Vadose Zone Modeling Applied to Stormwater Infiltration

Stormwater managers use infiltration to promote groundwater recharge in urban areas and to reduce the volume and flow rate of stormwater runoff created by post development conditions. However, the potential for groundwater contamination exists. Studies have shown that some areas are more prone to groundwater contamination than others. As a result, methods for easily predicting contamination potential beneath infiltration devices are being developed. One such method involves the use of computer programs, such as SESOIL — a one-dimensional vadose zone model. SESOIL was selected because of the multitude of processes available in the program for predicting pollutant removal. SESOIL was first evaluated using a full factorial design, considering six potential factors that were thought to influence the predictions (rainfall, concentration, vadose zone thickness, intrinsic permeability, soil pH, and soil organic matter. The results showed that, at the concentrations typical of urban stormwater runoff, only rainfall, concentration and intrinsic permeability were important. Difficulties of application were encountered due to a lack of available input values and data sets. Therefore, laboratory research on partitioning factors between the water and the soil were undertaken. The results showed that the partitioning coefficients calculated from the removal of pollutants from a multicomponent mix at typical stormwater concentrations were similar to the low range found in the literature. However, the literature did not have values for sparingly-sorbable compounds such as the nutrients. This research is using a vadose zone model, such as SESOIL which incorporates these numerous chemical reactions in the calculations, as a design tool for infiltration devices with both engineered and natural soils. This research will support the development of guidance for protecting groundwater from unintentional contamination by stormwater runoff passing through an infiltration structure.