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Vl-4 Summary Regarding Water Quality Parameters and Aquatic Life Through observation and monitoring of chemical parameters and aquatic life, a general conclusion regarding South Pl atte water qual ity is that from Chat- field Reservoir to approximately Bowles Avenue, the river is of good qua- lity, and offers excellent recreational potential; from Bowles Avenue to Oxford, degradation begins to occur, probably due to leachate inflows from solid waste landfills near the river; from Oxford to Bates the river is fur- ther impacted by non-point source pollution; and from Bates to Brighton qua- lity further diminishes due to urban runoff and sewage treatment plant dis- charges. Results of Runoff Monitoring Table IV-6 provides average event mean concentrations (EMC) in mill igrdms per liter of constituents monitored at the nine urban basins. Event mean concentrations of pollutants were calculated by dividing individual storm loads (in pounds) by individual storm runoff (in cubic feet) and converting the result to milligrams per liter (mg/1). The EMC's depicted in Table IV-6 were obtained by averaging values for all runoff studies(3). The signifi- cance of the values in Table IV-6 is perhaps best understood by contrasting val ues with those val ues found in secondary sewage treatment pl ant effl uent. Consequently, Table IV-7 provides average concentrations of selected pollu- tants in effluent from the Metropolitan Denver Sewage Disposal District Pl ant No. 1. UR~AN RUNOFF QUALITY DRCOG's goals in performing its recent study of stormwater quality and how it affects the South Platte River were the following: (2) Compare and contrast key water quality parameters in stormwater runoff wi th the 5 dme par ameters in second ar y sewage, As indicated in Tables VI-6 and VI-7, non-point source of contributions of lead and zinc in all nine basins monitored exceeded concentrations of lead and zinc in secondary wastewater. Additionally, concentrations of total copper were larger in runoff from two basins than in secondary sewage effluent. Concentrations of total Kjeldahl nitrogen and of total phos- phorous are significantly higher from all sewage treatment plants in the stUdy reach than were concentrations of the same constituents associated with stormwater. Nitrite and nitrate concentrations from the stormwater, however, were higher than secondary sewage effluent concentrations of nitrite and nitrate. Dissolved organic carbon measurements from stormwater tend to be relatively low as do chemical oxygen demand concentrations. Total suspended solids concentrations also tend to be relatively low when contrasted with normal river values. The DRCOG report indicates that very few EPA priority pollutants were detected in runoff silllples and that pollu- tant loading per runoff amount did not correlate well with basin impervious- ness or land use. Concentrations of pollutants did not vary in a predict- able pattern, nor did they vary in patterns which would normally be ex- pected. (1) Assess, through field sampling, the nature of stormwater runoff in the Denver area, (3) Describe impacts of stormwater runoff on the South Platte River, and (4) Suggest ways in which the non-point pollution problem can be miti- gated. To assess the magnitude of water quality problems associated with storm- water, DRCOG monitored runoff events in nine urban basins. These basins represented various land uses and intensities of development. Table VI-4 presents a listing of constituents in urban runoff which were analyzed by DRCOG. Runoff was obtained from thunderstorms, upslope rainstorms and snow- ~Lunll~. Table VI-5 pr"oviJes descr--iptions of urban runoff monitoring basins. Impact of Stormwater Runoff on South Platte River Water Qual ity Table VI-8 presents ambient streamflow concentrations of various parameters monitored between April 1980 and September 1981 at the South Pl atte River at 50th Avenue; these concentrations are those associated with normal (base)