Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2003
Publication Date: October 1, 2003
Citation: Hunter, W.J. 2003. Accumulation of nitrite in denitrifying barriers when phosphate is limiting. Journal of Contaminant Hydrology. 66:79-91. Interpretive Summary: Nitrate is used in and forms from a number of agricultural activities. It is both highly soluble in water, highly mobile, and persistent in deep groundwater. The presence of nitrate in drinking water is a concern because it may cause methemoglobinemia. Denitrifying barriers can remove nitrate from groundwater. Barriers may be constructed by filling an excavated area with a porous mixture of sand, fine gravel and substrate or by the injection of a non-aqueous phase liquid substrate into an aquifer. The substrate, usually a carbon source, stimulates denitrification by denitrifying microorganisms. This study indicates a need to ensure that adequate phosphate is present in denitrifying barriers. If adequate phosphate is not present, nitrite may form. The accumulation of nitrite is bad because it is more toxic that nitrate
Technical Abstract: Permeable in situ denitrifying barriers can remove nitrate from groundwater. Barriers may be constructed by filling an excavated area with a porous mixture of sand, fine gravel and substrate or by the injection of a non-aqueous phase liquid substrate into an aquifer. The substrate stimulates denitrification by denitrifying microorganisms. It was found that nitrite accumulated in laboratory denitrifying barriers when phosphat limited nitrate removal. Sand columns injected with soybean oil emulsion were used as laboratory models of denitrifying barriers. When a groundwater containing 17 mg L-1 nitrate-N and 0.009 mg L-1 phosphate-P was pumped through the columns only a small amount of nitrate was removed from the water and, in some effluent fractions, 52 to 88% of the influent nitrate had converted to nitrite. Nitrite also accumulated when the phosphate concentration of the groundwater was increased to 0.040 or 0.0 80 0mg L-1 phosphate-P. Only when a 0.160 mg L-1 phosphate-P supplement was added to the groundwater was there a loss of nitrate without a large accumulation of nitrite. The addition of solid calcium phosphate or rock phosphate to the sand columns was found to provide adequate phosphate for denitrification in short-term studies. The study shows the need to ensure that adequate phosphate is present in denitrifying barriers.