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VI-5 flow within the river. Such concentrations would be obtained during periods in which limited urban runoff would be entering the South Platte River. By contrast to Table VI-8, Table VI-9 provides storm runoff event mean concen- trations monitored between April 1980 and September 1981 at the South Platte River at 50th Avenue. Table VI-9 demonstrates the impact that stormwater has on the South Pl atte River for various magnitudes of storms. Many para- meters exhibit prominent increases in the aftermath of storm runoff input to the South Platte River, including the following: Total suspended sol ids, Total organic carbon, Total nitrogen, Total phosphorus, Fecal col iform bacteri a, Iron, Mang anese, Lead, and Zinc lity standards for ambient and storm event mean concentrations. This was done for April 1980 through September 1981 at the South Platte River at 50th Avenue, and the results are depicted in Table VI-10. Table VI-lO pre- sents stream standards as established by the Colorado Water Quality Control Commission for 19 parameters, and demonstrates that the South Platte River under flood conditions exceeded these 19 standards with greater frequency and with greater magnitude than under base flow conditions. In addition to urban runoff, other factors cause South Platte water quality to deteriorate, including: (1) Instream gravel mlnlng. (2) Dredging in areas of deposition of toxic materials. (3) Dumping of snow removed from streets. (4) Vegetation removal. In addition to affecting water in the South Pl atte River, urban storm water impacts sediments on the river bottom. DRCOG, in conjunction with Woodward- Clyde Consulting Engineers recently studied river bottom sediment chemistry at 20 monitoring points on the South Platte River. Locations of these moni- toring points are provided in Figure VI-2. River sediments are important because they serve as chemical sinks and sources (for example, metals and nutrients can be absorbed or released, depending upon various factors) and as habitat for benthic organisms. Figures VI-3 and VI-4 provide concentra- tions of selected chemical constituents in bottom sediments of the South Platte River on February 24, 1981. As would be expected, concentrations of lead, zinc and cadmilJll are relatively low from Stations 0 through 4; how- ever, as the river enters the more heavily urbanized area, concentrations of these metals steadily increase until they peak in downtown Denver near the Col fax vi aduct. POINT SOURCES OF POLLUTION The principal point sources of direct discharges into the South Platte River between Chatfield Reservoir and Brighton include: (1) The Littleton/Englewood Sewage Treatment Plant (2) Gates Rubber Company (3) Public Service Company (4) The Denver Metropolitan Sewage Disposal Plant (5) The South Adams County Sewage Treatment Plant (6) The Brighton City Sewage Treatment Plant (which was not studied because the plant is essentially at the end of the study reach.) A final means of assessing the impact of urban runoff on water quality in the South Pl atte River is to calcul ate the percent exceedence of water qua- Due to the limited scope of this water quality analysis, other point source discharges into tributaries which feed the South Platte River were not stud- ied. Additionally, because Lakewood now transmits all of its sewage to Denver Metro for treatment, the Lakewood Sewage Treatment Plant, which was operational recently, is not addressed herein. DRCOG's recent water quality study of the South Platte River indicated that neither Gates Rubber Company