Previous studies have shown that Fault H controls a significant part of the fluid recharge in the Cerro Prieto, Mexico geothermal field. The fault is a conduit for the upflow of deep hot fluids into the reservoir, and for the downflow of colder groundwaters from aquifers located above the producing formations. Further examination of the field data supports the hypothesis that under prior natural-state conditions some of the geothermal fluids flowing up Fault H discharged into the overlying groundwater aquifers. When pressures in the system decreased in response to commercial exploitation, fluid flow in the upper regions of the fault (i.e., above the level of the producing formations) reversed direction, that is, colder groundwaters began descending into the geothermal reservoir. Numerical simulation studies of these phenomena show similar physical and chemical changes to those observed. The analysis of the field data and the results of the numerical modeling study of the role of Fault H in the hydrology of the eastern CP-II and CP-III areas of the field are discussed.

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