Nutrient concentrations in surface and groundwater in the South Platte River Basin show statistically significant differences between urban and agricultural land-use settings. Concentrations in surface water were determined from 20 monthly samples obtained at the mouth of streams that discharge from individual small urban and agricultural basins. Concentrations in groundwater were determined from samples collected from 30 randomly selected alluvial wells in each land-use setting.
The median concentration of dissolved nitrate, as nitrogen, in surface water was 3.0 milligrams per liter (mg/L) in the urban setting and 6.6 mg/L in the agricultural setting. The median concentration of dissolved phosphorus, as phosphorus, in surface water was 0.28 mg/L in the urban setting and 0. 16 mg/L in the agricultural setting. For groundwater, the median concentrations for nitrate were 2.1 mg/L in the urban setting and 7.0 mg/L in the agricultural setting, and the median concentrations of phosphorus were 0.02 mg/L in the urban setting and 0.2 mg/L in the agricultural setting.
In both land-use settings, nitrate was the predominant nitrogen species in surface water, except during storm events when organic nitrogen concentrations increased and nitrate concentrations decreased. Ground water contributed substantially to nitrate concentrations in surface water, but the nitrate contribution from groundwater varied seasonally and was smaller than expected because of partial denitrification in anaerobic streambed sediments.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225