Uranium-thorium series radionuclides in brines and reservoir rocks from two deep geothermal boreholes in the Salton Sea geothermal field, southeastern California

摘要: Naturally occurring U and Th series radionuclides have been analyzed in high temperature brines (~300°C, 25 wt% dissolved solids) associated rocks from two deep geothermal wells located on the northeastern margin of Salton Sea Geothermal Field (SSGF). These data are part a study SSGF as natural analog possible radionuclide behavior near nuclear waste repository constructed salt beds, permit evaluation some characteristics water-rock interaction SSGF. Rock/Brine concentration ratios (Rc = (dpm/g)rock/(dpm/g)brine) were found to vary unity for isotopes Ra, Pb Rn about 5 × 105 232Th. The sorptivity 232Th is closely followed by that 238U 234U ~ 104), suggesting retained +4 oxidation state reducing conditions brines. relatively solubility 210Pb 212Pb attributed formation chloride complexes, while Ra complexing, lack suitable adsorption sites due brine salinity temperature, prevent MnO2 RaSO4 forming. The 228Ra226Ra approximately equal those their parents (232Th230Th) rocks, indicating equilibration brine-rock system achieved within mean life 228Ra (8.3 years). 224Ra228Ra these 0.7, either (1) composition not homogeneous 224Ra decays fracture zones deficient travels wellhead or (2) brine-host rock complete (5.2 days) because desorption solid phase impeded. 228Ac228Ra activity ratio studied <0.1, this residence time 228Ac before sorption onto surfaces estimated be <70 minutes, potential rapid removal reactive fom brines. The greatly enriched 226Ra (2–3 dpm/g, 104–105 times its parent 230Th), reservoir 10% relative 230Th. Material balance calculations 2226Ra 18O suggest reside 102–103 years, formed 10,000–40,000 years B.P., porosity cannot more than 20%.