In situ tritium production and fluid mean residence times in the two subsystems of the Valles Caldera hydrothermal system, New Mexico
— Lisa Shevenell and Fraser Goff

Tritium ( 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.) contribute 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 yrs, if all post-bomb ³H has entered and mixed with the reservoir waters. The results from this model indicate 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 yrs) than waters in the Sulphur Springs subsystem ( 2000 to 4400 yrs). When accounting for the likelihood that not all post-bomb recharge water has mixed with the reservoir fluids, much younger mean residence times are obtained (660 to 1280 yrs). The oldest water occurs in the Redondo Creek subsystem, but some waters in this subsystem are similar in age to those at Sulphur Springs, possibly suggesting stratification in the reservoir. 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 fluid's mean residence time and volumes within a reservoir.