A GIS-Based Model to Assess the Risk of On-Site Wastewater Systems Impacting Groundwater and Surface Water Resources

On-site systems are successfully used across Ontario and Canada in rural areas to treat and disperse wastewater in areas without access to centralized sewer facilities. Ontario has detailed technical guidelines for the design and installation of on-site systems to help safeguard against contamination to public health and the environment. However, since on-site systems are managed by private users, there are significant risks to either surface and groundwater resources when the systems malfunction or are improperly operated and maintained. This research assesses and models these risks on a regional basis utilizing a GIS-based assessment tool. The developed risk assessment model uses nine pertinent parameters to account for contaminant loading and pathways, and system characteristics. Risk parameters included soil type, slope, lot size, surface water proximity, floodplain, groundwater intrinsic susceptibility, recharge areas and water supply proximity. When applied to Huron-Kinloss Township in Ontario, at-risk areas were successfully determined by the model and then confirmed and validated by local experts. Soil type, groundwater intrinsic susceptibility and system age were the greatest contributors to the overall risk for this area. The GIS-based model is useful for decision-makers in identifying at-risk areas for targeted management strategies such as prioritized reinspection programs.