Geohydrology, water quality, and water budgets of Golden Gate Park and the Lake Merced area in the western part of San Francisco, California

The groundwater resources in the western part of the San Francisco, groundwater budgets for Golden Gate Park and the Lake Merced area, and a surface-water budget for Lake Merced are described. A continuous groundwater basin underlies a 39-sq-mi coastal strip in the San Francisco Peninsula south of the city. Basin fill consists largely of sand and silt. An extensive subsurface clay layer is present near Lake Merced. Recharge is principally from rainfall and irrigation-return flow, with lesser amounts from leaking water and sewer pipes, which were identified in part by stable-isotope and major ion analyses. In Golden Gate Park, about 1, 070 acre-ft/yr of groundwater flows to the ocean. Water levels are not declining, and pumpage could be safely increased. However, nitrate concentrations in excess of Federal drinking-water standards in water from many wells may limit potential uses of groundwater. Groundwater in the Lake Merced area is in a state of overdraft, as indicated by long- term declines in the level of Lake Merced and by groundwater levels persistently below sea level in deep wells. Seawater intrusion has not been detected, however. A surface-water budget for Lake Merced indicates that the largest inflow is from shallow groundwater and the largest outflow is loss by evaporation. (USGS)