DENITRIFICATION POTENTIAL IN ACID MINE DRAINAGE STREAMS

Nitrogen cycling in acid mine drainage (AMD) environments is poorly characterized. As nitrogen deposition rates increase, particularly in the Rocky Mountain Front Range, the question arises as to whether AMD streams can process nitrogen, specifically nitrate. Little is known about the fate of nitrate in these acidic, heavy metal laden streams. Studies have shown that denitrification (biological conversion of nitrate to nitrogen gas) is inhibited below pH 4 in many soil and groundwater environments. Our research examined several Colorado AMD sediments for denitrification potential. Microcosms were set up using sediment and surface water (amended with ~5mg/L NO₃-N) from AMD sites ranging in pH from 2.60 to 4.91. The microcosms were made anoxic and nitrate, nitrite, ammonium, nitrogen gas, and pH were monitored over a 7-day period. Autoclaved controls were used to verify that nitrate disappearance was biological. Nitrate was utilized fastest in the Cinnamon Gulch microcosm (pH 4.12), with all nitrate used after 80 hours. Denitrification potential in St. Kevin's Gulch (pH 4.2) and sediment and water seeping directly from a tailings pile (pH 2.60) were the slowest with 29.9% and 34.8% nitrate removal in ~150 hours. During the incubation, no ammonium was detected and varying nitrite concentrations were produced and utilized. The concentration and species of electron donors and metals, and species and number of microbes capable of denitrification present may explain differences in nitrate utilization and amounts of nitrite generated. After the ~150 hour incubation, pH in all microcosms increased, indicating denitrification as a possible remediation strategy for AMD streams. Current research in our lab is examining the preferred electron donors for denitrification in these environments. Further studies are planned to characterize the microorganisms involved in nitrate utilization in AMD environments.