Wolfsberg, A.V., Stauffer, P.H., Walvoord, M.A., Vadose-Zone Fluid and Solute Flux in Deep Arid Systems at the Nevada Test Site: Modeling the Effects of Climate Change, Plant-Soil Interactions, and Material Heterogeneity, Eos. Trans. AGU, 84(46), Fall Meet. Suppl., Abstract H32A-0519, 2003.

Abstract

Understanding liquid and vapor fluxes in the vadose zone is necessary for assessments of both groundwater recharge and contaminant migration. However, direct measurement of such fluxes in arid vadose zones is virtually impossible. Therefore, they are estimated in this study using simulation techniques integrated with field and laboratory measurements of material properties, matric potentials, and environmental tracers. Compounding the problem, present-day fluxes are not in steady state. Rather, they represent ongoing responses to climate and vegetation changes that occurred tens of thousands of years ago, the exact timing of which is uncertain. Therefore, the simulations seek to reduce time-varying boundary condition uncertainty by coupling independent data sets highlighting different processes reflective of when the climate changed to the present warm, arid conditions at Frenchman Flat on the Nevada Test Site. As the climate changed to arid conditions, vegetation changes caused a major shift in the vadose-zone liquid flow patterns, as observed in matric potential and chloride profiles. As the climate changed to warmer conditions, stable isotopes of oxygen and hydrogen became enriched in the shallow soils due to increased evaporation. Thus, simulations seeking to match field observations address independent processes with the different data sets. Whereas chloride serves as a tracer for liquid-phase water only, the oxygen and hydrogen stable isotopes trace water movement in both liquid and gas phases. Flux estimates based upon chloride data are low and most sensitive to the timing of the climate change. Transport simulations for oxygen and hydrogen stable isotopes, which are dominated by vapor diffusion, indicate a shorter post-climate-change warm, dry period. Thus, if climate warming occurred concurrently with end of the pluvial period, the stable isotope simulation results support the lower present-day flux predictions. Most importantly for this study, the low estimated liquid and vapor fluxes suggest that diffusion of gas-phase contaminants is the more important process for contaminant migration assessment associated with shallow waste disposal.