The effects of electron donor and granular iron on nitrate transformation rates in sediments from a municipal water supply aquifer

Abstract

A municipal water supply well for the town of Baden, located about 10 km southwest of Waterloo, ON, Canada, was forced to close due to unacceptably high concentrations of nitrate in the groundwater. Stimulated in situ denitrification was considered a possible solution to the problem. In advance of a planned field test, the effectiveness of various electron donors (acetate, hydrogen gas, elemental sulphur, thiosulphate, aqueous ferrous iron and pyrite) at stimulating denitrification was compared in microcosm experiments involving sediment from the Baden aquifer. All electron donors tested, with the possible exception of pyrite, stimulated nitrate removal from solution. Acetate was found to be the substance that stimulated the quickest initial removal rates, and denitrification was confirmed as the mechanism using the acetylene block technique. Nitrite accumulation was minimal in most systems, although the local water quality guideline limit of 1.0 mg/l NO 2 −–N was briefly and temporarily exceeded (maximum value was 1.2 mg/l) in some of the acetate amended microcosms. Granular iron was also considered as an electron donor or abiotic reducing agent, but was found to reduce nitrate predominantly to ammonium, in a neutral pH solution buffered with pyrite. In mixed granular iron aquifer sediment systems, where several electron donors were present (hydrogen, ferrous iron and pyrite) that could have supported denitrification, the abiotic reaction with the granular iron appeared to dominate other transformation pathways, and ammonium was again the major product. Based on the testing completed as part of this project, the aquifer at Baden is considered a good candidate for acetate-stimulated in situ denitrification for the removal of nitrate from the groundwater near the municipal water supply well.