Performance Of Bioretention Areas – The California Experience

In the last 15-years, the inclusion of stormwater treatment measures to passively treat rainfall runoff from developed areas in the State of California, the United States of America has become widespread. Stormwater treatment measures are intended to reduce impacts to downstream receiving waters to the maximum extent practicable for typical urban constituents such as; metals, fertilisers, and hydrocarbons that may comingle with rainwater during storm events and would otherwise be transported downstream. 

In California, bioretention areas have become the preferred method of treating urban stormwater. Bioretention areas use planted media to slowly filter urban rainwater runoff and remove typical urban pollutants through uptake of nutrients, adsorption, microbial activity, decomposition and volatilisation before ultimately discharging into downstream receiving waters. Although originally intended to meet broad “Maximum Extent Practicable” stormwater performance standards, there has recently been greater interest in understanding how bioretention installations perform in regard to treatment of selected stormwater constituents, necessitating sampling runoff from constructed bioretention facilities. This paper summarises the findings of four case studies performed throughout the State of California in regard to removal efficiency of typical urban stormwater constituents based on variables such as size of installation, size of catchment, type of runoff, and catchment land use. The locations of the bioretention facilities are listed below: 

• Serramonte Library, Daly City. Traditional Bioretention Facility at a Public Institution: One year of rainfall sampling performed by San Francisco Estuary Institute. 
• Bayport, Alameda. Small Proprietary Bioretention Facility in a Residential Housing Development: Five years of sampling performed by ENGEO Incorporated. 
• San Francisco-Oakland Bay Bridge. Regional Bioretention Facility for a State Highway: Three years of sampling performed by the California Department of Transportation. 
• Heritage Fields, Irvine. Regional Bioretention Facility for a Residential Housing Development: One year of sampling for dry weather flows (urban non-stormwater runoff) by Irvine Ranch Water District. 

Finally, this paper provides recommendations for policy makers and potential improvements to existing bioretention technology, based on the results of the study, in addition to wider applicability for bioretention utilisation in New Zealand.