Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Engineering Science


Civil and Environmental Engineering


Robinson, Clare

2nd Supervisor

Roy, James


Environment and Climate Change Canada



There is concern that septic systems may contribute nutrients, fecal pathogens, and emerging contaminants to tributaries, and thereby impair surface water quality. The objective of this thesis was to quantify the percentage of septic effluent reaching multiple streams and to evaluate whether this percentage varies based on the stream flow conditions and the physical and socioeconomic characteristics of a subwatershed. This was addressed by broad-scale sampling in 46 subwatersheds in the Lake Erie and Lake Simcoe Basins, Ontario, with data analyzed using statistical models. It was found that the percentage of septic effluent reaching subwatershed outlets, based on acesulfame stream loads, was higher under high flow conditions and in subwatersheds with older occupied homes and lower topographic wetness index. Fecal contamination in streams, possibly associated with underperforming septic systems, was observed in smaller subwatersheds with high septic system density, small setback distances, and high topographic wetness index. The findings of this research are needed to refine estimates on the contribution of septic systems to stream contaminant loads and to inform programs for locating, constructing, and maintaining septic systems.

Summary for Lay Audience

Septic systems are widely used to treat household wastewater in rural areas not serviced by municipal wastewater treatment plants. Septic systems work by partially treating the household wastewater before releasing it gradually into the soil. Household wastewater contains high concentrations of contaminants that can degrade surface water quality, including nutrients (nitrogen and phosphorus), bacteria, viruses, pharmaceuticals, and other contaminants of concern. A well-functioning septic system can limit the amount of these contaminants entering groundwater and surface waters, thereby limiting their adverse impacts on the environment. However, septic systems do not always perform as designed and can release excessive amounts of contaminants to the environment, including to groundwater and surface waters. Currently, it is not clear the amount of contaminants that are reaching streams from septic systems and how this may vary between geographical areas with different physical and socioeconomic characteristics (e.g., household age, soil permeability, and household income).

This study collected water samples from streams across the Lake Erie and Lake Simcoe Basins in Ontario, Canada. The samples were analyzed for multiple tracers for human wastewater, including artificial sweeteners and human-specific DNA markers. The data were then analyzed to determine how much wastewater from septic systems was reaching the stream sampling locations. Relationships were explored between the different subwatershed characteristics and the amount of septic system wastewater reaching the streams. Using statistical models, it was found that when the flows in streams were high (from rainfall or snowmelt), in areas with older occupied homes, and in higher sloped areas, there was a greater amount of septic system wastewater reaching the streams. In addition, it was found that in smaller subwatersheds, with smaller distances between the septic system and the stream, and low sloping terrain, there was more fecal contamination in the streams, which may be associated with a higher prevalence of failing septic systems. The results of this work help understand areas where more septic system wastewater is likely to reach streams, and therefore can aid in prioritizing septic system reinspection and education programs.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.