Process-Based Model to Describe Treatment of Nitrate-Rich Groundwater Using Emulsified Oil

Abstract

Permeable reactive barriers (PRBs) are being considered for treatment where the discharge of nitrate plumes contributes to eutrophication (e.g., Cape Cod, MA). PRBs enhance denitrification through the addition of carbon-based amendments such as the injection of emulsified vegetable oil (EVO). The use of EVO to stimulate denitrification foregrounds aspects of carbon utilization, dosing, longevity, and secondary effects in ways that differ from the application of EVO at hazardous waste sites. The overall objective of this study was to develop and evaluate a process-based modeling approach for simulating denitrification stimulated and supported by EVO. A series of one-dimensional column experiments assessed emulsion retention, production of soluble substrate, and utilization of carbon for nitrate reduction. Retention of 5.5 g dispersed phase emulsion resulted in sustained reduction of nitrate (∼43 mg/day) at ∼2 m/day porewater velocity. Biokinetic processes underlying the model are based on the two-step denitrification description of the Activated Sludge Model (ASM) No. 3. Biokinetic processes were integrated within the flow and transport simulator COMSOL to simulate the column experiment. The model capitalizes on the biokinetic parameters available in the ASM literature to limit the number of site-specific fits of model parameters. Simulation results demonstrate how this approach can result in reasonable predictions, although model performance was enhanced by fitting two parameters—yield coefficients for nitrate and nitrite. Comparisons with existing biokinetic transport models that were similarly fit to the column data suggest that the better overall descriptions of the column data using the process-based model stem from a more robust handling of spatial and temporal distribution of biomass. Sensitivity analyses highlight the importance of accurately describing the transformation of complex carbon into soluble substrate, and the subsequent utilization of that substrate. This research establishes a foundation for exploring implications of carbon processing on dosing, longevity, and effectiveness in denitrifying PRBs.