The application of borehole logging to characterize the hydrogeology of the faultless nuclear test site, Nevada, USA

Abstract

The Central Nevada Test Area was the site of the “Faultless” underground nuclear test, a potential source of radio nuclide contamination to aquifers in Hot Creek Valley, Nevada, USA. Field studies in 1992 and 1993 used hydrologic logging and water sampling to determine the adequacy of the current groundwater monitoring network. New data from the monitoring wells and an abandoned postshot hole raise questions about the ability of the current monitoring network to detect migration. Logging and sampling at monitoring wells HTH-1 and HTH-2 reveal that the construction of HTH-1 may be responsible for its elevated water level (as compared to pre-nuclear test levels) and may also create a local groundwater mound in the alluvium that accounts for higher post-test water levels at HTH-2. This mound would serve to divert flow around the monitoring wells, so that only migration of contaminants through the underlying, higher-pressure volcanic units is currently monitored. A hydraulic high observed in an abandoned hole between the nuclear test and the monitoring wells further reduces the likelihood that HTH-1 and HTH-2 can intercept contaminant migration. Hydrologic logs from a postshot well, drilled into the nuclear detonation zone, reveal inflow at about 485 m below land surface that drives strong vertical flow both up and down the well to outflow zones at about 350 m and 600 m. Although hydrologic relationships between the cavity, the rubble chimney, and the surrounding alluvium are unknown, migration from the site has probably not yet occurred, because regional hydraulic pressures are greater than those observed in this postshot hole.