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DIVISION S-1-SOIL PHYSICS

Recharge from a Subsidence Crater at the Nevada Test Site

^a Desert Research Institute, currently with the USDA-ARS National Sedimentation Laboratory, 598 McElroy Dr., Oxford, MS 38655 USA
^b Harry Reid Center for Environmental Studies, P.O. Box 4009, Las Vegas, NV 89119 USA
^c Desert Research Institute, Hydrologic Sciences Division, 755 E. Flamingo Rd, Las Vegas, NV 89119 USA

wilson{at}sedlab.olemiss.edu

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 was instrumented for neutron logging beneath the playa 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 ponding event of 63000 m³ 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 yr 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 playa 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.

Abbreviations: ET, evapotranspiration • NTS, Nevada Test Site • PA, performance assessment