Lawrence Berkeley National Laboratory

San Luis National Wildlife Refuge Complex to meet Reclamation s obligations for Level 4 water supply under the Central Valley Project Improvement Act. Hydrogeological assessment of the East Bear Creek Unit of the San Luis National Wildlife Refuge was conducted using a combination of field investigations and a survey of available literature from past US Geological Survey Reports and reports by local geological consultants. Conservative safe yield estimates made using the available data show that the East Bear Creek Unit may have sufficient groundwater resources in the shallow groundwater aquifer to meet about between 25 percent and 52 percent of its current Level II and between 17 percent and 35 percent of its level IV water supply needs. The rate of surface and lateral recharge to the Unit and the design of the well field and the layout and capacity of pumped wells will decide both the percentage of annual needs that the shallow aquifer can supply and whether this yield is sustainable without affecting long-term aquifer quality. In order to further investigate the merits of pumping the near surface aquifer, which appears to have reasonable water quality for use within the East Bear Creek Unit -- monitoring of the potential sources of aquifer recharge and the installation of a pilot shallow well would be warranted. Simple monitoring stations could be installed both upstream and downstream of both the San Joaquin River and Bear Creek and be instrumented to measure river stage, flow and electrical conductivity. Ideally this would be done in conjunction with a shallow pilot well, pumped to supply a portion of the Unit's needs for the wetland inundation period.