Old Mission Creek Summer Urban Runoff Facility

The purpose of the Old Mission Creek Summer Urban Runoff Facility (SURF) is to divert water in the Westside Storm Drain during low flow conditions to an active treatment facility that uses ultraviolet (UV) light to eliminate microorganisms, including pathogenic bacteria and viruses.  Operation of the SURF will improve water quality in Old Mission Creek and contribute to improved water quality in the coastal ocean at East Beach.  This project was identified for Phase I implementation in a study of pollution hotspots and treatment options that was completed for the Creeks Division.  Phase I projects were determined to be high priority, easily implemented, and highly effective in addressing program goals.

Relationship to Old Mission Creek at Bohnett Park Restoration, and Water Quality

Old Mission Creek is known to have high levels of bacterial pollution that contribute to unsafe creek water and ocean warnings at East Beach.  The SURF project will complement the creek restoration project at Bohnett Park that was completed in 2003.  Improved water quality entering the creek from the Westside storm drain will provide healthier and safer conditions for children that play in Old Mission Creek at Bohnett Park, along with residents and visitors that swim in the ocean at East Beach.  Upon completion of the underground SURF project, the City will install a pocket park on top of the facility, further enhancing the neighborhood.

Similar projects elsewhere have resulted in non-detectable levels of indicator bacteria after treatment with ultraviolet light, as long as the water had been filtered of fine particles before treatment.  Indicator bacteria can return to high levels downstream of a UV treatment facility.  However, it is important to note that this likely represents nonpathogenic indicator bacteria that are capable of growing in the natural environment, illustrating the problem with the use of indicator bacteria to assess the risk to human health.

Technical Project Description

Construction of the SURF project was completed in December 2006, and the project will be online beginning April 1, 2007. The project is located underground and within the boundary of the City-owned Bohnett Park.  A diversion structure was built into the existing 84” diameter storm drain.  Dry-weather flow up to 120 gallons per minute are diverted from the drain to an oil/water separator, and then a pump station passes the water to a vault housing the treatment facility. Water moves through solid-media filters, in order to remove particulates, and then passes under ultraviolet (UV) light bulbs.  The residence time of the water under the bulbs and the intensity of the UV light have been designed to eliminate bacteria and inactivate viruses in the water.  The treated water will be discharged back to the storm drain and then flow into Old Mission Creek in Bohnett Park.  A valve allows the facility to be closed during the rainy season. 

The concept work and subsequent design plans for the project also include a solid removal device (CDS unit) for removing trash and sediment from higher flows (up to 54 cfs).  Although not necessary for the operation of the SURF during low flows, the Creeks Division anticipated that the CDS would help reduce the discharge of trash and sediment in Old Mission Creek during average rainfall events.

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