Dissolved Kr and Xe isotope ratios in groundwater enable quantitative resolution of water-table depth at the time of recharge

Constraints on the magnitude of water-table depth fluctuations - on timescales from decades to millennia - are of great interest to the paleoclimate, hydrogeology, and water resource management communities. Quantifying glacial-interglacial shifts in water-table depth, for example, offers the potential to improve our understanding of long-term regional hydroclimate change. Additionally, resolving mean water-table depths from the centuries prior to modern human development in regions lacking historical records presents an opportunity to quantify anthropogenic drawdown of groundwater resources. Motivated by these applications, we have developed a technique to quantitatively reconstruct water-table depth at the time of recharge with high-precision measurements of dissolved Kr and Xe stable isotopes in groundwater.

This technique relies on the principle that gravitational settling in unsaturated zone air leads to a depth-proportional enrichment of heavy Kr and Xe isotopes relative to the well-mixed atmosphere, as verified by a recent study in the Amargosa Desert, Nevada (Seltzer et al., 2017). These unsaturated zone gases dissolve into groundwater at the water-table and thus "lock in" the signal of gravitational settling at the time of recharge. With an extraction and analysis system recently developed at Scripps Institution of Oceanography, our ±5 per meg amu^-1 precision measurements permit ±1-2 m accuracy estimation of past-water table depth.

We will describe the physical fractionation model and companion inverse model underpinning our technique, as well as results from proof-of-principle groundwater measurements in the Mojave Desert, San Diego, and Fresno, California. We observe a 20-m lowering of mean water-table in San Diego from the late Pleistocene to the Holocene, and will discuss its paleoclimatic implications. Future applications of dissolved noble gas isotopes will also be addressed, including the potential for excess ⁴⁰Ar as a dating tool and Ar, Kr, and Xe isotopes in seawater as tracers of air-sea gas exchange.

A. 2017. y-state-fractionation-heavy-noble-gas-isotopes-deep-unsaturated-zone">Steady state fractionation of heavy noble gas isotopes in a deep unsaturated zone</a>. Water Resources Research. 53:2716-2732.