Subsurface stratigraphy and hydrology of the Rillito Creek-Tanque Verde Wash area, Tucson, Arizona

Abstract

The Rillito Creek-Tanque Verde Wash area is divided into two physiographic and hydrologic areas by Rillito Creek. The northern or Catalina Foothills area is a dissected pediment composed of Pliocene(?) red beds which are relatively impermeable. South of Rillito Creek is an area of moderately permeable valley-fill deposits of Tertiary-Quaternary(?) age. These valley-fill deposits contain the major water supply for the City of Tucson. Because of the depletion of this reservoir, interest in its geologic and hydrologic nature is demanding more attention. It was found that generalized stratigraphic sections may be made from water-well drillers logs by grouping logs of somewhat similar appearance into composite logs from which correlations may be made. Well cuttings, when secured and handled carefully, provide much more useful data than well logs alone. Samples of this type give an indication of the porosity, permeability, petrology, and depositional environment of the sedimentary units represented. The thickness of the valley-fill sediments is as yet undetermined; the specific yield of the area is not known and can only be estimated with the incomplete data. The fine- grained character of the subsurface sediments and the large demand for water somewhat account for the rapidly declining water table. Surface runoff accounts for a large percentage of the total amount of water leaving the Tucson basin. Because of evaporation, a large part of this surface water is lost from any beneficial use. The future water-supply problem may be somewhat alleviated if this runoff water is injected into the subsurface reservoir as artificial recharge.