Prioritizing localized stormwater capture for water supply and water quality benefits

The Water Replenishment District of Southern California (WRD) manages the Central and West Coast Groundwater Basins (Basins) for nearly four million residents in 43 cities of southern Los Angeles County. In an effort to increase its sustainable water supply portfolio and to decrease its reliance on imported water, WRD is investigating alternatives to capture more stormwater for groundwater recharge. WRD's 420 square mile service area is located within Los Angeles' most urbanized watersheds including those of the Los Angeles and San Gabriel Rivers. Segments of these rivers and their receiving waters are impaired and subject to existing and proposed enforceable surface water quality regulations including multiple TMDLs.To identify catchments with greatest potential to provide distributed and subregional groundwater recharge and to help reduce pollutant loading of surface water bodies, an in-depth, regional assessment was conducted using spatial analyses and locally developed models. These models included the Structural Best Management Practices Prioritization and Analysis Tool (SBPAT), the Groundwater Augmentation Model (GWAM), and the WRD/U.S. Geological Survey (USGS) MODFLOW model. The assessment considered a suite of factors important to siting groundwater recharge projects (e.g. geologic conditions, pre-existing contamination, dewatering) and local water quality objectives.Analyses identified approximately 10% of the 270,000 acres within the WRD service area as opportunities for local and regional stormwater recharge where nearly 17,000 acre-feet per year of potential water supply benefits can be expected. Of those, nearly 8,000 acres were identified as high priority areas that could contribute more than 4,000 acre-feet per year to the local potable aquifers. In addition, the study identified that each acre of land in south Los Angeles County that receives appropriately well-sited retrofits could annually yield approximately 0.54 acre-feet of groundwater recharge and more than 200 pounds of pollutant reduction.