Presentations 2016

Groundwater remediation for nitrate contamination in public supply wells: Challenge of the non-point source

University of Waterloo

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Elevated nitrate concentrations in public supply wells are being reported on an increasing frequency globally. Long term agricultural nutrient management practices are often identified as a principal cause. Best management practices (BMPs) designed to limit the regional loss of nutrients on agricultural land in the vicinity of the supply wells are frequently implemented to reduce the groundwater nitrate concentrations. Due to the intrinsically slow movement of groundwater both through the saturated and unsaturated zones, the influence of the BMP activity is often slow to arrive at the wellheads and during this time the concentration of nitrate in the drinking water supply remains high. As an approach to temporarily reduce the nitrate concentrations in the wells prior to the point when the BMPs become fully effective, enhancing the natural process of denitrification in situ offers promise. In this study, denitrification is stimulated through the injection of a carbon amendment (acetate) the production aquifer of a well field in southern Ontario where nitrate concentrations exceed the drinking water limit. The acetate is introduced to the subsurface through an injection-extraction well doublet within a zone of high nitrate mass flux where the groundwater flow field converges near the wellheads. The acetate amendment is pulsed on a regular basis over a period of 60 days and the groundwater geochemistry down gradient of the injection site is monitored in detail to track the fate of nitrate and by-products of the denitrification reaction. The data indicate that the upstream nitrate concentrations averaging 13 mg/l are reduced to below 2 mg/l as a result of the enhanced in situ denitrification. Concentrations of nitrite and other redox products remain very low. The extended field trial illustrates the potential value of enhanced denitrification as a temporary remedial option until the permanent influence of the regional BMPs become fully effective.