RELATIVE ROLES OF ELECTRON DONORS IN AQUIFER DENITRIFICATION REACTIONS: INSIGHTS FROM GEOCHEMICAL MODELING

Denitrification is limited by electron donors. Hence, knowledge of the natural denitrification capacity of aquifers is required to manage the ongoing nitrate load into groundwater systems. At the national and regional levels many studies assessed nitrate vulnerability of aquifers; however, they typically consider only physical sediment properties and human activities and largely ignore the role of geochemical properties of aquifers. Groundwater denitrification can be estimated by computing the mass balance of electron donors contributing to the reaction. Denitrification by pyrite, organic carbon and ferrous iron is measured by the net sulfate, bicarbonate and Fe(III)-oxyhydroxides increases, respectively. However, the latter two electron donors sometimes give rise to precipitating reaction products. For such cases, carbonate precipitates can be estimated by computing the mass balance of the co-precipitating cations, Mg2+ and Ca2+, including the portion contributed via cation exchange from mineral surfaces. The amount of Fe(III)-oxyhydroxides that precipitate out from solution is hard to decipher and can only be predicted indirectly by computing the net nitrate lost due to denitrification but not accounted for by reactions with pyrite and organic carbon. Initially, the abundance of electron donors in aquifer sediments was determined using several techniques. Then, geochemical modeling with PHREEQC was employed to gain insights into the in situ denitrification processes that took place with all the common electron donors. The partial equilibrium geochemical modeling included the following major aquifer reactions: ion exchange, reversible reaction and redox reactions. Results confirmed that all common electron donors are important; however, the dominance of one electron donor over the other is site dependant.