GROUNDWATER RECHARGE AND MOVEMENT IN THE CENTRAL CHIRICAHUA MOUNTAINS, ARIZONA

The Chiricahua Mountains in southeastern Arizona are a major recharge zone for aquifers in several adjacent basins. The mountains are composed primarily of volcanics and volcaniclastics associated with Tertiary calderas, along with lesser volumes of Cretaceous and Paleozoic sedimentary rocks. The Tertiary rocks are predominantly rhyolitic, but characteristics such as mode of deposition and degree of fracturing are quite variable.

Stable isotope signatures from groundwaters in an adjacent basin suggest a relatively low-elevation origin for precipitation that produces recharge, and waters in some parts of the basin also appear to have undergone post-recharge evaporation. The apparent low-elevation origin of recharge is at odds with several facts: dissolved gas data suggest many waters in the range are recharged at high elevations; numerous springs are present at high elevations; precipitation increases with increasing elevation; and snowpack forms only above ~2400 m, providing a large pulse of infiltration in the spring that is not present at lower elevations.

Much of the high-elevation recharge in the Chiricahuas appears to originate in intercaldera ash-flow tuff facies that are relatively porous and transmissive compared to the more dense, unfractured lava deposits (primarily rhyolite, dacite, and monzonite). No known groundwater outflow to the surface occurs in the intercaldera facies, because their dip and the orientation of bedding planes and foliations channels water toward the range’s major axis. Most recognized springs in the range are related to formation boundaries. Numerous such springs are present in areas of the lava deposits that are adjacent to the intercaldera facies. Significant surface discharge also occurs in previously-unidentified springs located in drainage channels. The vast majority of these outflows are associated with fractures.

The enriched stable isotope signatures of basin groundwater (compared to the values for high-elevation precipitation) are explained by alteration of the snowpack prior to and during melt. Many basin waters with evaporative signatures appear to result from “re-recharge”—recharge at low elevation of water that originally recharges at high elevation, discharges from mid-elevation springs and experiences evaporation while flowing overland downslope.