GEOLOGIC FRAMEWORK OF AQUIFER UNITS AND GROUND-WATER FLOW PATHS NEAR THE OUTLET OF TWO SOUTHWESTERN ALLUVIAL BASINS---UPPER VERDE RIVER, ARIZONA

A detailed understanding of ground-water movement is critical for managing base flow in the upper Verde River, Arizona—an area of rapid population growth that is supplied almost entirely by ground-water pumping. Ground water supplying the upper Verde River near the confluence of two large alluvial valleys exits basin-fill sediment through bedrock via preferential flow paths. The largest springs lie at the intersection of the Verde River canyon and the trend of a basin-bounding fault. Regional ground-water movement is affected by factors such as the provenance and grain size of alluvial sediment, buried volcanic units, bedrock fractures, faults, and solution cavities in the regional carbonate aquifer. In this 3-year study, geologic framework and hydrogeology of water-bearing units were characterized for major basin-fill aquifers and the interconnected regional aquifer using a multi-disciplinary approach. Geophysical techniques (aero e.g. magnetic, gravity, and radiometric surveys) are used to interpret basin geometry and structural complexities. For example, magnetic anomalies effectively map hidden volcanic rocks lying at shallow depths beneath the ground surface. Semi-circular magnetic lows indicate the extent of shallowly buried (<200-300 m) lati-andesite plugs. These plugs probably act as barriers to groundwater flow because of their relatively unfractured and impermeable nature. Del Rio spring, a major discharge zone, is up gradient of numerous plugs. A conceptual model of geology at depth is developed by field reconnaissance, reinterpretation of geologic maps and existing driller’s logs—which has greatly improved understanding of shallow buried volcanic layers. Flow paths supplying major discharge springs are corroborated using water chemistry and environmental isotopes as natural tracers, including NETPATH modeling. A geochemical study of base flow and ground water in the Verde River identifies sources of major inflows to the river based on chemical differences in Sr, Ba, Li, HCO₃, dO¹⁸, dD, and C¹⁴, and quantified the discharge of each spring using tracer dilution. The integrated findings of all these approaches create a detailed geologic framework that will be applied to a ground-water flow model.