Delineation of subsurface deposits through the integration of geological, geophysical and hydrological data, Rillito Creek, Tucson, Arizona

Rillito Creek, near Tucson, Arizona is like many ephemeral streams in the Southwest in that flows within the creek are an important source of recharge to the regional groundwater system. Increasing demands on groundwater resources in the area have prompted proposals for artificial recharge along the 20-km-long channel. The U.S. Geological Survey, along with the Arizona Department of Water Resources, is developing a regional groundwater-flow model to determine recharge effectiveness. This study provides the geometry and a detailed characterization of recent alluvium and basin-fill deposits. The approach used to determine the geometry and physical and hydraulic properties of the deposits included the integration of surface and borehole geological data and geophysical data. Data were derived from electromagnetic, resistivity and seismic-refraction surveys, pre-existing geologic logs and analysis of cores and cuttings for grain-size distribution, particle and bulk density, porosity, saturated hydraulic conductivity and volumetric water content. A fine-grained layer in the lower reach of Rillito Creek may impede infiltration and decrease recharge rates in this area.