Recharge properties of the Tucson Basin aquifer as reflected by the distribution of a stable isotope

Abstract

The Tucson Basin is southeastern Arizona is a semi-arid alluvial basin. Winter precipitation is derived principally from the Pacific Ocean, and summer precipitation originates mostly from the Gulf of Mexico. Previous investigations have noted that the isotopic composition of the two rainy seasons and of associated floodflows were significantly different. Once infiltrated, the bimodally distributed isotopic waters represent excellent natural tracers for distinguishing seasonal and areal recharge to the groundwater regime. This study examines the distribution of the ratios of ¹⁸0/¹⁶0 in natural waters within the Basin and accounts for that distribution in terms of a possible model of groundwater recharge and circulation. The distribution of the oxygen-18 isotope is hypothesized to be principally a function of the location of groundwater recharge zones, and regional groundwater flow patterns. Active recharge zones are indicated by high isotopic gradients along the perimeter of the Santa Catalina Mountains and along the Santa Cruz River. The gradients also suggest that limited transverse groundwater mixing appears to be occurring within the Tucson Basin Aquifer. On a regional scale, recharge to the Tucson Basin appears to be predominately derived from winter precipitation.