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Title: Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2

item THOMAS, ROBERT - Us Environmental Protection Agency (EPA)
item Endale, Dinku
item SCHROER, KATHERINE - University Of Georgia
item SAMARKINA, LIDIA - Us Environmental Protection Agency (EPA)

Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2006
Publication Date: 9/15/2006
Citation: Washington, J.W., Thomas, R.C., Endale, D.M., Schroer, K.L., Samarkina, L.P. 2006. Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2. Geochimica et Cosmochimica Acta. 70:3533-3548.

Interpretive Summary: Nitrate contamination of groundwater is a major national public health concern, especially in areas where agriculture is practiced intensively and extensively. Scientists and action agencies are actively pursuing ways and means of reducing this public threat by trying to understand the processes that lead to contamination and developing management practices that limit it. Scientists from the Ecosystem Research Division of the US Environmental Protection Agency in Athens, GA, and the USDA-ARS, J. Phil Campbell Sr. Natural Resource Conservation Center in Watkinsville, GA, have been studying a natural phenomenon that appears to limit nitrate accumulation in ground water. In the southeastern US a mutually exclusive relationship appears to exist between nitrate and iron in ground water: no high nitrate in the presence of high iron and no high iron in the presence of high nitrate. The group monitored ground water for several solutes that included nitrate and iron. Thermodynamic examination of these data indicated that, under the prevailing conditions, nitrate reduction proceeded largely by oxidation of ferrous iron to ferric iron supporting the hypothesis that iron could play a role in reducing nitrate contamination of ground water. Large areas of the Southern Piedmont and Coastal Plain, regions that extend from Virginia to Alabama, have soils that are rich in iron. Understanding the role of iron in reducing nitrate contamination in these regions, where agriculture plays a pivotal role in the economy, is of considerable interest to all stakeholders impacted by or concerned about the nitrate problem.

Technical Abstract: Nitrate reduction in groundwater having low ppm-range dissolved O2 commonly is held to be mediated largely by microbial assimilatory nitrate reduction with Corg as the electron source. To evaluate this concept, groundwater was monitored for several solutes and these data examined thermodynamically. Also, aquifer solids were subjected to dissolution to evaluate the solubility controlling phase for Fe3+; for these aquifer solids, log[Fe3+][OH-]3 was found to be -40.3, a value closely consistent with several other studies. The speciation of NO3-, NO2-, and NH4+ were found to be nearly thermodynamically consistent with Fe2+ equilibrium with freshly precipitated Fe(OH)3 in the NO3- rich water studied, differing energetically by only as much as, or little more than, the amount typically needed to support microbial metabolism. These data support the hypothesis that nitrate reduction proceeds largely by oxidation of Fe2+ to an amorphous solid that subsequently recrystallizes to a metastable ferric hydroxide. An inverse relationship between [Fe2+] and [NO3-] in GA waters, noted in other studies, suggests that this phenomenon might exercise a regional control on [NO3-] for groundwater in the SE U.S. N transformation in the aquifer studied is sufficient to concentrate N2 fugacity to >1 atm so that N2 exsolution by bubble formation is favored thermodynamically. Roughly 99% of N added to the pasture is estimated to be lost to transformation followed by volatilization and storm-water runoff.