Understanding long-term groundwater flow at Pahute Mesa and vicinity, Nevada National Security Site, USA, from naturally occurring geochemical and isotopic tracers

Recently collected naturally occurring geochemical and isotopic groundwater tracers were combined with historic data from the Pahute Mesa area of the Nevada National Security Site (NNSS), Nevada, USA, to provide insights into long-term regional groundwater flow patterns, mixing and recharge. Pahute Mesa was the site of 85 nuclear detonations between 1965 and 1992, many of them deeply buried devices that introduced radionuclides directly into groundwater. The dataset examined included major ions and field measurements, stable isotopes of hydrogen (delta H-2), oxygen (delta O-18), carbon (delta C-13) and sulfur (delta S-34), and radioisotopes of carbon (C-14) and chloride (Cl-36). Analysis of the patterns of groundwater C-14 data and the delta H-2 and delta O-18 signatures indicates that groundwater recharge is predominantly of Pleistocene age, except for a few localized areas near major ephemeral drainages. Steep gradients in sulfate (SO4) and chloride (Cl) define a region near the western edge of the NNSS where high-concentration groundwater flowing south from north of the NNSS merges with dilute groundwater flowing west from eastern Pahute Mesa in a mixing zone that coincides with a groundwater trough associated with major faults. The Cl-36/Cl and delta S-34 data suggest that the source of the high Cl and SO4 in the groundwater was a now-dry, pluvial-age playa lake north of the NNSS. Patterns of groundwater flow indicated by the combined data sets show that groundwater is flowing around the northwest margin of the now extinct Timber Mountain Caldera Complex toward regional discharge areas in Oasis Valley.

Automated summary

By combining naturally occurring geochemical and isotopic groundwater tracers with historic data from the Pahute Mesa area in Nevada, USA, researchers gained insights into long-term regional groundwater flow patterns, mixing, and recharge dynamics. The study revealed that groundwater recharge is predominantly of Pleistocene age, except for localized areas near major ephemeral drainages, and identified a mixing zone where high-concentration groundwater merges with dilute groundwater, with a source of high Cl and SO4 traced back to a now-dry playa lake north of the study area. Patterns of groundwater flow indicate that the groundwater is flowing around the now-extinct Timber Mountain Caldera Complex towards discharge areas in Oasis Valley.