A wide range of isotopes in the 238U, 235U, and 232Th decay chains was measured in geothermal brines collected from two production zones at 1898 and 3220 m in the Salton Sea Scientific Drilling Project well. High concentrations of radium, radon, and lead isotopes are generated and maintained by the input of these isotopes from solid phases into brine by both recoil and leaching processes, by the high chloride content of the brine which complexes radium and lead, and by the apparent absence of suitable unoccupied adsorption sites. In contrast, uranium, thorium, actinium, bismuth, and polonium isotopes all have low concentrations due to their efficient sorption from brine to rock. Measurements of short-lived isotopes in these decay series yield insights regarding the mechanisms controlling radioisotope exchange, and they permit estimation of rates of brine-rock interaction. For example, the 228Ac/228Ra activity ratio of 0.2 in brines indicates that the mean residence time of actinium in solution before sorption onto solid surfaces is less than 2.5 hours.

If molecular diffusion to fracture walls limits the rate of actinium sorption, the maximum width of the larger fractures in which sampled brine resides is 1--2 cm. However, the mean width of fractures must be only 1--2 μm, in order to account for the recoil input of 223Ra. The ratios of radium isotopes in the brine provide information about the mechanisms of recoil and leaching in transferring radium from rock to brine. Brine/rock concentration ratios <(dpm/g)b/(dpm/g)r> of radium isotopes increase with increasing half-life, so that 223Ra (11 days)=0.05--0.09, 228Ra (5.8 years) =0.26--0.40, 226Ra (1600 years)=0.45--0.97. Two mechanisms could explain this dependence on half-life: (1) input of radium by the alpha recoil process occurs in microfractures and pore spaces, but the rate of diffusion down microfractures is so slow that it diminishes the effect of recoil input of the shorter-lived isotopes to the larger fractures where the brine sampled resides, or (2) weathering and leaching of radium from solid phases occurs on time scales comparable to the half-lives of 228Ra and 226Ra. The second mechanism is more likely because the 212Pb/224Ra activity ratios are within 20% of the value predicted by a simple recoil model with negligible adsorption, indicating that recoil input of 212Pb (10.6-hour half-life) is not limited by diffusion.

This model permits the rates of chemical exchange between brine and rock to be estimated. These rates indicate the mean residence times of lead and radium in solution are about 30 years,and the residence time of radium in associated solid phases is about 2000 years. The estimated uncertainty in these residence times is about a factor of 2. ¿ American Geophysical Union 1988