceptibility to future arsenic contamination, is more widely distributed in presumably pristine aquifers than anticipated.},
	keywords = {BANGLADESH, BENGAL BASIN, groundwater, PLEISTOCENE AQUIFER, POLLUTION, RELEASE, RIVER DELTA, Sediment, TRANSPORT, VIETNAM},
	issn = {2328-8930},
	doi = {10.1021/acs.estlett.0c0067210.1021/acs.estlett.0c00672.s00110.1021/acs.estlett.0c00672.s00210.1021/acs.estlett.0c00672.s00310.1021/acs.estlett.0c00672.s004},
	url = {https://apps.webofknowledge.com/InboundService.do?product=WOS\&Func=Frame\&DestFail=http\%3A\%2F\%2Fwww.webofknowledge.com\&SrcApp=search\&SrcAuth=Alerting\&SID=8AkJOJLKFU3j5nkGaRI\&customersID=Alerting\&mode=FullRecord\&IsProductCode=Yes\&AlertId=4d48b20a-7d27-4fa2-},
	author = {Nghiem, Athena A. and Shen, Yating and Stahl, Mason and Sun, Jing and Haque, Ezazul and DeYoung, Beck and Nguyen, Khue N. and Thi Mai, Tran and Trang, Pham Thi Kim and Pham, Hung Viet and Mailloux, Brian and Harvey, Charles F. and van Geen, Alexander and Bostick, Benjam{\'\i}n C.}
}
@article {2839,
	title = {Springwater provenance and flowpath evaluation in Blue Lake, Bonneville basin, Utah},
	journal = {Chemical Geology},
	volume = {529},
	year = {2019},
	month = {Jan-12-2019},
	pages = {119280},
	abstract = {Water in Utah and Nevada is important for agriculture, municipal use, solute transport, and ecosystem preservation. Large spring wetland systems occur on the playa margin of the Bonneville basin, including Blue Lake, 15 km south of the Bonneville Salt Flats on the Utah-Nevada state border. Large spring systems have historically been studied as water budgets don{\textquoteright}t apparently balance from direct mountain front recharge (Nelson and Mayo, 2014; Gardner and Heilweil, 2014; and others). Blue Lake is no exception; prior studies here have suggested discharge rates 0.04 - 0.05 km3/yr (Louderback and Rhode, 2009), greater than expected for modelled recharge in the surrounding mountain range (0.03 km3/yr). Three hypothesized recharge mechanisms for Blue Lake are tested: mountain-front recharge, interbasinal groundwater flow, and infiltration from historic Lake Bonneville. Remote sensing suggests that a conservative estimate of Blue Lake discharge constitutes 50-64\% of modelled mountain-front recharge (Flint et al., 2011). Major ions, δ18O, δ2H, dissolved gases (14C, 3He, 4He, Ne, Ar, Kr, Xe) and trace elements (Sr and 87Sr/86Sr isotopes) comprehensively constrain recharge conditions, water-rock interactions, flowpaths, and groundwater provenance of this large spring system. 14C signatures suggest that Blue Lake discharge has a transit time between 5,600 and 12,200 years, older than that of Fish Springs. Noble gas concentrations in Blue Lake water suggest an elevated recharge temperature greater than 19{\textdegree}C and low salinity, indicating a deep water table and high geothermal gradient in the recharge area. 87Sr/86Sr ratios of playa-margin springs are elevated from that of mid-playa groundwaters, springwaters from the adjacent mountain range, and alluvial fill groundwater from the valley directly south of Blue Lake. Playa-margin spring Sr isotope values (0.713-0.714) are most similar to direct runoff from the Deep Creek Range granodiorite outcrop (>0.713). Interbasinal groundwater flow in combination with mountain-front recharge is best supported by chemical data rather than mountain-front recharge alone or the slow discharge of regional aquifers recharged by lacustrine infiltration from Lake Bonneville.},
	keywords = {Bonneville basin, Geochemistry, groundwater, Isotope chemistry},
	issn = {00092541},
	doi = {10.1016/j.chemgeo.2019.119280},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0009254119303870},
	author = {Lerback, Jory Chapin and Hynek, Scott A. and Bowen, Brenda B. and Bradbury, Christopher D. and Solomon, D. Kip and Fernandez, Diego P.}
}
@article {2767,
	title = {The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35-km Transect Perpendicular to the Upper Brahmaputra River in Assam, India},
	journal = {Water Resources Research},
	volume = {54},
	year = {2018},
	month = {Jun-10-2019},
	pages = {8160 - 8173},
	abstract = {Well testing in the floodplain of the Brahmaputra River in Golaghat and Jorhat districts of Assam, India, shows that groundwater arsenic (As) concentrations increase with distance from the river. To establish the origin of this pattern, an additional 900 wells <60-m deep were tested for As and nine sites were drilled along a 35-km transect perpendicular to the river. The field data show no relation between groundwater As concentrations ranging from <1 to 660 μg/L along the transect and (a) As concentrations of <1{\textendash}5 mg/kg in cuttings of aquifer sand recovered while drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates, however, a marked increase in the thickness of a clay layer capping the aquifer starting from <1{\textendash}5 m near the river to over 60 m at the most distant site toward the base of the Naga foothills. Organic radiocarbon ages of 18{\textendash}46 kyr obtained from all but one of 13 clay samples indicate pre-Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of dissolved inorganic carbon of 0.2, 4.7, and 17.8 kyr were measured in groundwater from three monitoring wells installed to 30{\textendash}60-m depth at distances of 10, 20, and 40 km from the river, respectively. A conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.},
	keywords = {aquifer flushing, Arsenic, groundwater, South Asia},
	issn = {0043-1397},
	doi = {10.1029/2017WR022485},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2017WR022485},
	author = {Choudhury, R. and Nath, B. and Khan, M. R. and Mahanta, C. and Ellis, T. and Geen, A.}
}
@article {1021,
	title = {Alabama Gulf Coast groundwaters: 4He and 14C as groundwater-dating tools},
	journal = {Geology},
	volume = {32},
	year = {2004},
	note = {id: 1222},
	pages = {289-292},
	abstract = {Analyses of 4He and 14C in groundwaters from a Miocene quartz aquifer on the Alabama Gulf Coast show the usefulness of 4He for dating these Holocene groundwaters. In this aquifer system of low alkalinities and low pHs, radiocarbon ages can be used without model correction. The groundwaters studied ranged from 42.3\% to 95.5\% modern carbon and yielded uncorrected ages of 375{\textendash}6790 yr. Radiogenic 4He ages ranged from >50{\textendash}7500 yr. },
	keywords = {14C, 4He, age dating, Baldwin County Alabama, groundwater},
	doi = {10.1130/G20081.2 },
	author = {Carey, Anne and Dowling, Carolyn B. and Poreda, Robert J.}
}
