David E. Prudic Robert G. Striegl David W. Morganwalp Herbert T. Buxton David A. Stonestrom 1999 <p>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 (<span size="-2">¹⁸</span>O), deuterium (D), and tritium (^{<span size="-2">3</span>}H) 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 <span size="-2">¹⁸</span>O. The vapor-phase concentrations of D and <span size="-2">¹⁸</span>O have remained stable through repeated samplings. Vapor-phase ^{<span size="-2">3</span>}H concentrations have generally increased with time, requiring synchronous sampling of liquid and vapor to assess equilibrium. Below 30 meters, concentrations of D and <span size="-2">¹⁸</span>O in pore water become approximately equal to the composition of ground water, which is isotopically lighter than modern precipitation and has a carbon-14 (<span size="-2">¹⁴</span>C) 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. </p> application/pdf en U.S. Geological Survey Isotopic composition of water in a deep unsaturated zone beside a radioactive-waste disposal area near Beatty, Nevada reports