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2. Project Description San Leandro Treatment Wetland IS/MND 2-13 February 2024 a) Interior Dikes The basin will be divided into five equally sized cells using dikes constructed in various possible configurations. As water flows toward the outfall, it will pass through or over one or more dikes, each incorporating wood chips for denitrification, similar to the side slopes. Treatment wetlands with multiple cells in series consistently yield better effluent quality, as noted by the US Environmental Protection Agency (EPA) and supported by the Iowa Department of Natural Resources, which recommends a minimum of two cells.7,8 The wetland will be segmented into four cells using dikes to achieve these treatment objectives. Mattress-style dikes will be constructed from a single layer of polystorm crates filled with wood chips, arranged in three rows. This design aims to minimize hydraulic short-circuiting, which is the establishment of preferential flow paths that reduce treatment efficiency. Influent will be distributed through seepage slopes with terraced bioreactors before it reaches a shallow (~18- 24 inches) FWS treatment component. The impermeable liner prevents infiltration and unwanted plant growth outside designated areas. The FWS component will be divided into multiple cells using interior dikes to maximize retention time and minimize preferential flow pathways. This arrangement will enhance denitrification and aid in the photolysis of micro-organic contaminants like pharmaceuticals and pesticides. Topsoil shall cover the terraced bioreactors and interior dikes and be planted with native species. a) Open Water Areas The open water areas, excluding the side slopes and dikes, aim for an adjustable depth of 18” to 24” for optimal denitrification. The proposed bottom elevation is 6.5 feet (NAVD88) to increase hydraulic head relative to tidal elevations at the outfall and provide some margin of safety for sea level rise. The outfall configuration will maintain the water at this depth. Open water/FWS areas offer both nitrification and denitrification potentials due to the aerobic conditions at the top and anaerobic conditions at the base of the water column. Shallow FWS wetlands, such as unit-cell process wetlands enhance nitrate removal through photolysis, biotransformation, and biofilm formation, where anaerobic denitrifying bacteria thrive.9 Adding plant carbon sources like woodchips can improve nitrate removal efficiency. Optimal carbon: nitrogen ratios for treatment wetlands and bioreactors vary, and additional liquid carbon sources can further augment denitrification efficiency. If supplemental carbon is required in the future, a system would be needed to introduce this external carbon at the basin's upstream end. 7 United States Environmental Protection Agency (EPA), 2000. Constructed Wetlands Treatment for Municipal Wastewaters, EPA 625/R- 99/010. 8 Iowa Department of Natural Resources, 2007. Constructed Wetlands Technology Assessment and Design Guidance. August. 9 Jasper, J.T., Z.L. Jones, J.O. Sharp, and D.L. Sedlak. 2014. Nitrate Removal in Shallow, Open-Water Treatment Wetlands. Environmental Science and Technology. 48: 11512–11520.