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1 <br /> I • Particulates will be physically strained out of the stormwater through the filtration <br /> process. Some filtration occurs as storm water moves through the plants in the ponding <br /> area, but the amended soil is the primary filtering media. 90 percent of the particulates (6 <br /> to 41 microns) can be trapped by 18-inches of sand. Bioretention soils are typically high <br /> in sand content so this level of performance can be anticipated. <br /> • Ions and molecules will be bound to electrostatic receptor sites on the filter media <br /> particles (amended soil) through the adsorption process. This process is the primary <br /> mechanism for removing soluble nutrients,metals, and organics that occur in storm flows <br /> as it percolates through the amended soils of the bio-retention area. This process <br /> increases with increased organic matter, clay and a neutral to slightly high alkaline PH. <br /> The percolation of the stormwater initiates the adsorption and microbial action for <br /> pollutant removal. <br /> • The vegetated soils allow the phytoremediation processes to occur. These processes <br /> include degradation, extraction by the plant, containment within the plant (assimilation) <br /> or a combination of these mechanisms. It has been shown through studies that vegetated <br /> soils are capable of more effective degradation, removal, and mineralization of total <br /> petroleum hydrocarbons (TPHs), polycyclic aromatic hydrocarbons (PAHs), pesticides, <br /> chlorinated solvents, and surfactants than non-vegetated soils. <br /> • Thermal attenuation is provided which reduces stormwater temperature as storm flows <br /> move through subsurface soil layers. <br /> The rain garden provides excellent water quality advantages. Section 6.1.4 of the LID Technical <br /> Guidance Manual for Puget Sound outlines in more detail the above listed water quality <br /> I provisions provided in our proposed bioretention system. <br /> t <br /> Minimum Requirement#7: FLOW CONTROL <br /> Basin I: The developed basin is a small basin of 6,812 SF (0.16acres) located on the east <br /> end of the developed site. This basin as with the existing basin flows east to the 7th Ave <br /> ' SE existing drainage system. The basin includes the front landscaped area 4.452 SF <br /> (0.10acres), pervious sidewalk (476 SF), pervious driveway to unit 1 (454 SF) all on <br /> site. The site has 1,884 SF (0.04acres) of impervious area including the private road. The <br /> impervious area of Basin I is less than 2,000 SF, of impervious area and therefore no <br /> detention is required City of Everett Storm Water Manual 2.2.4, Supplemental <br /> Information. There are 2 catch basins with oil water separators in the private roadway to <br /> collect pollutants for water quality and there is less than 0.10 cfs storm water impact. <br /> 9 <br />