Concentrations of dissolved nitrate in recharge-area water from a 200-km² segment of the South Platte River alluvial aquifer near Greeley, Colorado, ranged from less than 0.1 to 58 milligrams per liter as nitrogen, and the median concentration was 26 milligrams per liter as nitrogen. Hydraulic-head data indicated that this nitrate-enriched groundwater moved toward the river. However, a nitrate mass balance of surface-water inflows and outflows to the river indicated that the concentration of nitrate in discharging groundwater was only about 6 to 8 milligrams per liter as nitrogen. Measurements of dissolved oxygen, nitrogen gas, nitrate, and nitrate-nitrogen isotope ratios showed that microbial denitrification in riverbed sediments removed nitrate from groundwater before the water entered the river and may have caused the difference in nitrate concentrations between recharge- and discharge-area water. The availability of solid organic carbon in the sediments to support denitrification was limited; however, infiltration of surface water into the sediments may have provided dissolved organic carbon to support microbial activity. Comparison of chloride and silica concentrations in river water and in groundwater demonstrated that mixing occurred to depths of at least 2.4 meters below the bottom of the river. These data also indicated that groundwater, which had discharged to the river, may have reentered the riverbed sediments and undergone further denitrification. Results from this study indicated that estimates of nitrate loads delivered to rivers from large aquifer systems need to account for biogeochemical reactions occurring in the near-river environment.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225
2 U.S. Geological Survey, Reston, Virginia