Uncertainty in recharge from subsidence craters at Frenchman Flat, Nevada Test Site: Impacts of initial and boundary conditions and media properties

David Matthew Ely, University of Nevada, Las Vegas

Abstract

The Nevada Test Site has over 400 subsidence craters formed by the collapse of overlying rock following underground nuclear tests. Under natural conditions, infiltration and recharge would not be considered likely due to infrequent precipitation events and extreme evapotranspiration. Crater U5a in Frenchman Flat was chosen for study because it intercepts significantly more drainage than surrounding craters. Vadose zone modeling was conducted to test the sensitivity of water movement to boundary and initial conditions. The effects of ponding depth was negligible but depth of wetting front movement was highly dependent on the lateral extent of the ponds. The model was insensitive to the initial conditions tested due to the extreme dryness of the soil; Field and laboratory observations provide evidence for a 63,000 m{dollar}\sp3{dollar} pond occurring on the initial crater surface. Model results predict such a wetting front reaching the water table in 30 years. Sediment deposited by this large pond, however, provides an effective barrier to future infiltration and recharge.