The Influence of pH on Subsurface Denitrification Stimulated with Emulsified Vegetable Oil

Abstract

Treatment of nitrate rich groundwater using permeable reactive barriers (PRBs) established with injection of emulsified vegetable oil is receiving attention in areas where groundwater discharges contribute to eutrophication (e.g., Cape Cod, MA). To better understand the biogeochemical process kinetics when emulsified vegetable oil (EVO) is used to stimulate denitrification within the subsurface, microcosm experiments and process-based modeling were conducted for pH conditions ranging from 4 to 8. Biomass variability in soil and pH variations were found to affect denitrification, with limited nitrate reduction observed below pH 5.0. Different rates for denitrification associated with biomass variability suggest that a greater characterization of the indigenous biological community may improve PRB design and operation. Process-based modeling employed the activated sludge model No 3 (AMS3) framework that assumes denitrification as a two-step anoxic process dependent primarily on heterotrophic bacteria, soluble substrate, nitrate, and nitrite. Experimental data were used to calibrate the model under neutral to low pH, resulting in a robust set of equations that can be coupled with transport in future research to improve PRB effectiveness.