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

Edward Kwicklis,Ronald L Hershey,John Hoaglund,Ate Visser,Irene Farnham

doi:10.1007/s10040-021-02397-x

Abstract

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 (δ2H), oxygen (δ18O), carbon (δ13C) and sulfur (δ34S), and radioisotopes of carbon (14C) and chloride (36Cl). Analysis of the patterns of groundwater 14C data and the δ2H and δ18O 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 36Cl/Cl and δ34S 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.

Highlights

Pahute Mesa, located in the northwest corner of the Nevada National Security Site (NNSS), formerly known as the Nevada Test Site, was the location of 85 underground nuclear tests conducted between 1965 and 1992 (US Dept. of Energy 2015; Fig. 1)
Understanding of the groundwater flow system near Pahute Mesa at the NNSS was updated with new groundwater hydrochemical and isotopic data collected as part of a phase II characterization effort
Groundwater flow paths defined by Cl and SO4 contours indicate convergent flow into a hydraulic trough in western Pahute Mesa that is created by a combination of low- and high-permeability faults

Summary

Introduction

Pahute Mesa, located in the northwest corner of the Nevada National Security Site (NNSS), formerly known as the Nevada Test Site, was the location of 85 underground nuclear tests conducted between 1965 and 1992 (US Dept. of Energy 2015; Fig. 1). These trends suggest that dilute groundwater flowing west or southwest from high-elevation recharge areas on eastern Pahute Mesa mixes with more concentrated groundwater from Gold Flat in western Pahute Mesa and the Thirsty Canyon area.

Results

Conclusion