Two Approaches to Harnessing Stormwater for Groundwater Recharge and Water Supply
With the recent 4-year drought in California, attention has been focused on capturing and reusing non-traditional sources of water. While in some places capture and recharge/reuse of excess water flows has been practiced for decades, stormwater is a new water resource for many water agencies. This presentation will highlight two water agencies in California with different approaches to capturing stormwater within an urban landscape to protect groundwater and surface water resources and supplement water supply.
The Scotts Valley Water District (SVWD) in Scotts Valley, Santa Cruz County, California is an agency reliant solely on groundwater for potable water supply. During a period of increasing population in the 1990’s the Santa Margarita groundwater basin became overdrafted. Urbanization of the area likely contributed to reduced recharge and compounded the strain on the aquifer. The overdraft has been stabilized by implementing recycled water for non-potable uses and strict water conservation. However, groundwater levels have not recovered to historic levels and SVWD considered additional resources to take advantage of the aquifer storage capacity. To that end, Kennedy/Jenks and SVWD completed a parking lot retrofit to facilitate capture, treatment, and recharge of stormwater funded by a State Water Resources Control Board Proposition 84 stormwater grant. The project location is a commuter parking lot in partnership with the Santa Cruz County Metropolitan Transportation District and includes biofiltration cells integrated into the landscape, pervious concrete pavement, and a detention/infiltration tank with a water level sensor. The project also included development of an excel data analysis tool to estimate the runoff and infiltration quantities that is adaptable to other sites by allowing for multiple tributary areas, multiple infiltration tanks, and incorporates the local precipitation data. The tool also includes a dashboard with summary information as well as daily and monthly reports and graphs for rainfall vs transducer information, rainfall vs runoff captured and rainfall vs infiltration.
Similar to SVWD, the Las Virgenes Municipal Water District (LVMWD) in Calabasas, Los Angeles County, California faces water supply challenges, especially during drought. LVMWD relies on imported surface water for potable water and treats and delivers recycled water for irrigation. As part of a recycled water study funded by the US Bureau of Reclamation under their Title XVI program, LVMWD commissioned an evaluation of stormwater as a source of supply in addition to wastewater. The Title XVI study evaluated the use of advanced water purification of recycled water with the anticipated end use of surface water augmentation as a potable drinking water supply. A range of stormwater projects were identified, evaluated, and screened. The most promising projects appear to be those associated with the Malibu Creek Enhanced Watershed Management Plan to comply with total maximum daily loads (TMDLs) in Malibu Creek. Therefore, projects implemented to supplement water supply may have multiple benefits, also improving downstream water quality and contributing to TMDL goals. However, diverting stormwater to the water supply system also poses challenges, requiring a balance of increased capture volumes with distance from the treatment plant and pump costs as well as careful analysis to ensure downstream ecosystems are not negatively impacted. The presentation will discuss how potential projects were identified and evaluated to aid audience members in determining if stormwater may be a viable resource in their community, and the audience will be invited to share existing projects (such as a traditional detention pond or a flood control project) that could be re-imagined as a capture and reuse project. While no two cities are the same, this evaluation process can be utilized to help any city connect drops from summit to sea.