A stable increase in the lake s level requires a corresponding increase in the level of its underlying aquifer. Groundwater is pumped from the aquifer for domestic water use by Daly City, San Bruno and South San Francisco and for irrigation by several golf courses and cemeteries, San Francisco Zoo and Recreation and Parks, San Francisco State University and others. Three sewer pipes are cosmetically hidden in a walking bridge that separates the South Lake and the Impound Lake. If the pipes are not well maintained or if there is a major earthquake, there is a possibility that raw sewage could be dumped into the lake. Aquifer is the name given to a body of underground soil or rock that stores groundwater.
Aquifers typically consist of gravel, sand, or fractured rock such as limestone and sandstone.
Water leaving an aquifer naturally or through pumping is called discharge water. The Westside Basin consists of two aquifers, an unconfined shallow sand aquifer and a fine-grained to clay deep aquifer that extends to bedrock. These two aquifers are separated by a clay layer of varying depth, thickness, and permeability. The Westside Basin is an elongated trough that extends from northwestern Golden Gate Park southwest to San Francisco International Airport. It is bounded on the northeast by a bedrock ridge from Twin Peaks to San Bruno Mountain and on the southwest by the San Andreas Fault.