Ground water in terrace deposits of the South Platte River alluvial aquifer near Greeley, Colorado, USA, had a median nitrate concentration of 1,857 µmol/L. Median nitrate concentrations in groundwater from adjacent floodplain deposits (468 µmol/L) and riverbed sediments (461 µmol/ L), both of which are downgradient from the terrace deposits, were lower than the median concentration in the terrace deposits. The concentrations and delta-15N values of nitrate and N2 in groundwater indicated that denitrifying activity in the floodplain deposits and riverbed sediments accounted for 15 to 30 percent of the difference in nitrate concentrations. Concentrations of Cl-1 and SiO2 indicated that mixing between river water and groundwater in the floodplain deposits and riverbed sediments accounted for the remainder of the difference in nitrate concentrations. River flux measurements indicated that groundwater discharge in a 7.5-km segment of river had a nitrate load of 1,718 kg-N/d and accounted for about 18 percent of the total nitrate load in the river at the downstream end of that segment. This nitrate load was 70 percent less than the load predicted on the basis of the median nitrate concentration in the terrace deposits and assuming no denitrification or mixing in the aquifer. Water exchange between the river and aquifer caused groundwater that originally discharged to the river to reenter denitrifying sediments in the riverbed and floodplain thereby further decreasing the nitrate load in this stream-aquifer system. Results from this study indicated that denitrification and mixing within alluvial aquifer sediments may substantially decrease the nitrate load added to rivers by discharging groundwater.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225
2 U.S. Geological Survey, Reston, Virginia