The Fort Huachuca Military Reservation, on the northeast flank of the Huachuca Mountains, is in the southern part of the San Pedro River drainage in the Basin and Range physiographic province in Arizona. The main sources of water available in the reservation area are ground water stored in two unconsolidated sedimentary deposits filling the San Pedro basin, and spring flow in Garden ·and Huachuca Canyons in the Huachuca Mountains.
The unconsolidated deposits are divided into the upper and lower units of basin flu. These units yield the major part of the fort's water supply, and pumping from them has caused the water level in the Fort Huachuca well field to decline 3 feet per year. All the upper unit and 40 feet of the 220-foot-thick lower unit have seen dewatered in post wells 1 and 2. In Garden Canyon, spring flow is derived from solution channels and fractures in carbonate rocks; and in Huachuca Canyon, from fractures in mudstone, sandstone, carbonate rocks, ·and granite. The flow from springs generally is not used by the fort, but it is sufficient to supply the entire water demand during some periods.
Spring flow, if used to supplement the ground-water supply, will decrease the draft on the ground-water reservoir in the two basin-fill units; or it could be used for artificial recharge to these aquifers. A second well field, if developed in the North Gate-Libby Field area, would partly accomplish the same result by decreasing the heavily concentrated draft on the ground-water reservoir of the Fort Huachuca well field, and by utilizing ground water that now moves unused northeastward to the San Pedro River.
