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presented in the water quantity modeling results, where specific storm events were <br />considered. <br />Water quality was modeled for several pollutants for two runoff scenarios. Both <br />scenarios consider the likely treatment that runoff would receive in stormwater BMPs <br />located along the route that the water would follow. For example, the runoff routed into a <br />properly designed detention pond would lose about 75% of the total suspended solids it <br />carries. This water can then be routed downstream, where it might encounter another <br />detention pond or infiltration system where another increment is removed. <br />In the first scenario, runoff is stored only in the detention ponds and infiltration basins <br />within the central drainage corridor. In the second scenario, extra storage that would <br />exist elsewhere on the site in small ponds is considered. In this case, runoff is stored, but <br />does not infiltrate into the groundwater. <br />The exact nature of the primary solids removal BMPs located at the storm sewer inflows <br />to various drainageways has not yet been determined. These could be a mix of forebays <br />created from earthen material, catch basin inlet filters, all the way to sub -grade treatment <br />train systems. <br />Table 17.6 presents the results of water quality modeling for total phosphorus (TP). TP <br />was chosen to present the quality results because it is one of the more difficult pollutants <br />to remove. That is, if effective removal of TP occurs, the other pollutants will have equal <br />or better removals. The table shows that with storage and treatment in the central <br />drainage corridor facilities, the total phosphorus load leaving the RTC site (out of <br />subwatersheds 26 and 31) is approximately 20 lbs/year. This figure is cut in half when <br />additional site storage is considered. In terms of a per unit area loading rate, the first <br />scenario yields 0.053 lbs TP/acre-year; that figure is approximately halved with the <br />addition of extra storage. These areal loading rates are reflective of the numerous <br />detention ponds and the natural infiltration occurring throughout the RTC site. <br />Table 17.6. Average Annual Total Phosphorus in runoff leaving RTC site <br />With storage in <br />the central <br />corridor <br />With additional on -site storage <br />lbs TP/yr <br />lbs TP/ac-yr <br />19.6 10.3 <br />0.053 0.028 <br />The modeled phosphorus removals are contained in Table 17.7. These results are <br />presented to show the reductions that the water quality treatment system used on the RTC <br />site can achieve. <br />17-12 <br />