Modeling of Water Quality Downgradient of Mulch Biowall with Nearby Surface Water Receptor

Groundwater treatment biowalls are commonly placed at a property boundary to prevent contaminant transport off-site or adjacent to a surface water body to protect from contaminant discharges to the surface water. Groundwater contaminants passing through the biowall are treated within the biowall or immediately downgradient of the biowall. A biowall typically contains bark mulch or other agricultural waste material that serves as an electron donor to the degradation process. One of the results of biowall treatment activity is the generation of anaerobic groundwater, which may result in increased sulfide levels if influent groundwater contains moderate to high sulfate concentrations. Sulfide represents a degraded water quality of the groundwater exiting the biowall, which is of significant concern if the biowall is located in close proximity to a groundwater discharge to surface water. A time-dependent one-dimensional fate and transport model was developed to evaluate concentrations of sulfide that may be generated in such a situation. The model was developed by applying mass governing equations over a control volume of the aquifer between a biowall and a surface water body receptor. COMSOL Multiphysics was used to solve the family of equations for a timescale extending to approximately 4 years, using appropriate parameters and initial / boundary condition values. The mass species glucose (surrogate for carbon source upon degradation of solid carbonaceous material), lactic acid (surrogate for fatty acids), and electron donor sulfate were modeled with the partial differential equations. The model results provide insight into the advance of sulfide levels in groundwater downgradient of a biowall, and in close proximity of a surface water receptor. The carbon source, sulfate and sulfide data is compared to actual site data obtained from literature or other sources.