Other
Scientific paper
Oct 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.2719z&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 10, pp.2719-2731
Other
3
Scientific paper
Naturally occurring U and Th series radionuclides have been analyzed in high temperature brines (~300°C, 25 wt% dissolved solids) and associated rocks from two deep geothermal wells located on the northeastern margin of the Salton Sea Geothermal Field (SSGF). These data are part of a study of the SSGF as a natural analog of possible radionuclide behavior near a nuclear waste repository constructed in salt beds, and permit evaluation of some characteristics of water-rock interaction in the SSGF. Rock/Brine concentration ratios ( R c = (dpm/ g ) rock /(dpm/ g ) brine ) were found to vary from near unity for isotopes of Ra, Pb and Rn to about 5 × 10 5 for 232 Th. The high sorptivity of 232 Th is closely followed by that of 238 U and 234 U ( R c ~ 5 × 10 4 ), suggesting that U is retained in the +4 oxidation state by the reducing conditions in the brines. The relatively high solubility of 210 Pb and 212 Pb is attributed to formation of chloride complexes, while the high Ra solubility is attributed to chloride complexing, a lack of suitable adsorption sites due to the high brine salinity and temperature, and the reducing conditions that prevent MnO 2 and RaSO 4 from forming. The 228 Ra / 226 Ra ratios in the brines are approximately equal to those of their parents ( 232 Th / 230 Th ) in associated rocks, indicating that Ra equilibration in the brine-rock system is achieved within the mean life of 228 Ra (8.3 years). The 224 Ra / 228 Ra ratios in these brines are about 0.7, indicating that either (1) brine composition is not homogeneous and 224 Ra decays in fracture zones deficient in Ra and Th as the brine travels to the wellhead or (2) Ra equilibration in the brine-host rock system is not complete within the mean life of 224 Ra (5.2 days) because the desorption of 224 Ra from the solid phase is impeded. The 228 Ac / 228 Ra activity ratio in the SSGF brines studied is <0.1, and from this ratio the residence time of 228 Ac in the brine before sorption onto solid surfaces is estimated to be <70 minutes, indicating the potential for rapid removal of reactive isotopes fom brines. The brine is greatly enriched in 226 Ra (2-3 dpm/g, about 10 4 -10 5 times that of its parent 230 Th), while reservoir rocks are about 10% deficient in 226 Ra relative to 230 Th. Material balance calculations for 2 226 Ra and 18 O suggest that brines reside in the reservoir for 10 2 -10 3 years, that the SSGF formed 10,000-40,000 years B.P., and that porosity cannot be more than 20%.
Elders Wilfred A.
Hammond Douglas E.
Ku Teh-Lung
Zukin Jeffrey G.
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