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Colorado Water Science Center


National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Bret W. Bruce and Peter B. McMahon 1


The occurrence and distribution of a broad range of water-quality constituents in the primary shallow groundwater aquifers in the South Platte River Basin were determined for three study areas that have specific combinations of land-use and aquifer settings: (1) urban-crystalline bedrock, (2) urban-alluvium, and (3) agricultural-alluvium. Twenty-seven wells in the urban-crystalline bedrock study area and 30 wells in the urban-alluvium and agricultural-alluvium study areas each were sampled once for about 170 constituents, including major ions, nutrients, volatile organic compounds, pesticides, trace elements, and radon. Wells in the urban-crystalline and agricultural-alluvium study areas also were sampled for tritium. Results of water analyses indicate that natural rock/water interactions and anthropogenic land-use activities affected the quality of shallow groundwater. Major-ion data indicated an increase from small concentrations of dissolved solids in the upstream urban-crystalline area to large concentrations of dissolved solids in the downstream agricultural-alluvium study area. This downstream increase in dissolved solids concentrations generally was correlated with a change from bicarbonate to sulfate as the dominant anion and by increasing concentrations of calcium, sodium, and sulfate. Ground water in the agricultural-alluvium study area had higher nitrate concentrations (median = 9.35 milligrams per liter as nitrogen) than the other study areas (14 of 30 samples exceeded the U.S. Environmental Protection Agency drinking-water standard of 10 milligrams per liter as nitrogen). Volatile organic compounds were detected only in water samples from the urban-crystalline and urban-alluvium study areas. Thirty-one of 59 analyzed volatile organic compounds were detected at least once in the urban-alluvium study area, and concentrations occasionally exceeded maximum contaminant levels. Pesticides were detected only in the urban-alluvium (27 of 30 wells) and agricultural-alluvium (29 of 30 wells) study areas. No measured pesticide concentration exceeded maximum contaminant levels of health advisory levels for that compound. The concentration of the trace elements, manganese, selenium, and uranium, exceeded established or proposed drinking-water standards in one or more study areas. Background concentrations of radon in the shallow aquifers of much of the South Platte River Basin were substantially larger than the national average. The presence of large tritium concentrations and anthropogenic compounds in groundwater samples indicates that the water being sampled in each study area recently was recharged (after 1952), with the possible exception of six wells in the urban-crystalline study area that had concentrations of tritium small enough to indicate some water of pre-1952 origin.

In general, large uranium and radon concentrations occur naturally in groundwater throughout the South Platte River Basin. In the urban-crystalline study area, the presence of anthropogenic volatile organic compounds in groundwater indicates that the crystalline-bedrock aquifer is vulnerable to affects from land use. In the urban-alluvium study area, pesticides and volatile organic compounds were widespread in groundwater indicating the affect of high density urban land use on water quality. In the agricultural-alluvium study area, large concentrations of dissolved solids and nitrate were measured and pesticides were frequently detected.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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