9507991 Gillespie This study will examine the significance of fracture-directed flow on recharge patterns in an arid, alluvial aquifer. Alluvial basin aquifers in the western U.S. are dependent on recharge derived from adjacent mountain ranges. A significant amount of this recharge may be transported in the subsurface via fractures within the bedrock aquifers and along basin-bounding faults. We propose to determine the extent to which fracture systems within the southern Sierra Nevada Range and the Owens Valley Fault Zone redistribute recharge from the Sierras into the western part of the Indian Wells Valley alluvial aquifer. The proposed research is a logical extension of the investigators' interest in the architecture and hydrogeochemistry of alluvial aquifers and petroleum reservoirs. It may have a broader significance in the development of improved conceptual models of recharge patterns in arid alluvial aquifers and lead to the discovery of ground water resources in areas normally considered poor candidates for ground water development. Our methods will include remote sensing, surface and subsurface field studies to determine the location of phreatic divides, the orientation of fracture patterns, the hydraulic gradient along the fractures and gradients within the alluvial aquifer. Chemical and isotopic analyses, including age-dating along the major flow paths, will be used to delineate flow systems, assess the role of fractures as barriers or conduits and constrain the relative contribution of fracture-directed flow versus surface recharge. Finally, geologic and hydrologic data will be incorporated into existing numerical flow models and the amount and distribution of subsurface recharge to the alluvial aquifer quantified. The investigators will employ undergraduate and graduate students and provide training in the qualitative and quantitative techniques of classis field geology, hydrogeology and hydrochemistry. The students will play an ac tive role in the dissemination of the information via abstracts, poster sessions, presentations and travel to regional professional society meetings.
