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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. McMahon1


As part of the National Water-Quality Assessment Program's South Platte River Basin study, the U.S. Geological Survey has sampled shallow alluvial groundwater beneath urban (Denver, Colo.) and agricultural (South Platte River alluvium from Brighton, Colo. to North Platte, Nebr.) land-use settings to determine the effect of land use on water quality. Thirty randomly distributed wells in each land-use setting were sampled for nutrients, trace elements and radon, pesticides and volatile organic compounds (VOC's).

Nutrient species concentrations generally were lower in the urban setting than in the agricultural setting. The median concentration of dissolved nitrite plus nitrate as nitrogen in the urban setting was 2.1 milligrams per liter. Preliminary results indicate a median concentration of 7.0 milligrams per liter for nitrite plus nitrate in the agricultural setting.

Trace-element and radon concentrations indicated no correlation with land-use setting. However, uranium and radon occurred at elevated concentrations in both land-use settings (median concentrations of uranium and radon were 25 micrograms per liter and 1100 picoCuries per liter). The distribution of uranium and radon in groundwater probably was affected mostly by local geology rather than by land use.

Pesticide compounds detected in groundwater samples from both land-use settings generally occurred at low concentrations. Prometon, a nonselective herbicide, was the most frequently detected pesticide in the urban setting and atrazine was the most frequently detected pesticide in the agricultural setting. However, both of these compounds were detected in each of the land-use settings.

VOC's were detected in 86 percent of the urban wells sampled, and concentrations of specific compounds frequently exceeded National Drinking Water Standards. Preliminary results indicate that VOC's are almost nonexistent in alluvial groundwater of the agricultural setting. The presence of VOC's affected redox conditions in the alluvial aquifer in the urban setting and, consequently, the chemistry of the urban groundwater.

Data indicated that land use does have a measurable effect on the quality of groundwater in the South Platte River alluvial aquifer and that groundwater quality differs between urban and agricultural land-use settings.

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

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