Arsenic stratification in the Santa Fe Formation, Bernalillo, New Mexico
— T. E. Kelly and Scott Reinert

Valles caldera hosts a high-temperature geothermal system (220-300°C), which has surface expression in hot springs, fumaroles and other features in the area. In order to trace the sources of solutes in the hydrothermal fluids, we analyzed springs and well waters in the region for ³⁶Cl and chloride. The fluids sampled are of two types, one characterized by relative high ³⁶Cl/Cl ratios and low CI concentrations, and the other with low ³⁶Cl/Cl ratios and high Cl concentrations. The results for the first group (Cl <10 mg/L; 36Cl/Cl of 300-1000 x 10 ^-15) are typical of rainwater, whereas the second group of samples (Cl, 800 to 9400 mg/L; ³⁶Cl/Cl, 11-26 x 10 ^-15) are geothermal brines reflecting longer residence times in the subsurface, with associated leaching of solutes from subsurface sources. The history of the first group of samples is fairly uncomplicated, reflecting the recent meteoric nature of the waters with perhaps some anthropogenic addition, but the second group must derive its salts from non-meteoric sources which are assessed here. We calculated ³⁶Cl/Cl ratios in the major formations in the region, based on estimated subsurface neutron fluxes for these formations. A comparison of calculated ³⁶Cl/Cl ratios with measured ³⁶Cl/Cl ratios in the hydrothermal brines eliminates the Quaternary volcanic rocks that the waters currently reside in as a likely source for chloride in the waters. More likely sources are formations at depth, such as the Precambrian basement and Paleozoic rocks, especially the Madera and Abo Formations. A likely scenario is that meteoric waters penetrate to depths of 2-3 km, and leach solutes from rocks there before coming to reside in near-surface volcanics (residence time <0.l Ma) after convective upwelling.