HISTORIC ANOMALIES AND RECENT REGULARITIES (AND IRREGULARITIES) IN GAS TRANSPORT BEHAVIOR AT THE AMARGOSA DESERT RESEARCH SITE, NV

The USGS Amargosa Desert Research Site (ADRS) in southwestern Nevada has a well-documented history of anomalous unsaturated-zone gas transport. The Nation’s first commercial low-level radioactive waste (LLRW) facility (1962-1992) serves as a continuing source of organic, inorganic, and radioactive contaminants to the adjacent ADRS. Waste emplacement initiated contaminant plume migration into the surrounding 110-m thick unsaturated zone. Predictive models of varying complexity have failed to account for observed migration rates and spatial patterns of gaseous contaminants, including tritium, radiocarbon, and mercury, on the multi-decadal timescale of contaminant transport. In contrast, recent monitoring of contaminant plumes over the past decade as well as a SF₆ tracer injection test indicate more predictable gas-phase transport behavior, though some inconsistencies still persist. Reconciliation of historical anomalous transport and recent more typical transport observations requires consideration of both spatial and temporal variability of gas behavior. Analytical results from the SF₆ tracer study reveal a substantial increase in estimated effective diffusion coefficient with transport distance, suggesting an apparent scale-dependence of diffusion that may help explain apparent discrepancies in data interpretations. The influence of historical waste-disposal activities on apparent anomalous transport is a major understanding gap since site conditions prior to ~1994 are generally unknown. We consider aspects associated with site history, early-time perturbations to the system, and the potential role of advection. Further, we evaluate the roles of geologic heterogeneity and barometric pumping in supporting anomalous gas transport. This synthesis of historic and recent results provides context for evaluating the evolution of contaminant gas transport in arid unsaturated zones where much of the Nation’s low-level radioactive waste is interred.