Project Specifics

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 decrease its reliance on imported water, as a subconsultant to the Council for Watershed Health, Geosyntec investigated 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.

Geosyntec Scope of Services

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. Using these existing tools, datasets, and analyses, Geosyntec prioritized individual catchments for stormwater capture for groundwater recharge. Through a targeted outreach, Geosyntec identified possible sites for pilot projects and, after selection, provided concept designs within a focused area.

Notable Accomplishments

Analyses identified approximately 10% of the 270,000 acres within the WRD service area as providing 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 priority pollutant reduction.

Benefits and costs of the pilot project, advanced to the 5-10% design level, were estimated by Geosyntec. The residential and subregional BMPs have roughly equal cost to benefit ratios. Street BMPs have somewhat higher costs than benefits, while the commercial green street has significantly higher cost than benefit. It is critical to note that these benefits only include water quality and supply, and ignore other values such as aesthetic appeal, green job creation and increased economic activity.

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