Project 1: Westside Basin Aquifer Recharge
By restoring the San Francisco peninsula’s Westside Basin Aquifer, Project One will demonstrate the feasibility and environmental benefits of restoring the groundwater basins 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 by rainwater seeping through vast areas of sand dunes, collecting over bedrock hundreds of feet below the surface, flowing laterally through the sands, then escaping into the Pacific Ocean. Urban development is starving the aquifer of its rainwater source, since there is less unpaved area for water to seep through, into the ground. Municipal and private pumping simultaneously draws from the aquifer. As a result, its natural stability has been thrown off.
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 the valuable rainwater resource that could be filtering through the ground to recharge the aquifer.
During large storms, the sewer system can become overwhelmed. An excess in rainwater forces the treatment plants to release untreated sewage straight into the Pacific Ocean or into the Bay. And this happens several times every year! Today, our rainwater resource is wasted as storm water – and the further insult is that it picks up pollutants, pesticides and fertilizers on its way into the sewers.
Westside Water Resources is committed to change 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.
Rainwater harvesting cisterns at Heron Head’s Park Ecocenter
How much water are we talking about?
The volume of rainwater that can be transitioned from rainwater to groundwater is astonishing! San Francisco receives an average of 21” of rain per year. Every 1” of rain falling on 1000 sq ft of impervious surface produces 623.42 gallons of water. For 1000 sq ft of rooftop for example, that adds up to 13,092 gallons of water. That’s a lot of water!
By working with property owners, property by property, we can accomplish groundwater recharge in an astonishing volume. We will accomplish this through outreach, education and organization, to transition the flow of rainwater by directing it away from the sewer system and instead into the ground.
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Additional Benefits of Project One
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.
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Historically when it rained, the water seeped directly into the ground. Below, Lake Merced today is surrounded by urban development, offering less surface area for groundwater recharge.
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 the 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, but 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 as to 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.
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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 Tuolume 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.
Contact us to learn more about how to get involved.
