2725 32ND AVE NE 2018-01-02 MF Import

wEYERHA usEN SMITH ISLAND LANDFILL CLOSURE IMPROVEMENTS AND NPDES OUTFALL 005 ammTION SEPA ENVIRONMENTAL CHECKUST Ecology has recommended that sediments from the west and north end of the perimeter ditch be removed and disposed. The estimated volume of the ditch sediments to be removed is 500 cubic yards. The ditch material will be properly disposed in the landfill. Construction of the west access road into the landfill will require installation of a culvert in the bottom of the existing perimeter ditch and filling of the ditch in a localized area. The volume of fill material for the new access road is estimated;, t 300 cubic yards. As part of the stormwater lift station construction, minimal amount (less than 25 cubic yards) of ditch material will be removed and replaced with large rip rap. This work is limited to the area around the wet well inlet pipe. The rip rap will be imported from commercial sources. 4) Will the proposal require surface water withdrawals or diversions? Give general descriptions, purpose, and approximate quantities if known. No surface water would be diverted to any natural stream or other water bo6y. The proposed stormwater lift station will redirect existing stormwater flows from the areas surrounding the landfill and the log sort yard south of the landfill into the primary settling basin for treatment and ultimate discharge through Outfall 001. The stormwater flows from these two areas currently discharge through Outfall 005, which is being eliminated as required by NPDES Permit No. WA-003197-6. The lift -tation is sized to ha,.ile approximately 600 gallons per minute (gpm) or 116,000 cubic feet per day (cf/day). The 6-month storm runoff volume is estimated at 1M,000 cf/day. Runoff in excess of the lift station capacity will be temporarily stored in the ditch that has an estimated total storage capacity of over 350,000 cf. 5) Does (lie proposal lie within a 100-year floodplain? If so, note location on the site plan. The entire site is mapped as within the FEMA 100-year fluodplain. However, the dike surrounding the landfill is above the 100-year flood elevation (9 feet NGVD). When closure improvements are completed, the landfill will remain at elevation 9 feet or higher. 6) Does the proposal involve any discharges of waste materials to surface waters? If so, describe the type of waste and anticipated volume of discharge. Discharge of additional treated storm water into Steamboat Slough, including any runoff from the landfill, will be routed through Outfall 001 in accordance with Permit No. WA-003197-6. This storm water would be treated in the wastewater treatment system. Solids settle out in the primary settling basin and the wastewater flows into secondary treatment basin. Stormwater passes'hrough the secondary treatment basin and is discharged into Steamboat Slough through Outfall 001. The storm water lift station is designed to handle the projected combined 24-hour storm runoff volume for the 6-month storm event (0.84 acre-feet). The discharge water quality will comply with the requirements of the NPDF.S permit (WA 003197-6). The proposed change will improve water quality by treating storm water runoff from the existing landfill perimeter that is currently discharged untreated to the Snohomish River. 14