WESTSIDE BASIN AQUIFER RECHARGE
What is the Westside Basin Aquifer?
The Westside Basin Aquifer is a groundwater reservoir covering nearly 40 square miles and extending from Point to Lobos to the bay south of San Francisco Airport.
By restoring the San Francisco Peninsula’s Westside Basin Aquifer, Westside Water Resources demonstrates the feasibility and environmental benefits of restoring groundwater through distributed aquifer recharge.
San Francisco’s west side sits over a large underground water reservoir called the Westside Basin Aquifer. Historically, this aquifer maintained stability as rainwater seeped through vast areas of sand dunes, collected over bedrock hundreds of feet below the surface, and was bounded by the denser saltwater of the Pacific Ocean. Urban development is starving the aquifer of its rainwater source. Over 60% of the area is covered by rooftops and paving tied to hard piping, preventing water from seeping through into the ground. Municipal pumping draws from the aquifer at the same time. As a result, natural stability has been thrown off. For over one hundred years, the Westside Basin Aquifer has been in a state of overdraft.
Every time it rains, we miss the opportunity to recharge the Westside Basin Aquifer. Instead, rainwater flows into the City’s combined sewer system where it is treated along with sewage and discharged into the Pacific Ocean. This process wastes a valuable rainwater resource that would otherwise filter through the ground and recharge the aquifer. How much fresh rainwater is this? The San Francisco Public Utilities Commission estimates that approximately eleven billion gallons of water per year that would otherwise filter through the ground into the Westside Basin Aquifer is instead piped to the ocean. Eleven billion gallons a year is approximately two and a half times the entire City of San Jose, California’s, annual water consumption. So, a lot of water!
Watershed Map courtesy of SFWater.org
The blue shaded area represents San Francisco’s portion of the watershed that feeds the Westside Basin Aquifer. 11,000,000,000 gallons of rainwater per year are diverted from this area and piped to the ocean.
Raw sewage overflows at Ocean Beach during heavy rains.
During large storms, the sewer system becomes overwhelmed. Untreated sewage spills straight onto Ocean Beach and into the Pacific Ocean (on the east side of the city, into San Francisco Bay). This happens many times every year!
Westside Water Resources is committed to changing this system. By working together, we will dramatically increase groundwater recharge to enhance the health of San Francisco’s Westside Basin Aquifer by disconnecting our roof rainwater from the sewer system to allow it to naturally filter back into the ground.
How much water are we talking about?
The volume of rainwater falling on impervious surfaces that can be diverted to groundwater is astonishing! San Francisco’s west side receives an average of 23” of rain per year. Every 1” of rain falling on 1,000 sqft of impervious surface produces 623.42 gallons of water. For 1000 sq ft of rooftop, for example, that adds up to 14,339 gallons of water. A house with 2,000 sqft of impervious surfaces (rooftop, driveway, patios, etc.) could conserve 28,678 gallons a year. In San Francisco, a family of four uses approximately 60,000 gallons per year. Thus, water diverted to the groundwater reservoir would supply nearly half of that household’s total water use.
By working with property owners, property by property, we can achieve groundwater recharge in astonishing volumes. We will do this through outreach, education, organization, and actual reconfiguring where the water goes.
Rainwater harvesting cisterns at Heron Head’s Park Ecocenter
The Benefits of Aquifer Recharge
Disconnecting gallons of rainwater from the sewer system will benefit the regional environment, residents of San Francisco and residents of other municipalities served by the San Francisco Public Utilities Commission (SFPUC) in multiple ways:
Benefits to San Francisco’s Oceanside Wastewater Treatment Plant
By reducing the volume of wastewater flowing to San Francisco’s Oceanside Wastewater Treatment Plant, Project One will help reduce electricity consumption and increase the efficiency of the treatment plant by lessening the volume of wastewater flowing to the plant. The reduction of stormwater runoff into the combined sanitary sewer/stormwater system will benefit the environment and the residents of San Francisco by effectively lowering climate change greenhouse gas emissions as well as lowering public costs associated with operating the treatment plant.
By reducing the volume of stormwater flowing to the treatment plant during the rainy season, Project One will improve ocean near-shore and deep-water quality by reducing or eliminating treatment plant overflows which drain directly onto public beaches several times per year.
