Biofiltration systems like rain gardens and bioswales are an important tool for capturing andinfiltrating polluted runoff, but little data exists on their efficiencies within Mediterraneanclimates. A two-year study initiated in 2015 investigated water retention and pollutant loadingand retention in the Ballona Creek Rain Garden (BCRG). This 300 by 3 m biofiltration systemwas constructed by The Bay Foundation in 2011 along Ballona Creek in Culver City, Los AngelesCounty, California. The purpose of the garden was to capture and infiltrate runoff from lightindustrial and commercial operations bordering the Creek, thus reducing pollutants enteringthis waterway and flowing into Santa Monica Bay 9 km downstream. During storm events,runoff enters the garden via five inlets, and when filled, flows into the creek via two outlets.The goal of this study was to sample flows and pollutant concentrations in runoff entering andleaving the garden and then integrate these to calculate mass loading estimates. Flows weremeasured at all inlets and outlets using 90° V-notch weirs outfitted with Hobo water levelsensors to produce hydrographs. The following pollutants were measured at all flowing inletsand outlets two to three times per storm depending on its duration and intensity: fecalindicator bacteria (E. coli and enterococci), total suspended solids, metals (copper, zinc, andlead), and semivolatile hydrocarbons (polyaromatic hydrocarbons, diesel hydrocarbons, andmotor oil hydrocarbons). The summation of load method was used to calculate the mass ofcontaminants entering and leaving the garden for each storm event, and their percent capturewithin the garden. The BCRG was very effective at infiltrating runoff and sequesteringpollutants. The garden’s infiltration rates ranged from 73% to 100% (with 100% for many of thesmaller storms percent retentions were in the 80-90% range for all pollutants, with an average of 90% for allnine pollutants sampled. This suggests rain gardens and other Low Impact Development (LID)systems can be used successfully in urban Mediterranean climates like Los Angeles to promoteinfiltration, capture pollutants, and prevent polluted stormwater from reaching impaired waterbodies.