Effect of ground-water/surface-water interactions on nitrate concentrations in discharge from the South Platte River alluvial aquifer, Colorado

Concentrations of dissolved nitrate in recharge-area water from a 200-km2 segment of the South Platte River alluvial aquifer near Greeley, Colorado, range from less than 0.1 to 58 mg/l as nitrogen, and the median concentration is 26 mg/l as nitrogen. Hydraulic-head data indicate that this nitrate-enriched ground water move toward the South Platte River. However, the median concentration of nitrate in ground water from the discharge area is only about 9 mg/l as nitrogen. Moreover, measurements of dissolved oxygen, nitrogen gas, nitrate, and nitrate-nitrogen isotope ratios are then taken, and comparison between chloride and silica concentrations in river and ground waters are also made.