Evaluation of Groundwater Flow Paths and Travel Times in Frenchman Flat Using Carbon-14 and Other Geochemical Indicators

Frenchman Flat at the Nevada Test Site was the location of ten underground nuclear tests between 1965 and 1971. To help evaluate how far contamination associated with these tests will move over the next 1,000 years, groundwater geochemical and isotopic data were examined in the context of the overall hydrologic setting and paleoclimate history of Frenchman Flat to estimate groundwater flow directions and velocities in the shallow alluvial and tuff aquifers. Carbon-14 (¹⁴C) data are used to calculate groundwater ages and define evolutionary trends in other groundwater components (such as cation compositions) that help to identify groundwaters that lie along a flow path. The evolutionary trends in cation compositions are, in turn, used to provide estimates of groundwater ages at locations where ¹⁴C data are lacking. Selected flow paths identified from relative groundwater ages and cation compositions are also tested with geochemical inverse models to determine whether down gradient changes in other geochemical components could be explained through water/rock interactions. The accuracy of the estimated groundwater ¹⁴C ages and the mobility of groundwater ¹⁴C in the presence of potentially ¹⁴C-sorbing mineral phases were evaluated by comparing the temporal variability in groundwater ³⁶Cl/Cl ratios based on these ¹⁴C ages against theoretical reconstructions of past atmospheric ³⁶Cl production and against 40,000-year-long archives of meteoric ³⁶Cl/Cl deposition preserved in packrat middens. The calculated groundwater ages span a range of about 8,000 to 40,000 years and demonstrate that most of the groundwater in Frenchman Flat was recharged during the cooler, wetter conditions that prevailed in the late Pleistocene. The low velocities of groundwater movement (0.1 to 1.1 m/yr) estimated between well pairs are consistent the near-absence of recharge under the present-day arid climate and result in predictions of likely transport distances of 100 to 1,100 m from test locations over the next 1,000 years.