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AEROBIC DENITRIFICATION: IMPLICATIONS FOR THE MOM RIVER BASIN
Citation:
THOMAS, R., J. W. WASHINGTON, AND L. SAMARKINA. AEROBIC DENITRIFICATION: IMPLICATIONS FOR THE MOM RIVER BASIN. Presented at EPA Science Forum, Washington, DC, May 16 - 18, 2006.
Impact/Purpose:
Improve the scientific understanding of the processes controlling nutrient distributions in surface waters. Produce a suite of enhanced models for characterizing nutrient distributions in surface waters by incorporating improved process understanding in existing models (e.g., WASP), by developing new models (e.g., WHAM, reactive transport), and improving linkages between model components.
Description:
Each year about 1.6 million metric tons of nitrogen, mostly from agriculture, is discharged from the lower Mississippi/Atchafalaya River Basin into the Gulf of Mexico, and each spring this excess nitrogen fuels the formation of a huge hypoxic zone in the Gulf. In the Mississippi, as well as most nitrogen-degraded rivers and streams, NO(3-) is the dominant N species and therefore understanding its biogeochemical behavior is critical for accurate nitrogen fate modeling. Based on thermodynamic modeling of data representing samples from natural freshwater systems, dissolved O(2) and NO(3-) were reduced to nearly equal redox potentials in every sample we analyzed, suggesting simultaneous reduction of O2 and NO(3-). In an effort to evaluate implications of these thermodynamic data experimentally, five headspace-recirculating microcosm reactors were constructed using engineered glass jars, copper tubing, and a peristaltic pump. An O(2) saturated, sterile synthetic river water solution (0.13 mM NO(3)) was added to the reactors aseptically along with three different inoculums taken from (1) natural river water, (2) wetland surface water, and (3) wetland sediment slurry. Two sterile controls were also employed. The reactors were sealed and headspace gas, originally of atmospheric composition, was recirculated through the solution. Within approximately three weeks, the N(2)O concentration in the headspace increased greater than five fold, while the nitrate concentration in the slurry decreased concomitantly. At the end of the experiment, the dissolved O(2) concentration in the slurry was approximately that of water equilibrated with atmospheric O(2). Although aerobic denitrification recently has been shown to take place under non-environmental conditions such as sewage treatment facilities where NO(3-) and C(org) are very concentrated, to our knowledge this is the first work to report aerobic denitrification under conditions typically found in environmental settings, such as the Mississippi River Basin. Our discovery of aerobic denitrification is expected to have a high impact on N-fate modeling as most existing models call for denitrification to take place in anoxic settings, yet most impacted surface waters are aerobic.