InTech

JAN-FEB 2019

Issue link: http://intechdigitalxp.isa.org/i/1079769

Contents of this Issue

Navigation

Page 36 of 55

INTECH JANUARY/FEBRUARY 2019 37 SPECIAL SECTION and local water systems. WSIP's primary goals included: l seismically strengthening the regional water system by rebuilding existing facilities in vul - nerable areas, such as fault zones and beneath San Francisco Bay l maintaining the high quality of the drinking wa- ter supply by rebuilding water treatment plants and constructing new water treatment facilities l increasing the reliability of San Francisco's water supplies by adding water wells under two groundwater projects These two groundwater projects are the SFGW Project and the Regional Groundwater Storage and Recovery Project (GSR Project). The SFGW Project is located wholly in San Francisco, while the GSR Project is located in northern San Ma teo County, south of San Francisco. Both projects tap groundwater from the Westside Basin aqui- fer. The SFGW Project is designed to supplement the city's drinking water at all times and is the focus of this article. The GSR Project will provide groundwater during droughts or other emergen- cies to both wholesale customers in the project area and to San Francisco. With groundwater supplies, SFPUC's customers are less vulner- able to disrupted services, whether from a major earthquake, drought, or other future unknowns. The SFGW Project consists of six groundwa- ter well facilities, each consisting of a well and pump station, and five miles of a new ground- water transmission pipeline constructed for the project (figure 2). Five of the wells supply groundwater directly to the Sunset Reservoir via the new groundwater pipeline, where it is mixed with the Hetch Hetchy Regional Water System supply. The sixth well, located near Lake Merced, connects to the SFPUC's Lake Merced Pump Station, where the groundwater is mixed and distributed to both the Sunset and Sutro Reservoirs. In addition to mixing with the sur- face-water supplies, the groundwater is treated using chlorination and pH adjustment. Four of the well facilities (phase I in figure 1) are complete and currently pumping groundwater to the Sunset and Sutro Reservoirs. The remaining two well facilities (phase II in figure 1) will begin pumping to the Sunset Reservoir by 2021. The av erage groundwater production will increase incrementally, with a goal of 1 million gallons per day (mgd) (1,120 acre-feet per year) during the first year and 4 mgd (4,480 acre-feet per year) when all six wells are operating. The full 4-mgd production rate represents a blend of approximately 13 per - cent groundwater in the Sunset Reservoir. The quality of groundwater from the Westside Basin aquifer is different from the surface-water supplies from the Hetch Hetchy Regional Water System. The resulting groundwater-surface wa- ter blend, therefore, is also different from what customarily has been served to SFPUC custom- ers. The key water quality variations are in gen- eral mineral content, pH, nitrate, hexavalent chromium, and manganese. l The general mineral content of the groundwa- ter—primarily total dissolved solids, hardness, and alkalinity—is higher than San Francisco's main supply from the Hetch Hetchy Reservoir. l The pH of groundwater from individual wells ranges from 7.6 to 8.0 (slightly alkaline). This range is lower than the pH of SFPUC's treated surface-water supply, which is maintained at a range of 8.8 to 9.4 for optimal corrosion con - trol. (This is required by the drinking water Lead and Copper Rule, which is enforced by the State Water Resources Control Board.) l Nitrate and hexavalent chromium concen- trations in the groundwater are higher than in the surface-water supply. Nitrate has a health-based drinking water standard (pri- mary maximum contaminant level [MCL]). In addition, the state of California is in the pro- cess of adopting a primary MCL for hexava- lent chromium, expected in 2019. l Manganese is often found in groundwater throughout the San Francisco Bay Area at concentrations higher than the aesthetically Figure 1. SFGW project elements

Articles in this issue

Archives of this issue

view archives of InTech - JAN-FEB 2019