EVALUATION OF GROUNDWATER MIXING, FLOW PATHS AND TRAVEL TIMES AT THE NEVADA TEST SITE USING GEOCHEMICAL AND ISOTOPIC TRACERS

The Nevada Test Site (NTS) in southern Nevada was the location of 828 underground nuclear tests conducted between 1951 and 1992. The Underground Testing Area (UGTA) program led by the U.S. Department of Energy (DOE) is currently undertaking studies to understand the extent of groundwater contamination caused by radionuclides introduced into the subsurface by these tests. As part of these studies, natural variations in groundwater chemical and isotopic compositions are being used along with groundwater modeling to interpret the groundwater flow patterns and velocities in each of the major testing areas at the NTS. The geochemical studies have allowed aspects of the NTS flow system to be identified that would not be apparent from hydraulic data alone.

The geochemical studies rely on the concept that variations in the groundwater chemical and isotopic compositions are compatible with some flow system interpretations but not with others. The chemical and isotopic species considered in the analyses typically include the stable isotopes of water (²H and ¹⁸O), major cations and anions, carbon isotopes (¹³C and ¹⁴C), chlorine-36 (³⁶Cl) and strontium-87 (⁸⁷Sr). The geochemical studies conducted thus far for the major testing areas each use a combination of graphical and geochemical inverse modeling approaches to explain the downgradient changes in groundwater composition as a function of groundwater mixing and water/rock interaction. The flow system interpretations permitted by the geochemical studies are evaluated in the context of prevailing hydraulic gradients, the overall hydrogeologic setting, and the long-term climatic history of the area to further evaluate their likelihood.