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EFFECT OF NITRATE, ORGANIC CARBON, AND TEMPERATURE ON DENITRIFICATION RATES IN AN ALLUVIAL AQUIFER

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Karla Pfenning and Peter B. McMahon 1

ABSTRACT

A study conducted in 1994 as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program, South Platte River Basin investigation, examined the effect of certain environmental factors on potential denitrification rates in nitrate-rich riverbed sediments. The acetylene block technique was used to measure nitrous oxide (N2O) production rates in laboratory incubations of riverbed sediments to evaluate the effect of varying nitrate concentrations, organic-carbon concentrations and type, and water temperature on potential denitrification rates. Sediment incubations amended with nitrate, at concentrations ranging from 357 to 2,142 µmol/l (as measured in the field), produced no significant increase (p>0.05) in N2O production rates, indicating that the denitrification potential in these sediments was not nitrate limited. In contrast, incubations amended with acetate as a source of organic carbon, at concentrations ranging from 0 to 624 µmol/l, produced significant increases (p<0.05) in N2O production rates with increased organic-carbon concentration, indicating that the denitrification potential in these sediments was organic-carbon limited. Furthermore, N2O production rates also were affected by the type of organic carbon available as an electron donor. Acetate and surface-water-derived fulvic acid supported higher N2O production rates than groundwater-derived fulvic acid or sedimentary organic carbon. Decreasing incubation temperatures from 22 to 4 °C resulted in about a 77% decrease in the N2O production rates. These results help to explain findings from previous studies indicating that only 15 to 30% of nitrate in groundwater was denitrified prior to discharging to the South Platte River and that nitrate concentrations in the river generally were higher in winter than in summer.

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

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