Stable Isotopes and Ground-Water Chemistry as Indicators of Mountain Front Recharge, Tucson Basin, Pima County, Arizona

The relative importance of mountain front recharge as compared to total recharge was determined for a portion of the Tucson basin aquifer margin by interpretation of chemical and isotopic data. Concentrations of 180/160 lower than 6 -10.7 0 /0o as compared with a background of about 6 -9.3 0 /oo in ground water from the base of the mountains in the gneissic rock suggest the presence of recharge from significantly higher elevations. The trilinear diagram of major ions dissolved in ground water from 123 wells in the Santa Catalina foothills indicates three water types. Water from wells in gneissic rock is high in sodium and potassium content and low in calcium and magnesium. Wells in the gypsiferous Pantano Formation yield water high in sulfates. The majority of wells in the study area, which are along major streams and in the regional aquifer, have calcium carbonate type water. Their chemistry indicates only minor contributions from the gneissic mountain block and the underlying Pantano Formation. Funding for this project came from the Spanish Project Register T377017.