The hydrogeology of Picacho Basin was studied to define the stratigraphy, basin structure, physical
and hydraulic properties of the basin sediments, and predevelopment and postdevelopment conditions of
ground-water flow as of 1985. The study area includes about 900 square miles and contains a sedimentfilled
asymmetric graben. The greatest sediment thickness occurs along the east margin of the graben.
Basin sediments contain the principal water-bearing units and are separated into lower, middle, and upper
units. The lower unit is several thousand feet thick and contains a conglomerate facies and a playa facies
that contains a thick evaporite sequence. The middle and upper units contain alluvial and playa facies that
are as much as 1,500 feet thick. Ground water occurs in lower and upper aquifer systems separated by a
middle confining unit that comprises the playa facies of the three units. Hydraulic properties and
compressibility of the middle and upper units are much greater than those of the lower unit. Vertical-head
gradients exist, and vertical flow occurs within and between the aquifer systems.
Early development of surface-water supplies resulted in increased recharge through deep percolation
of irrigation water. Later development of the ground-water supplies resulted in extensive water-level
declines, changes in the direction of ground-water flow, removal of water from storage, aquifer
compaction, land subsidence, and earth fissures. Dewatering of pore spaces in the upper unit has been the
primary source of water; however, as much as 80 percent of the water derived from storage in the Eloy
area has resulted from compaction of pore spaces.
