A MULTIPLE-TRACER APPROACH TO UNDERSTANDING REGIONAL GROUNDWATER FLOW IN THE SNAKE VALLEY AREA OF THE EASTERN GREAT BASIN, USA

Groundwater in Snake Valley and surrounding basins in the eastern Great Basin province of the western United States is being targeted for large-scale groundwater extraction and export. Concern about declining groundwater levels and spring flows in western Utah as a result of the proposed groundwater withdrawals has led to efforts that have improved the understanding of this regional groundwater flow system. In this study, environmental tracers (d²H, d¹⁸O, ³H, ¹⁴C, ³He, ⁴He, ²⁰Ne, ⁴⁰Ar, ⁸⁴Kr, and ¹²⁹Xe) and major ions from 142 sites were evaluated to investigate groundwater recharge and flow-path characteristics. With few exceptions, d²H and d¹⁸O show that most valley groundwater has similar ratios to mountain springs, indicating recharge is dominated by relatively high-altitude precipitation. The spatial distribution of ³H, terrigenic helium (⁴He_terr), and ³H/³He ages shows that modern groundwater (<60 yr) in valley aquifers is found only in the western third of the study area. Pleistocene and late-Holocene age groundwater is found in the eastern parts of the study area. The age of Pleistocene groundwater is supported by minimum adjusted radiocarbon ages of up to 32 ka. Noble gas recharge temperatures (NGTs) are generally 1–11 ^oC in Snake and southern Spring Valleys and >11 ^oC to the east of Snake Valley and indicate a hydraulic discontinuity between Snake and Tule Valleys across the northern Confusion Range. The combination of NGTs and ⁴He_terr shows that the majority of Snake Valley groundwater discharges as springs, evapotranspiration, and well withdrawals within Snake Valley rather than continuing northeastward to discharge at either Fish Springs or the Great Salt Lake Playa. The refined understanding of groundwater recharge and flow paths acquired from this multi-tracer investigation has broad implications for interbasin subsurface flow estimates and future groundwater development.