Extended pilot test of a cross-injection in situ denitrification system for pre-emptive treatment of municipal well water

Abstract

Elevated groundwater nitrate concentrations have been linked to deleterious health and environmental effects. A significant source of the nitrate is nitrogen fertilizers applied to agricultural landscapes. Beneficial Management Practices (BMPs), including the optimization of fertilizer use and selective crop rotations, have proven to be effective in some cases. The city of Woodstock in southern Ontario relies on public wells for all of its municipal supply. Several of the wells have experienced chronic increases in nitrate concentrations exceeding the maximum allowable limit of 10 mg/L N-NO3−. While BMPs are established, an interim reduction plan based on enhanced in situ denitrification (Cross Injection System, CIS) in a 15 m thick zone of high nitrate mass flux within the aquifer zone was evaluated. Based in the results of preliminary acetate injection experiments, a C:N ratio of 2.35, (approximately 260 mg acetate/L), was selected to optimize the denitrification reaction. Injections were performed for six hours a day every day for a period of approximately two months. Dissolved oxygen (DO) and nitrate concentrations recorded over time indicated that reduction of both commenced within a few days of the beginning of the acetate injections and reduced levels were maintained for the remainder of the two-month injection period. Denitrification occurred throughout the profile although nitrate reduction was the highest in the lower groundwater velocity zones. An overall reduction of nitrate of 50% was achieved through the treated section of the aquifer. It is estimated that an upscaled treatment system utilizing a treatment width of only 70 m would be sufficient to reduce the nitrate concentrations to below the drinking water limit demonstrating the potential for the CIS method to functions as an interim groundwater nitrate reduction strategy.