Over 100 stable isotope and 45 tritium analyses from thermal and nonthermal waters of the Jemez Mountains region, New Mexico, have been used to define the hydrodynamics of the Valles caldera (Baca) geothermal system and related geothermal fluids of the region. Evaluation of 36 cold meteoric waters yields an equation for the Jemez Mountains meteoric water line of ΔD=8Δ18O+12, while further evaluation of nine cold meteoric waters yields an equation relating recharge elevation to deuterium content of E(meters)=-44.9 (ΔD)-1154. Based on the deuterium content of five Baca well waters (223¿--294¿C), the average recharge elevation of the Valles geothermal system ranges from 2530 to 2890 m. This range of elevations falls between the elevations of the lowest point of the caldera floor (2400 m) and the summit of the resurgent dome inside the caldera (3430 m). Thus stable isotopes indicate that the caldera depression probably serves as a recharge basin for the deep geothermal system. Although cold spring waters of the Jemez Mountains region consist of meteoric water, tritium analyses show that most of them contain water between 20 and 75 years old.

The two major streams draining the Valles moat zone, San Antonio Creek and East Fork Jemez River, may contain more than 50% of this relatively old groundwater depending on the season. In contrast, streams draining the central resurgent dome of the caldera contain present-day meteoric water. Using piston flow and homogeneously mixed reservoir models as end-member cases, the tritium contents of the baca fluids (0.18--1.10 tritium units (TU)) indicate that the means residence time of water in the reservoir is between 60 and 10,000 years old. Deep geothermal fluids display a positive oxygen 18 shift of not less than 2%0 because of rock-water isotopic exchange at 220¿--300¿C. The Valles geothermal system is capped by a vapor zone that is roughly 600 m thick and best developed at Sulphur Springs. Fumarolic steam at Sulphur Springs is unusually depleted in oxygen 18, suggesting that it is probably derived by boiling of near-surface groundwater at 200¿C. Surface acid-sulfate waters are mixtures of condensed steam and surficial groundwaters that display the isotopic processes of evaporation and exchange between H2O and CO2.

A lateral outflow plume discharges from the Valles geothermal system down the Jemex fault zone and feeds two sets of thermal springs in San Diego Canyon. Isotopic evidence shows that these springs consist of three components: (1) deep geothermal fluid, (2) surficial and/or near-surface groundwater, and (3) relatively old, but cold, mineralized water. This latter component presumably circulates in Paleozoic strata underlying the western caldera flank. Hot, Precambrian, pore fluid brine occurs beneath the main Valles caldera hydrothermal system and may be generated by metamorphic processes in the relatively impermeable conductive regime above the magnetic heat source of the caldera.