THE USE OF TRITIUM IN GROUNDWATER TO DETERMINE FLUID MEAN RESIDENCE TIMES OF VALLES CALDERA HYDROTHERMAL FLUIDS, NEW-MEXICO, USA

Naturally and artificially produced H-3 in precipitation is used to pl ace limits on mean residence times of fluids associated with the Valle s caldera geothermal reservoir. The H-3 concentrations in precipitatio n are used to calculate fluid mean residence times using two end-membe r cases: piston-flow and well-mixed reservoirs. These simplified model s using H-3 yield relative fluid mean residence times in different por tions of the system. In-situ 3H production in the reservoir through ne utron interaction with Li-6 is considered, and only very low levels of H-3 (less than or equal to 0.01 T.U) are contributed to the waters by this process. Calculations using the end-member cases at Valles calde ra indicate that mean residence times of geothermal waters in the rese rvoir are between 3000 and 10,000 years. The results also show that th e two distinct subsystems of the main reservoir are associated with di fferent fluid mean residence times. Waters in the Redondo Creek subsys tem are older (similar to 4300 to > 10,000 years) than waters in the S ulphur Springs subsystem (similar to 2000-4400 years). Comparison of t he results of the end-member cases with those obtained using more soph isticated methods indicates that the two end-member cases, with the us e of H-3, can provide acceptable limits (order of magnitude) on a flui d's mean residence time and volumes within a reservoir.