STABLE ISOTOPE STUDY OF PRECIPITATION AND RECHARGE PROCESSES IN CENTRAL NEVADA

Precipitation, spring water, and shallow infiltration samples were collected at four locations in central Nevada on a biannual basis starting in 1999. Sample sites are at an altitude of 2100 to 2300 m. Stable isotope analyses are used to assess the contribution of cool and warm season precipitation to the recharge budget, and to evaluate the timing and extent of evaporation during recharge. Cool season precipitation (mostly snowfall) accounts for 75 to 80% of the annual precipitation total, and has an average deuterium value that is 40 to 70 permil lighter than average summer rainfall. Perched mountain springs located near each precipitation gauge show small isotopic variations over time (generally less than 5 permil), and are reasonable proxies for average local recharge. Comparison of the stable isotope data for precipitation and springs suggests more than 90% of recharge originates from cool season precipitation.

Three of the four springs show isotopic evidence of moderate evaporation, inferred to occur during recharge. Shallow soil infiltration samples collected in lysimeters show variations in infiltration amount from year-to-year, independent of the total precipitation amount. The rate at which the snowpack melts may be a key control on the net infiltration flux. Smaller net infiltration volumes tend to show large evaporation effects whereas larger volumes show little evidence of significant evaporation. Water vapor loss through the soil zone during slow infiltration is inferred to be the dominant control on evaporation during recharge.

This work was funded by the Hydrologic Resources Management Program, National Nuclear Security Administration, Nevada Site Office, and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-Eng-48.