10632 7TH AVE SE 2025-07-28 - Laserfiche WebLink

I WATER QUALITY All available means to remove pollutants from stormwater flows are active in the proposed bio- retention systems. The removal processes provided are as follows: • Sediment removal. Sedimentation will occur within the Type "A" catch basins, located within the proposed private road, that direct road runoff to each individual rain garden. Oil/Water separation. A frop-tee oil/water separator will be located within the overflow catch basins of each individual rain garden. This will ensure that all runoff from pollutant generating surfaces, namely the private road and driveways, encounter 1 or more oil/water separation devices before discharging to the wetland buffer. • Particulates will be physically strained out of the stormwater through the filtration process. Some filtration occurs as storm water moves through the plants in the ponding- area, but the amended soil is the primary filtering media. 90 percent of the particulates (6 to 41 microns) can be trapped by 18-inches of sand. Bioretention soils are typically high in sand content so this level of performance can be anticipated. • Ions and molecules will be bound to electrostatic receptor sites on the filter media particles (amended soil) through the adsorption process. This process is the primary mechanism for removing soluble nutrients, metals, and organics that occur in storm flows as it percolates through the amended soils of the bio-retention area. This process increases with increased organic matter, clay and a neutral to slightly high alkaline PH. The percolation of the stormwater initiates the adsorption and microbial action for pollutant removal. 111 • The vegetated soils allow the phytoremediation processes to occur. These processes g include degradation, extraction by the plant, containment within the plant (assimilation) or a combination of these mechanisms. It has been shown through studies that vegetated soils are capable of more effective degradation, removal, and mineralization of total petroleum hydrocarbons (TPHs), polycyclic aromatic hydrocarbons (PAHs), pesticides, chlorinated solvents, and surfactants than non-vegetated soils. • Thermal attenuation is provided which reduces stormwater temperature as storm flows move through subsurface soil layers. The rain garden provides excellent water quality advantages. Section 6.1.4 of the LID Technical Guidance Manual for Puget Sound outlines in more detail the above listed water quality provisions provided in our proposed bioretention system. 1 5 I