Submitted to: Georgia Water Resources Conference
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2003
Publication Date: 4/15/2003
Citation: Endale, D.M., Washington, J.W., Samarkina, L. 2003. Role of iron-rich georgia soils in controlling nitrate contamination of ground water. Georgia Water Resources Conference. p. 879-883.
Interpretive Summary: 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. Why? 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 this natural phenomenon to understand the reasons. The group monitored ground water for several solutes that included nitrate and iron. When the data were examined thermodynamically, it lead to the conclusion that nitrate reduction proceeded largely by oxidation of ferrous iron to ferric iron. This determination supports the hypothesis that iron helps reduce nitrate contamination of ground water. Iron is a common mineral in soils of large areas of the Southern Piedmont and Coastal Plain, regions that extend from Virginia to Alabama. Agriculture and related industries play pivotal role in the region's economy, with potential for nitrate release into surface and groundwater resources. Understanding the role of iron in reducing nitrate contamination in these regions is of considerable interest to all stakeholders impacted by or concerned about the nitrate problem.
Technical Abstract: Nitrate contamination of groundwater is widespread in the USA, and is often associated with nutrient losses from grazing and row-crop agriculture. Studies in Georgia find, however, generally lower levels of nitrate in groundwater than in many other parts of the country. The mechanism controlling the fate of nitrate and related N compounds in groundwater are poorly understood but the commonality of iron minerals in Georgia soils suggests iron may play a role. We monitored ground water for several solutes for a year and examined the data thermodynamically. The redox states quantified as electron activity pE between couples of N-species (NO3-, NO2-,and NH4+) were found to lie on the intersection of the stability field of freshly precipitated Fe(OH)3 with Fe2+ on a Pourbaix (pE-pH) diagram. Moreover, The evident redox potential relationship between these couples seems stable through time as well. This strongly suggests that the energy-generating nitrification and denitrification reactions are being drawn toward equilibrium with Fe(OH)3/Fe2+ redox couple. These observations support the hypothesis that nitrate reductions proceed largely by oxidation of Fe2+ to an amorphous solid that subsequently recrystallizes to a meta-stable 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-] in ground waters of the southeastern USA.