Unconventional water quality project benefits area with high groundwater
Marina del Rey is the world’s largest man-made small craft harbor. It serves 6,500 boats and is considered “Los Angeles County’s most valuable resource,” according to the Los Angeles Times. However, the harbor waters have been identified as an impaired water body for bacteria and pollutants, such as copper, lead, zinc and organic pesticides. Keeping the harbor free of pollutants is important for the hundreds of thousands of tourists who visit the harbor and the aquatic wildlife that inhabits it.
Typical beach parking lots in Marina del Rey consist of an area paved with black asphalt and painted with white stripes. They are usually an afterthought; in an area with inviting beaches, it is unusual to see people spending time in a parking lot. But the newly renovated Marina del Rey Parking Lot 9 is not a typical parking lot.
The existing parking lots provide an opportunity to tackle pollutants, as they are some of the only areas in the urban watershed that provide an open space to implement storm water quality measures. The county developed a plan to incorporate water quality best management practices (BMPs) at the parking lots to address potentially polluted storm water while providing additional benefits to the community. The recently completed Parking Lot 9 Project, developed by Los Angeles County, accomplished these goals by incorporating aesthetic, recreational and habitat benefits while addressing storm water quality with biofiltration BMPs in an area with high groundwater.
Prior to the project, storm water runoff drained from the 1.5-acre parking lot into two small catch basins that discharge directly into the Marina del Rey harbor without treatment. The challenge is that Parking Lot 9 sits directly over the Marina, which makes the construction of any BMP difficult due to the tidal influence that causes the groundwater to be less than 5 ft below the surface. In addition, underground utilities, cathodic protection, seawall tie-backs, the adjacent promenade and nearby boaters inhibit construction in the parking lot. Finally, the marina was constructed on a historic landfill, which meant additional challenges in dealing with potential contaminants in the sediment.
“The biggest challenge to the design of this project was determining the best treatment system to adequately capture the water quality storm runoff volume within the restrictions of the project site,” said Charles Chen, design engineer for the project. “Having to fit our BMP components within those restrictions and minimize the elimination of parking stalls made the design of this project extremely challenging.”
Many BMPs were considered, and two BioClean modular wetlands units were selected. The modular wetlands biofiltration unit consists of a pretreatment chamber to treat the heavy sediment and trash with filter media before the runoff flows through a wetlands chamber where vegetation and soil media treat the runoff using natural processes, all while encased in a 30-ft-long, 5-ft-wide precast concrete chamber.
The enclosed concrete chamber separates the tidal-influenced groundwater from the treated runoff. High groundwater has always been a challenge in constructing projects in the harbor and makes retention-infiltration BMPs infeasible.
“The primary reason that we chose the modular wetland units versus other biofiltration systems is that the modular wetlands are able to treat lateral flow,” said Donna Tran, design engineer for the project. “Most biofiltration systems treat vertical flow, and, for this particular project, we were aiming to capture runoff water that flows within the parking [lot] to the catch basin; hence, we needed a system that treated the flow horizontally.”
The modular wetlands units accomplish this and add landscaping to the parking lot. Initial effectiveness studies have shown that the modular wetlands units are effective in reducing bacteria, sediment, metals and petroleum hydrocarbons.
Aesthetics Meet Function
Water quality was not the only goal of the project. “Since the promenade is a highly used area by joggers, visitors to the marina, and nearby residents, the county wanted to come up with a design that addressed both the water quality component and provided aesthetic enhancements to the promenade, such as bench seating and interpretive signage at the parklets,” said Hannah Dewey, project manager.
These “parklets” are composed of wooden decking and benches that provide ocean-side seating. They were strategically placed immediately next to the modular wetlands units and included interpretative signage.
“The interpretive signs offer the public a chance to learn about these low-impact development components and how they work to improve the water quality,” Chen said. Additional improvements included colored stamped concrete over the promenade, bio-swales to treat runoff before it enters the modular wetlands units, native vegetation landscaping, bike racks, planter pots, handicap ramps, and repaving of the entire parking lot. The project has been operational since December 2016 and is helping preserve the aquatic habitat of Marina del Rey Harbor while providing recreational and aesthetic benefits to the surrounding community.