A Conceptual Model and Preliminary Estimate of Potential Tritium Migration from the Benham (U-20c) Site, Pahute Mesa, Nevada Test Site

U-20c is the site of a large below-water-table nuclear test near the Nevada Test Site boundary. A conceptual model of potential groundwater migration of tritium from U-20c is constructed and quantitatively evaluated in this report. The lower portion of the collapse chimney at Benham is expected to intersect 200 m of permeable rhyolite lava, overlain by similar thicknesses of low-permeability zeolitized bedded tuff, then permeable welded tuff. Vertical groundwater flow through the chimney is predicted to be minimal, horizontal transport should be controlled by the regional groundwater flow. Analytic solutions treating only advective transport indicate 1 to 2 km of tritium movement (95% confidence interval 0.7--2.5 km) within 5 years after test-related pressure-temperature transients have dissipated. This point lies at the axis of a potentiometric surface trough along the west edge of Area 20, Nevada Test Site. Within 25 years, movement is predicted to extend to 3 km (95% confidence interval 2--5 km) approximately to the intersection of the trough and the Nevada Test Site boundary. Considering the effects of radioactive decay, but not dispersion, plume concentration would fall below Safe Drinking Water Act standards by 204 years, at a predicted distance of 11 km (95% confidence interval 7--31 km). This point is located in the eastern portion of the Timber Mountain Caldera moat within the Nellis Air Force Range (military bombing range)