A RISK ASSESSMENT MODEL FOR ONSITE SYSTEMS APPLIED TO THE CITY OF OTTAWA, CANADA

Failing onsite wastewater systems can pose a significant risk to public health and to the environment. An easy to use risk assessment model using readily available information would be a useful tool for regulators. Such a model has been developed and successfully applied to 19 villages within the City of Ottawa. The model is being used to identify high risk areas in order to prioritize re-inspection programs and to develop a management strategy. The model will also be useful to planners in developing environmentally sustainable growth plans for village areas. The model is comprised of a series of weighted risk factors applied to lot parcels in a GIS database. The factors were developed using existing data readily available to the City (soils, floodplain, parcel and building mapping, census data, aquifer vulnerability study, local hydro geological knowledge). The factors attempt to account for contaminant loading, contaminant pathways and operational life of onsite systems. Data from system inspection reports, replacement permits, local knowledge, groundwater quality and surface water quality can be used for model validation. Total risk was found to correlate well with the number of high risk factors per village identified by the local regulatory authority (Correlation Coefficient = 0.67). An analysis of recent system replacement permits in the study area indicates that 70% of failed conventional leaching bed systems were greater than 25 years old while 65% were in impermeable soils, suggesting a good correlation with the risk model factors for system age and soil type. An analysis of well water data over a 3 year period indicates a low occurrence of E.coli contamination (2%) and does not show any significant correlation with risk model factors.