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<br />0007,)3 <br /> <br />"r <br /> <br />in the system is satisfied by return flows, <br /> <br />Water Quality <br />Water quality issues in the basin include point and nonpoint source urban and mine drainage pollutants, <br />salt accumulation on lower basin irrigated lands, elevated levels of nutrients and total dissolved solids in <br />surface water and the alluvial aquifer, and localized areas of high nitrogen concentrations in the ground water <br />in the central portion of the basin. Metals contamination resulting from historic mining activities is observed <br />in several headwater streams, The problem is especially apparent in Clear Creek from Idaho Springs to the <br />South Platte, Water quality in the main stem of the South Platte River and its tributaries to the north is <br />degraded significantly when it reaches the plains by discharges from municipal waste treabnent facilities and <br />urban runoff. Water in stream reaches immediately below these urban-industrial corridors often fails to meet <br />the state's water quality standards for dissolved oxygen, fecal coliform bacteria, ammonia, and phosphorus, <br />Quality improves in the main stem farther downstream because of irrigation diversions, which tend to <br />consume nitrogen and phosphorus, and dilution effects from incoming streams draining less urbanized areas <br />of the northern front range, <br />The use of ground water return flows for irrigation in the lower basin contributes to higher dissolved <br />solids concentrations and can affect stream water quality, especially during low-flow conditions, Dissolved- <br />solids concentrations of the return flows are increased because of the concentrating effects of consumptive <br />use losses through evapotranspiration, dissolution of soil minerals, and their association with a variety of <br />agricultural chemicals, Median dissolved-solids concentration in surface water within the basin increases in a <br />downstream direction from 395 mgIL at Denver to 1,550 mgIL near Julesburg (Dennehy et al" 1993), <br />Contaminant. in alluvial ground water of the basin occur largely as a result of agricultural activities and <br />municipal and industrial wastewater discharges, The most serious problem is high nitrate concentrations, <br />which are occasionally above the EP A safe drinking water standard of 10 mgIL, The highest levels in the <br />shallow alluvial aquifer are observed near Greeley, The major source of ground water nitrate contamination <br />in this area is manure generated by concentrated animal feeding operations and applied to surrounding fields, <br />Farther downstream, both manure and agricultural fertilizers are believed to be the primlll)' sources of nitrates <br />in groundwater, Median dissolved-solids concentration from recent surveys of ground water from the South <br />Platte alluvial aquifer was about 1,000 mgIL (USGS, 1988), <br /> <br />20 <br />