3003 W CASINO RD BOEING STORMWATER MGT SYSTEM 2023-12-28

Kennedy Jenks Technical Memorandum Kyle Bringedahl, The Boeing Company 13 April 2021 KJ 2096009.00 Page 3 Discharges from the detention basin to the proposed conveyance system will be controlled with an actuated sluice gate to ensure no more than 49 cfs discharges to the stilling basin for up to the 25-year storm event. Storms larger than the 25-year event will exit the detention basin via the spillway, matching existing conditions. Other proposed improvements include enhanced pretreatment and increased treatment volume. For pretreatment, two continuous deflective separators will be installed upstream of the enhanced media filter for suspended solids removal. To increase the volume treated, one of the existing sluice gates on the detention basin outlet riser will be actuated to allow treatment down to water surface elevation 345.2, four feet deeper than existing conditions. When the detention basin water level elevation is between 345.2 and 348, detention basin discharges will be pumped to the enhanced media filter for treatment. Operational controls will be implemented to ensure that spill protection is maintained as in existing conditions. 1.4 Design Validation Validation efforts were performed to ensure that the proposed system will meet or exceed existing flow control and treatment criteria. The removal of constituents of concern by the proposed enhanced treatment media was determined by media pilot testing, and results were compared to effluent monitoring data from the existing peat filter. To address hydrologic and hydraulic criteria, the existing and proposed drainage systems were modeled using EPA SWMM5 to compare treatment capture efficiency and outflows to the Powder Mill Gulch wetland and the Powder Mill Creek. 1.4.1 Media Pilot Testing Media research and pilot testing was performed to confirm that the selected media is suitable for treating the parameters of concern and outperforms the existing peat filter. Pilot testing was performed at the site during winter 2020-2021. The selected media mixture consists of 60% sand, 15% biochar, 15% coconut core pith and 10% zeolite, and is a significant upgrade over the existing peat filter. The new media removed copper to well below the numeric effluent limits of 70 pg/L for total zinc (zinc) and 10 pg/L for total copper (copper). The existing peat filter was designed to meet the previous permit limits of 117 pg/L and 14 pg/L for zinc and copper, respectively. A table summarizing media performance is provided below. 1\kjc.locallkjc-rootlkj-projectslfederalwayl202012096009.00 boeing everett sw treatment109-reports19.08-reports-tmslcity of everelt design tmlboeing everett sw design tech memo.docx ©Kennedy/Jenks Consultants, Inc.