Description of hydrogeologic heterogeneity and evaluation of radionuclide transport at an underground nuclear test

Realistic models of contaminant transport in groundwater demand detailed ch aracterization of the spatial distribution of subsurface hydraulic properti es, while at the same time programmatic constraints may limit collection of pertinent hydraulic data. Fortunately, alternate forms of data can be used to improve characterization of spatial variability. We utilize a methodolo gy that augments sparse hydraulic information (hard data) with more widely available hydrogeologic information to generate equiprobable maps of hydrog eologic properties that incorporate patterns of connected permeable zones. Geophysical and lithologic logs are used to identify hydrogeologic categori es and to condition stochastic simulations using Sequential Indicator Simul ation (SIS). The resulting maps are populated with hydraulic conductivity v alues using field data and Sequential Gaussian Simulation (SGS). Maps of su bsurface hydrogeologic heterogeneity are generated for the purpose of exami ning groundwater flow and transport processes at the Faultless underground nuclear test, Central Nevada Test Area (CNTA), through large-scale, three-d imensional numerical modeling. The maps provide the basis for simulation of groundwater flow, while transport of radionuclides from the nuclear cavity is modeled using particle tracking methods. Sensitivity analyses focus on model parameters that are most likely to reduce the long travel times obser ved in the base case. The methods employed in this study have improved our understanding of the spatial distribution of preferential flowpaths at this site and provided the critical foundation on which to build models of grou ndwater flow and transport. The results emphasize that the impacts of uncer tainty in hydraulic and chemical parameters are dependent on the radioactiv e decay of specific species, with rapid decay magnifying the effects of par ameters that change travel time. (C) 2000 Elsevier Science B.V. All rights reserved.