Estimation of spatial recharge distribution using environmental isotopes and hydrochemical data, II. Application to Aravaipa Valley in Southern Arizona, U.S.A.

In Part I^* * Adar et al. (this volume). a mathematical model was developed to estimate the spatial distribution of annual recharge rates into an aquifer using environmental isotopes and hydrochemical data. In Part II the model is applied to real data from the Aravaipa Valley in southern Arizona. These data suffer from a paucity of hydrologic information but contain numerous isotopic and hydrochemical analyses. The model is able to extract from the available data both qualitative and quantitative information about recharge into, and flow through the shallow unconfined aquifer in the valley. The model shows that this aquifer is replenished primarily by lateral recharge from the pediments and the tributary Stowe Gulch Basin, as well as by upward leakage from the underlying confined aquifer. Recharge from the Aravaipa Creek is relatively minor. Lateral recharge from the eastern pediments into the unconfined aquifer exceeds that from the western pediments by a factor of two. The western pediments are a major source of recharge for the confined aquifer. These results are tentative because independent means to verify them are currently lacking. We are encouraged, however, that the mass balance errors of water and dissolved constituents do not exceed 4%, and that the flow rate obtained from Darcy's law at one location within the unconfined aquifer in which transmissivity is known, agrees with model prediction. While chemical reactions are ignored in the model, some of the isotopic and chemical species show sufficient spatial variability due to mixing to justify the approach. This is supported by analyses of equilibrium water-mineral interactions conducted by WATEQF.