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January 31, 2024 Risk-Based Evaluation for Reuse of Stockpile Material for the Proposed Treatment Wetland Project – City of San Leandro Water Pollution Control Plant 20240131-Terraphase-SanLeandro-TreatmentWetland-Risk-TM-DRAFT.docx Page 2 of 10 salt pond with little to no vegetation except for weedy plants along its banks that are typical of heavily disturbed sites along the Bay (LSA 2020b). Proposed Project The general proposed Project design is to add (or mix) stockpiled soil to the existing sludge in the storage basin, cover the soil/sludge with clean fill, and place an impermeable liner on top of the clean fill, which will become the bottom of the treatment wetland. As an additional measure, stabilization of the soil/sludge with cement to provide a stable construction foundation is also being considered. The goal of the treatment wetland is to reduce nitrogen (nitrate) and micro-organic constituents in the wastewater treatment plant effluent prior to discharge to the Bay. Upstream of the wetland will be an engineered membrane-aerated bioreactor designed to support nitrification of the ammonia in the wastewater plant effluent to nitrate. This water will then be conveyed through a hydraulic distribution system around the perimeter of the wetland basin. The open-water wetland and terraced bioreactors buried in the side slopes (filled with woodchips) will provide the carbon source to support de- nitrification (conversion of nitrate to nitrogen gas) prior to discharge. The impermeable liner is expected to develop an algal mat to help support de-nitrification. The water depth of the treatment wetland will be maintained at a relatively shallow level (12-18 inches) and the treated water will be discharged through a one-way outlet pipe with a valve that will prevent any tidal water from entering the basin. The treatment wetland will only be planted with riparian/wet meadow vegetation on the side slopes and potentially along the base of the slopes. This vegetation will be mowed and maintained to minimize the development of habitat for birds (bird strikes are an issue at Oakland Airport, which is directly adjacent to the Project site). Open-water areas will not be planted with emergent vegetation and floating plants will be discouraged through routine maintenance to support denitrification and photolysis of micro- organic contaminants. Historical Investigations Terraphase conducted a review of historical data for the stockpiled soil, the storage basin sludge, and groundwater, collected by others in 2014 to assist the City in developing an approach for the management of soil and sludge at the WPCP. Terraphase screened the data against applicable San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) screening guidelines. The maximum detected concentrations for select metals in samples collected from stockpiled soil and sludge were greater than the recommended sediment chemistry screening guidelines for beneficial reuse of dredged material (Wetland Foundation Material Concentration Guidelines; SFBRWQCB 2000). This included lead, silver, and zinc in the stockpiled soil and cadmium, copper, lead, mercury, nickel, silver and zinc in the sludge. Only four sludge samples had been previously collected for chemical analysis and all four samples were from the west end of the sludge pond. Therefore, because this information was insufficient to adequately characterize the nature and extent of COPC in sludge, Terraphase performed additional sludge sampling in other areas of the polishing pond for analysis.