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~, wetland area. Pretreatment in this upland. area also minimizes the area of wetland impact from the proposed secondary treatment cells described below. Treated runoff from Cell 1 overflows into Cell 2, which also receives runoff from the 10 acres of residential area west of Farrington Street (Figure 3) and outflow from the stormwater pond to the north on South Owasso Boulevard. This existing discharge at Farrington Street will be directed into the western end of Cell 2. The majority of the area of Cell 2 is excavated within the wetland area and will impact 0.63 acres of wetland. However, wetland impacts have been minimized by locating the pond in the area of the existing excavated channel to the greatest extent possible. The grades and shallow cover over the existing storm sewer system do not allow runoff from the western portion of the contributing area to be directed to Cell 1 for pretreatment prior to discharge into Cell 2. Nor is there sufficient upland area available to provide pretreatment of this runoff prior to discharge into the wetland. Other alternatives considered to this wetland impact are described in Section 3 below. --~ Outflow from Cell 2 will be directed over the northern edge of the excavated 3 cell into the adjacent wetland. The lack of a discrete outlet from Cell 2 is intended to direct treated discharge, by sheet flow, into the adjacent wetland area instead of continuing the more discrete flow of the existing channel to Lake Owasso. Cell 3 receives runoff from the 12 acres of residential area that currently -7 discharges into the wetland at Marion Street. This third cell is located completely within the wetland area and will impact 0.28 acres of wetland. However, wetland impacts have been minimized by locating the pond in the --I area of the existing excavated channel, as shown on Figure 4, to minimize i impacts to the adjacent, vegetated areas of the wetland. The grades and shallow cover over the existing storm sewer system do not allow runoff from ~ the eastern portion of the contributing area to be directed to Cell 1 for pretreatment. Nor is there sufficient upland area available to provide pretreatment of this runoff prior to discharge into the wetland. Other ~ alternatives considered to this wetland impact are described in Section 3 ' below. A storm water quality analysis of the proposed pond system was performed using the P8 computer model. The model results indicate that the three cell system will remove approximately 81 percent of total suspended sediment (TSS) and 52 percent of total phosphorus (TP). For comparison, if only Cell 1, located outside of the wetland area is considered, the TSS removal is reduced to 66 percent and the TP removal is reduced to 37 percent. _ The table below provides a summary of pollutant removal compared to a wetland impacts. Although the preferred 3-pond system results in some wetland impact, it provides treatment of currently untreated runoff from a larger contributing area and provides more effective pollutant removal prior -- { to discharge into the adjacent wetland and Lake Owasso. Construction of the 3-pond system will require excavation in the wetland, however the wetland disturbance has been minimized as much as possible and is located in an area _, Wetland Permit Application A-ROSEV0501.00 - South Owasso Boulevard Water Quality Improvements Page 8