Role of Iron-Rich Georgia Soils in Controlling Nitrate Contamination of Ground Water
Endale, Dinku M.
Washington, John W.
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Nitrate contamination of ground water 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 ground water than in many other parts of the country. The mechanisms 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⁻, 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 reduction proceeds 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.