Reducing the treatment plant overflows will significantly reduce run-off pollutants being discharged from the San Francisco Oceanside Wastewater Treatment Plant’s ocean outfall, which discharges adjacent to the Monterey Bay National Marine Sanctuary and the Gulf of the Greater Farallones National Marine Sanctuary.
These reductions will improve the wildlife habitat affected by these overflows and discharges, and will subsequently benefit the health of people using these ocean waters for commercial and recreational purposes, as well as ease the destruction of the natural habitat.
The Oceanside Treatment plant in San Francisco sits directly on the coast. During storm events, overflow is released directly into the ocean.
Lake Merced in undeveloped landscape, far from nearby roads, c. 1900s. Photo: Private Collection, San Francisco, CA, courtesy of Foundsf.org
Restoration of the water level of Lake Merced, the physical expression of the Westside Basin Aquifer
Westside Water Resources will assist in the restoration of Lake Merced, the large freshwater lake located south of Ocean Beach, which is the surface expression of the Westside Basin Aquifer below. Lake Merced has experienced declining water quality due to diminished groundwater recharge as a result of ongoing urban development, and ongoing pumping from the Westside Basin Aquifer by both San Francisco and San Mateo counties.
Lake Merced is surrounded by a 4.5 mile paved pathway and several golf courses. The lake is popular with boaters, hikers, bicyclists, and bird waters, and is an important stop for migrating birds. The stability and health of the Westside Basin Aquifer maintained the water quality of Lake Merced, and historically the lake has served as a freshwater supply for the City of San Francisco from 1868 through 1908. This was a contributing factor in a steady decrease in the lake’s water level, along with pumping of the lake’s water to irrigate several golf courses nearby. A build-up of impermeable surfaces around the lake has effectively diverted rainwater into the sewer system rather than into the lake. After the drought between 1989 and 1993, the lake hit its lowest water levels.
While it has shown an improvement in water levels over the past few years, the lake’s water level is an important indicator of the general health of the Westside Basin Aquifer. Restoring Lake Merced will benefit the public generally by enhancing the lake as a wildlife refuge and a public recreation resource.
Historically when it rained, the water seeped directly into the ground. Above, Lake Merced today is surrounded by urban development, offering less surface area for groundwater recharge.
Reduce San Francisco County’s Reliance on the Hetch Hetchy Reservoir
Since 1934, the City of San Francisco has received its water from the Hetch Hetchy reservoir located 167 miles away in the Sierra Nevada mountains. In 2017 the SPUC began drawing on the Westside Basin Aquifer as a source for high-quality water, in order to diversify San Francisco’s water supply and provide a local water source in the case of earthquake or drought. Initially, only 4% of water was drawn from the aquifer to be blended with water from the distant Hetch Hetchy reservoir, but the SFPUC plans to increase that amount by as much as 15% by the year 2024, which will accelerate decline in its water level. By assisting in the recharge of the Westside Basin Aquifer, Project One will assure that the aquifer will continue to provide a sustainable alternative water supply for the county.
This reduction lessens the need for diversion of surface water flows from the Sierra Nevada Mountains. This will help stabilize the natural, seasonal flow of the Tuolumne River, which in turn improves and benefits the river biome, including historic runs of salmon and steelhead trout. The lessening of the water diversion also increases freshwater flow, and decreases saltwater intrusion into the San Francisco Bay Delta system.
The lessening of diversion will have the important added benefit of reducing climate-change greenhouse gas emissions, and lower costs associated with the pumping required for such diversions, as well as reducing harm to the specific species and habitat of the Tuolumne River. Project One will increase the resilience of San Francisco’s water supply against delivery disruption due to drought, earthquake and wildfire by increasing the amount of local groundwater that can be tapped as an alternative water supply source – a practice that in fact, the SFPUC has already begun.
It is our mission to enhance the health of the Westside Basin Aquifer, and ensure its abundance for years to come.
Map of San Francisco’s Groundwater Supply Project. Courtesy of the SFPUC
The long journey of water from the Hetch Hetchy Reservoir to the San Francisco Bay Area.