BASELINE CONCEPTUAL MODEL OF HYDROLOGY AND GEOCHEMISTRY OF THE PATAGONIA MOUNTAINS, ARIZONA

The United States Forest Service needs to evaluate the impacts of previous mines and predict the effects of mineral exploration and mining on the biological and hydrological resources of the Patagonia Mountains of southeast Arizona. To support these efforts, we are using watershed models and existing geologic, hydrologic, and geochemical data to develop a baseline conceptual model of the hydrology and geochemistry of the Patagonia mountains. We use geology, water levels, isotopes, and watershed models to infer sources, directions, and timing of groundwater flow. Metals transport is being studied with the additional tools of watershed models and spatial variability of metal concentrations. Although detailed conceptual models of water flow through this system are difficult to construct because of diverse rock types and long-term structural deformation, we can still use this information to infer direction and sources of flow. Low permeability igneous rocks in the center and east side of the range may decrease flow and storage. Sedimentary rocks on the east and shear zones in the north likely promote infiltration and groundwater flow. Faults may contribute to significant flow of water along the main north-south axis of the mountain range. The east-west Patagonia Fault on the north end of the range may limit groundwater flow toward Sonoita Creek and the town of Patagonia. Water levels generally indicate flow from higher to lower elevations, and stable hydrogen and oxygen isotopes indicate that most recharge occurs in months ranked in the top 30% in terms of precipitation. Modeling indicates higher infiltration rates in the northwest part of the range. In the Harshaw watershed, metal concentrations in runoff decline in the middle section of the watershed, possibly due to high alkalinity of stream sediments. Higher concentrations downstream could be caused by inputs from metal-rich soils on altered, mineralized slopes of the Red Mountain porphyry copper system.