Recharge from a subsidence crater at the Nevada Test Site

Current recharge through the alluvial fans of the Nevada Test Site (NTS) is considered to be negligible, but the impact of more than 400 nuclear subsidence craters on recharge is uncertain, Many of the craters contain a playa region, but the impact of these playas has not been addressed, It was hypothesized that a crater playa would focus infiltration through the surrounding coarser-grained material, thereby increasing recharge. Crater U5a was selected because it represented a worst case for runoff into craters. A borehole mas instrumented for neutron logging beneath the plays center and immediately outside the crater. Physical and hydraulic properties were measured along a transect in the crater and outside the crater. Particle-size analysis of the 14.6 m of sediment in the crater and morphological features of the crater suggest that a large pending el ent of approximate to 63 000 m(3) had occurred since crater formation. Water flow simulations with HYDRUS-2D, which were corroborated by the measured water contents, suggest that the wetting front advanced initially by as much as 30 m yr(-1) with a recharge rate 32 Zr after the event of 2.5 m yr(-1). Simulations based on the measured properties of the sediments suggest that infiltration will occur preferentially around the play a perimeter. However, these sediments were shown to effectively restrict future recharge by storing water until removal by evapotranspiration (ET), This work demonstrated that subsidence craters may be self-healing.