The use of tritium in groundwater to determine fluid mean residence times of Valles caldera hydrothermal fluids, New Mexico, USA

Naturally and artificially produced 3H in precipitation is used to place limits on mean residence times of fluids associated with the Valles caldera geothermal reservoir. The 3H concentrations in precipitation are used to calculate fluid mean residence times using two end-member cases: piston-flow and well-mixed reservoirs. These simplified models using 3H yield relative fluid mean residence times in different portions of the system. In-situ 3H production in the reservoir through neutron interaction with 6Li is considered, and only very low levels of 3H (≤ 0.01 T.U) are contributed to the waters by this process. Calculations using the end-member cases at Valles caldera indicate that mean residence times of geothermal waters in the reservoir are between 3000 and 10,000 years. The results also show that the two distinct subsystems of the main reservoir are associated with different fluid mean residence times. Waters in the Redondo Creek subsystem are older (~4300 to > 10,000 years) than waters in the Sulphur Springs subsystem (~2000–4400 years). Comparison of the results of the end-member cases with those obtained using more sophisticated methods indicates that the two end-member cases, with the use of 3H, can provide acceptable limits (order of magnitude) on a fluidʹs mean residence time and volumes within a reservoir.