Agenda - Council - 02/26/2019 - Laserfiche WebLink

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• Automobile, truck and bus traffic (oil, exhaust, vehicle decomposition); • Lawn and landscaping chemicals (fertilizer and pesticide); • Litter; • Vegetative debris; • Pet waste; • Fueling spillage from the convenience stations; • Increased sanding and salting; and • New construction (erosion, debris). The pollutant removal efficiencies of the proposed stormwater management practices were assessed using the P8 Urban Catchment Model (Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds, developed by William Walker). This approach allowed for the evaluation of different runoff scenarios, as well as the prediction of pollutant loads passing through the proposed development and eventually into the Mississippi River. Model results presented are for a complete year with a long term average precipitation depth (23.85 inches). This scenario is different than those presented in the water quantity modeling results, where specific storm events were considered. Water quality was modeled for several pollutants for two runoff scenarios. Both scenarios consider the likely treatment that runoff would receive in stormwater BMPs located along the route that the water would follow. For example, the runoff routed into a properly designed detention pond would lose about 75% of the total suspended solids it carries. This water can then be routed downstream, where it might encounter another detention pond or infiltration system where another increment is removed. In the first scenario, runoff is stored only in the detention ponds and infiltration basins within the central drainage corridor. In the second scenario, extra storage that would exist elsewhere on the site in small ponds is considered. In this case, runoff is stored, but does not infiltrate into the groundwater. The exact nature of the primary solids removal BMPs located at the storm sewer inflows to various drainageways has not yet been determined. These could be a mix of forebays created from earthen material, catch basin inlet filters, all the way to sub -grade treatment train systems. Table 17.6 presents the results of water quality modeling for total phosphorus (TP). TP was chosen to present the quality results because it is one of the more difficult pollutants to remove. That is, if effective removal of TP occurs, the other pollutants will have equal or better removals. The table shows that with storage and treatment in the central drainage corridor facilities, the total phosphorus load leaving the RTC site (out of subwatersheds 26 and 31) is approximately 20 lbs/year. This figure is cut in half when additional site storage is considered. In terms of a per unit area loading rate, the first scenario yields 0.053 lbs TP/acre-year; that figure is approximately halved with the addition of extra storage. These areal loading rates are reflective of the numerous detention ponds and the natural infiltration occurring throughout the RTC site. 17-12