Geochemistry and occurrence of ground water in the Allen Flat Basin, Arizona

Abstract

Allen Flat is a small, intermontane basin in southeastern Arizona. The subsurface hydrology of the region was characterized by integrating geologic, hydrologic, and chemical data. Three aquifer systems were identified. Mountains and upland areas that border Allen Flat store ground water in bedrock aquifers and perched aquifers along washes. Basin alluvium constitutes the largest aquifer. Head measurements and drilling records indicate that the basin aquifer is semi-confined, heterogeneous, recharged near mountain fronts, discharged largely as underflow, and in hydrologic equilibrium. Groundwater quality is generally good, with local evidence of high fluoride waters and septic tank contamination. Major ion and stable isotope data were used to describe the chemical evolution of both mountain-front and basin wide groundwater systems. A qualitative and quantitative analysis of these data provided better definition of recharge zones and groundwater flow patterns, identified distinct water-bearing zones, explained the dynamics of a small geothermal system, and suggested regional paleoclimatic changes.