This presentation discusses the design, construction and performance of a stormwater treatment project needed to preserve the last vernal marsh in the South Bay area of Los Angeles while exceeding stormwater quality regulations. The audience is introduced to the challenges of designing a nutrient removal treatment system to treat urban runoff and stormwater from a large detention basin used to supplement water in the Madrona Marsh for nesting birds.
The City of Torrance Madrona Marsh Preserve is an Environmentally Sensitive Area and valuable habitat for a variety of birds, insects, mammals and plant species. Once sought out to be a condominium development, the City along with the Friends of Madrona Marsh saved the delicate habitat by forming an agreement with the developer to donate 54 acres surrounding the marsh.
Upon acquiring the land, the City hired a professional naturalist to institute programs for restoring the preserve. With funding and support from the California Coastal Conservancy and Santa Monica Restoration Commission, the City began researching methods to reduce nutrients in the water before entering the marsh. The main concern was phosphates causing rapid algal growth in the marsh leading to oxygen depletion, otherwise know as hypoxia.
After reviewing several ineffective plans, the City looked to Modular Wetland Systems, Inc. (MWS). The company’s WetlandMod is a self-contained treatment train system that incorporates media filtration for pre-treatment and a subsurface flow wetland (wetland). MWS tweaked its traditional system incorporating a much larger scale wetland to treat various flow volumes. The City incorporated this design into a project that prevents trash from entering the Marsh and aerates the treated water.
The Marsh receives water via two large storm outfalls collecting in a large detention basin. The basin contains three pumps. Two pumps deliver water into marsh and the third into the WetlandMod where water flows through a proprietary media removing high levels of TSS, hydrocarbons, particulate heavy metals and nutrients. Reducing particulates before entering the wetland media minimizes loading and prevents it from clogging. Water is then distributed through a manifold creating an even flow of water across the wetland. Treated water is diverted back into the detention basin using a rip rap waterfall. Within 24 hours following filtration, samples were pulled resulting in a 37% nitrate reduction, over 50% phosphate reduction and 87 % reduction in turbidity. Results will continue to improve as plant root systems establish growth within the wetland. Madrona Marsh staff will continue to monitor water quality.
The project included replacing pumps, installing retractable curb grates on catch basins to prevent trash from entering the detention basin, installing Bio Clean filters at the storm drain outfalls to remove oil and grease, removing trees around the outfalls providing UV exposure and constructing a rip rap waterfall to aerate treated water flowing back into the basin. Project benefits include keeping the City of Torrance in compliance and exceeding water quality regulations to restoring a preserve that continues to be a valuable habitat for the plants and animals as well as a sanctuary for bird watchers, college professors and children.