RESTORATION OF DECLINING LAKE LEVELS IN THE WESTSIDE GROUNDWATER BASIN, SAN FRANCISCO AND SAN MATEO COUNTIES, CALIFORNIA

Lake Merced is popular urban fishing and boating lake located in the southwest corner of the City of San Francisco and within the Westside Groundwater Basin. The 1.2 km² fresh water lake is a surface expression of the shallow groundwater table. Lake levels dropped 4.5 meters, beginning in the mid-1980's, partially related to groundwater pumping, as well as the historic diversion of surface water tributaries. The Westside Groundwater Basin underlies portions of San Mateo County (73 km²) and the City and County of San Francisco (31 km²). Total groundwater pumping for municipal and irrigation purposes averages about 1.5x10⁶ m³ /year. The Westside Basin provides up to 50% of the drinking water for 200,000 residents in northern San Mateo County. In San Francisco, groundwater is currently only pumped for non-potable purposes, primarily irrigation. However, additional municipal pumping is planned. Geologic investigations have resulted in a conceptual geologic model that is being used to 1) begin restoring declining lake levels, 2) develop additional groundwater resources in San Francisco and 3) develop a conjunctive use program in the San Mateo County portion of the Westside Basin.