Isotopic Composition of Water in a Deep Unsaturated Zone Beside a Radioactive-Waste Disposal Area Near Beatty, Nevada

By David A. Stonestrom, David E. Prudic, and Robert G. Striegl

ABSTRACT

The isotopic composition of water in deep unsaturated zones is of interest because it provides information relevant to hydrologic processes and contaminant migration. Profiles of oxygen-18 (¹⁸O), deuterium (D), and tritium (³H) from a 110-meter deep unsaturated zone, together with data on the isotopic composition of ground water and modern-day precipitation, are interpreted in the context of water-content, water-potential, and pore-gas profiles. At depths greater than about three meters, water vapor and liquid water are in approximate equilibrium with respect to D and ¹⁸O. The vapor-phase concentrations of D and ¹⁸O have remained stable through repeated samplings. Vapor-phase ³H concentrations have generally increased with time, requiring synchronous sampling of liquid and vapor to assess equilibrium. Below 30 meters, concentrations of D and ¹⁸O in pore water become approximately equal to the composition of ground water, which is isotopically lighter than modern precipitation and has a carbon-14 (¹⁴C) concentration of about 26 percent modern carbon. These data indicate that net gradients driving fluxes of water, gas, and heat are directed upwards for undisturbed conditions at the Amargosa Desert Research Site (ADRS). Superimposed on the upward-directed flow field, tritium is migrating away from waste in response to gradients in tritium concentrations.