@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 {70,
	title = {A Geochemical Approach to Determine Sources and Movement of Saline Groundwater in a Coastal Aquifer},
	journal = {Groundwater},
	volume = {52},
	year = {2014},
	pages = {756-768},
	abstract = {Geochemical evaluation of the sources and movement of saline groundwater in coastal aquifers can aid in the initial mapping of the subsurface when geological information is unavailable. Chloride concentrations of groundwater in a coastal aquifer near San Diego, California, range from about 57 to 39,400 mg/L. On the basis of relative proportions of major-ions, the chemical composition is classified as Na-Ca-Cl-SO4, Na-Cl, or Na-Ca-Cl type water. ?2H and ?18O values range from ?47.7? to ?12.8? and from ?7.0? to ?1.2?, respectively. The isotopically depleted groundwater occurs in the deeper part of the coastal aquifer, and the isotopically enriched groundwater occurs in zones of sea water intrusion. 87Sr/86Sr ratios range from about 0.7050 to 0.7090, and differ between shallower and deeper flow paths in the coastal aquifer. 3H and 14C analyses indicate that most of the groundwater was recharged many thousands of years ago. The analysis of multiple chemical and isotopic tracers indicates that the sources and movement of saline groundwater in the San Diego coastal aquifer are dominated by: (1) recharge of local precipitation in relatively shallow parts of the flow system; (2) regional flow of recharge of higher-elevation precipitation along deep flow paths that freshen a previously saline aquifer; and (3) intrusion of sea water that entered the aquifer primarily during premodern times. Two northwest-to-southeast trending sections show the spatial distribution of the different geochemical groups and suggest the subsurface in the coastal aquifer can be separated into two predominant hydrostratigraphic layers.},
	issn = {1745-6584},
	doi = {10.1111/gwat.12108},
	url = {http://dx.doi.org/10.1111/gwat.12108},
	author = {Anders, Robert and Mendez, Gregory O. and Futa, Kiyoto and Danskin, Wesley R.}
}
