Evaluation of Recharge Potential at Crater U5a (WISHBONE)
Radionuclides are present both below and above the water table at the Nevada Test Site (NTS), as the result of underground nuclear testing. Mobilization and transport of radionuclides from the vadose zone is a complex process that is influenced by the solubility and sorption characteristics of the individual radionuclides, as well as the soil water flux. On the NTS, subsidence craters resulting from testing underground nuclear weapons are numerous, and many intercept surface water flows.
... craters collect surface water above the sub-surface point of device detonation, these craters may provide a mechanism for surface water to recharge the groundwater aquifer system underlying the NTS. Given this situation, there is a potential for the captured water to introduce contaminants into the groundwater system. Crater U5a (WISHBONE), located in Frenchman Flat, was selected for study because of its potentially large drainage area, and significant erosional features, which suggested that it has captured more runoff than other craters in the Frenchman Flat area. Recharge conditions were studied in subsidence crater U5a by first drilling boreholes and analyzing the collected soil cores to determine the soil properties and moisture conditions. This information, coupled with a 32-year precipitation record, was used to conduct surface and vadose zone modeling. Surface water modeling predicted that approximately 13 ponding events had occurred during the life of the crater. Vadose zone modeling indicated that since the crater's formation approximately 5,900 m 3 of water were captured by the crater. Of this total, approximately 5,200 m 3 of potential recharge may have occurred, and the best estimates of annual average potential recharge rates lie between 36 and 118 cm of water per year. The term potential is used here to indicate that the water is not technically recharge because it has not yet reached the water table. Using a conservative conceptual model (worst-case scenario for deep movement of moisture), the wetting front may have reached a depth of approximately 130 m below the crater bottom. Wetting front advancement occurred almost exclusively following ponding events, indicating that without additional ponding events, movement of the wetting front would slow down dramatically or for all practical purposes stop. Modeling indicates that to date, water has not yet intersected the water table. Additionally, if contaminates are transported to the water table, the hydrology of Frenchman Flat is such that transport off of the NTS would likely require thousands of years. iii CONTENTS ABSTRACT .