Validation of an Onsite Wastewater Risk Assessment Model
Draft Final Report
Canadian Mortgage and Housing Corporation
Ontario Rural Wastewater Centre
Collège d’Alfred - University of Guelph
Authors: Chris Kinsley & Doug Joy
July 19th, 2005
Table of Contents
Appendix B – Water Softener Field Evaluation Study Data
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. 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 a municipality (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 was collected from two field inspection campaigns and from replacement system records of the City of Ottawa to validate model parameters. The field inspection campaigns found no correlation between ground and surface water quality and indications of system malfunction; however, both system age and clay soils were found to be significant indicators of system failure. An analysis of onsite system replacement records indicates that system age is the determinant factor in hydraulic system failure, with the relative risk increasing by a factor of 5 for systems of 10-29 years and by a factor of 12 for systems 30 years and older. Soils play an important, yet comparatively smaller role, with risk of failure generally increasing by a factor of 2 for systems installed in areas of impermeable soil.
The Risk Assessment Model was simplified and transformed to reflect our better understanding of the impact of system age and soil type on system failure rate. The revised Risk Model includes six factors: System Age, Soil, Lot Size, Depth to High Groundwater Table, Aquifer Conductivity, and Proximity to Surface Water.
We would like to acknowledge the significant help of the Ottawa Septic System Office in providing access to their database and in particular the time and input of Terry Davidson, P.Eng., Manager of Septic System Approvals Office and Rosalind Kee. We would also like to thank Sarah Willie (Ottawa Septic System Office and Mississippi Valley Conservation Authority) for incorporating our research needs into her re-inspection program and for collecting water quality samples for us. Finally we would like to thank René Goulet of René Goulet Septic Tank Pumping for conducting a series of field inspections of onsite systems on our behalf.
We gratefully acknowledge the financial support of the Canadian Mortgage and Housing Corporation, the City of Ottawa and the Eastern Ontario Water Resources Committee in funding this research project.
There is a growing acceptance that onsite wastewater systems form an integral and permanent part of the wastewater management infrastructure in rural areas. Traditionally, the maintenance of onsite systems (septic systems) was the responsibility of the home owner. With increasing development of non-centrally serviced areas and enhanced public concern for environmental protection and human health, local governments are taking a more active role in the management of onsite systems. A risk assessment model can provide a useful tool to help local governments develop rational management plans for decentralized systems. There are three types of risk which may be prioritized using models and addressed in a management plan: public health risk, ecological risk and financial risk (Jones et al., 2000).
Public health risk is a driving force behind most onsite system regulations. Contamination of drinking water by pathogens and nitrate are two major public health issues commonly related to onsite systems. Traditionally, prescriptive regulations attempt to assure sufficient depths of unsaturated soil and adequate horizontal separation distances between an onsite system and water supply wells or water bodies to protect public health from pathogen contamination. In Ontario, subdivision plans must ensure adequate dilution of nitrate through infiltration of precipitation and appropriate lot sizes. These regulations, coupled with system inspections at time of construction, attempt to minimize public health risks from onsite system effluent.
Ecological risk is a macro-level risk that considers the health of a watershed or an eco-system. This is often related to nutrient loading of surface water bodies and to cultural eutrophication. An evaluation of ecological risk must consider all sources of contamination including agricultural runoff, sewage plant discharges, industrial and storm water outfalls, and natural sources in addition to on-site systems.
The financial risks of onsite systems can be evaluated at both the community and the individual property scales. At the community scale, public health crises arising from contaminated communal water supplies or risks to tourism, fishing industries and to recreational water use from surface water contamination are all recent Canadian examples. At the scale of the individual property owner, system failure and its replacement cost, reduction in property value and alternative costs for a centralized system all represent significant financial risks for the individual owner.
Risk assessment methodologies have been developed to address one or more of these types of risk as they relate to onsite systems. The Onsite Wastewater Treatment Manual (USEPA, 2002) describes several model approaches including: a subjective vulnerability assessment, a probability analysis of water resources impact from wastewater discharges, contaminant transport modeling, and the DRASTIC model (Aller et al., 1987), which was developed by the USEPA to rate groundwater vulnerability using weighted factors of hydrogeologic settings. The factors included in the DRASTIC model are: depth to ground water, net recharge, aquifer media, soil media, topography, impact of the vadose zone media, and hydraulic conductivity of the aquifer. The Risk Assessment System Handbook (Government of New South Wales, 2001) provides a comprehensive approach to risk assessment from onsite systems up to the watershed scale. Contaminant fate from onsite systems is well described in the Guidelines for Assessing the Risk to Groundwater from On-Site Sanitation (British Geological Survey, 2001).
All of the models listed above provide information to decision makers, enabling them to relate a risk of surface or groundwater pollution to mitigating actions such as mandating a higher level of technology or conducting more frequent inspections in high risk areas. Examples of high risk areas could include aquifer recharge zones, high density developments, village cores, waterfront areas, or areas with poor soils. A risk assessment model can be a useful management tool for regulatory authorities.
A risk assessment model for onsite systems was developed utilizing readily available sources of data to create a useful management and planning tool for use by regulatory authorities. The model was applied to 19 villages within the City of Ottawa, Ontario, Canada, with the purpose of prioritizing an inspection program of existing systems.