Noble gas isotopes in groundwater as a proxy for past water table depth

Noble gases (NGs) in groundwater have been widely used as a quantitative, physical proxy for mean-annual surface temperatures in recharge areas. In this study, we investigate whether isotope ratios of heavy NGs may also enable precise estimation of past water table depths. Together, paleothermometry and water table depth reconstruction from NG measurements in groundwater would present a unique tool for quantifying past hydroclimate change. We measured isotope ratios of argon, krypton and xenon throughout a 110-m deep unsaturated zone (UZ) at the United States Geological Survey (USGS) Amargosa Desert Research Site (ADRS) in Nevada, USA. Based on previous measurements of gases in porous media together with theoretical considerations, we expected that gravitational settling would cause a nearly linear increase in heavy-to-light isotope ratios with depth. After correcting for some atmospheric contamination, our measured UZ isotope ratios confirm the hypothesis that gravitational settling dominates isotopic fractionation of heavy NGs in the UZ (along with the weaker influences of thermal diffusion and non-conservative fluxes of water vapor and CO2). Our study validates the expectation that heavy NG isotopes are sensitive indicators of depth within the UZ at steady state. We introduce an inverse model capable of resolving paleo-UZ depth from gas-phase Ar, Kr and Xe isotope ratios either measured in occluded UZ gas or inferred from dissolved NG measurements in confined groundwater.