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Project Description
<br />b) Impermeable Liner
<br />An impermeable synthetic liner will be placed over the compacted subgrade along the base of the basin and up
<br />the side slopes to prevent infiltration and any bypass of polished wastewater through the underlying soils.
<br />Additionally, it will inhibit the growth of unplanned plant species, which could create preferential pathways and
<br />shade the open water areas, hindering photolysis. The liner will likely consist of an impermeable 40-ml Linear
<br />Low -Density Polyethylene commonly used in lining open -water treatment wetlands, lagoons, and ponds.
<br />c) Hydraulic Distribution System
<br />Water will enter the wetland basin via a hydraulic distribution system, regulated by the effluent from the
<br />upstream membrane aerated biofilm reactor (MABR) nitrification system, not to exceed 0.95 mgd. This system
<br />will use perforated HDPE pipes along approximately 1,320 linear feet of the basin's edge to distribute water to
<br />the terraced denitrification bioreactors on the side slopes.
<br />Additionally, approximately 1,200 ft of smaller irrigation lines will wrap around the remaining extent of the
<br />basin's perimeter to water wetland plants on slopes not covered by the bioreactors. Separate irrigation lines
<br />will supply water to plants around the basin, outside the bioreactor areas. These lines will contribute negligible
<br />volumes of inflow to the basin.
<br />d) Side Slope Terraced Bioreactor
<br />Terraced crates will be filled with wood chips along the basin's side slopes to serve as bioreactors for
<br />denitrification (Figure 2.13, Wetland Side Slopes). Each crate will be lined with an impermeable material both
<br />beneath and on the downstream end to manage the water flow residence time. To expand vegetated habitats,
<br />the basin perimeter slopes will be reshaped to a uniform steepness, even in areas without bioreactors.
<br />Topsoil will cover the seepage slopes/bioreactor terraces, varying in depth from 6 to 24 inches (averaging 15
<br />inches), topped with erosion control matting, and planted with suitable native species. This bioreactor cell
<br />design is an innovative take on the ecotone levee concept, integrating global insights on using wood chips as a
<br />conductive carbon source for enhanced den itrification.4,5,6
<br />This Proposed Project's seepage slopes will differ from those in OLSD or other regional projects. Unlike the
<br />relatively flat slopes at OLSD (30H:1V), the proposed treatment wetland's existing slopes are steep (3:1). Early
<br />results from OLSD suggest rapid denitrification at the upper reaches of its levee, with nitrate becoming
<br />undetectable after about 10 horizontal feet. This is attributed to the combination of plant roots and low
<br />permeability soils creating long retention times. The Proposed Project explores the effectiveness of high
<br />permeability alluvium in supporting riparian scrub and native grasses typical of wet meadow habitats.
<br />4 Schipper, L.A., Robertson, W.D., Gold, A.J., Jaynes, D.B., Cameron, S.C., 2010. Denitrifying bioreactors - An approach for reducing nitrate
<br />loads to receiving waters. Ecological Engineering, 36:11, 1532-1543. Leveren 2010, Sere§ 2019, Halaburka 2017)
<br />5 Leveren, H.L., Haunschild, K., Hopes, G., Tchobanoglous, G., Darbya, J.L. 2010. Anoxic treatment wetlands for denitrification. Ecological
<br />Engineering, 36:11, 1544-1551.
<br />6 Seres, M., Mocova, K.A., Moradi, J., Kri§ka, M., Koff, V., Hnatkova. T., 2019. The impact of woodchip-gravel mixture on the efficiency and
<br />toxicity of denitrification bioreactors. Sci. of the Total Env. 647:10, 888-894.
<br />San Leandro Treatment Wetland
<br />IS/MND
<br />uary 2024
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