Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Engineering Science


Civil and Environmental Engineering

Collaborative Specialization

Environment and Sustainability


Robinson, Clare

2nd Supervisor

Roy, Jim


Environment and Climate Change Canada



Freshwater systems worldwide are threatened by excessive nutrient (nitrogen and phosphorus) loading. This study evaluated the contribution of septic systems to stream nutrient loads in nine subwatersheds. Stream sampling was conducted during low and high discharge conditions, with samples analyzed for total phosphorus (TP), soluble reactive phosphorus (SRP), nitrate (NO3-N), and acesulfame (ACE; wastewater tracer). Higher septic effluent reached the subwatershed outlets during high discharge conditions. Subwatersheds with newer households had a lower percentage of septic effluent reaching the stream compared with subwatersheds with older households. Seasonal and event-based ACE concentration-discharge relationships revealed that the hydrological pathways delivering septic effluent to the outlets are different between the subwatersheds. Finally, the contribution of septic systems to TP, SRP, and NO3-N stream loads were greater during low discharge conditions, with the percentage contributions greater in more urban subwatersheds with higher septic density. The study provides new information needed to guide septic system management strategies.

Summary for Lay Audience

Human activities have greatly impaired water quality. High inputs of nutrients (nitrogen and phosphorus) to surface waters, such as lakes, streams and rivers, can lead to rapid growth of algae in surface waters. Large algal blooms have many negative impacts on the environment, economy, and human health. To reduce excessive amount of nutrients entering surface waters, all human activities contributing nutrients to surface waters need to be identified and assessed. Septic systems, which are on-site domestic wastewater treatment units, are a potentially important, yet poorly understood, source of nutrients to surface waters. Septic systems work by treating domestic wastewater that enters the septic tank. Septic tanks partially treat the wastewater (the “cleaned” water is referred to as septic effluent) which is then slowly released from septic tanks through perforated pipes into the soil. Properly functioning septic systems can effectively remove nutrients and other pollutants from the wastewater limiting their release to surface waters. However, septic systems failure can occur and this can release high levels of nutrients into the soil, groundwater and surface water.

This study collected and analyzed field data to determine the amount of septic effluent that reaches streams in nine subwatersheds in Southern Ontario, Canada, as well as the ways by which the septic effluent reaches the streams. Furthermore, this study examines the contribution of septic systems to the total amount of nutrients in the stream. The main findings of the study were: (i) the amount of septic effluent that enters streams is higher in areas with older households and septic systems compared to areas with newer households and septic systems; (ii) more septic effluent is delivered to streams during wet weather (i.e. snow melt and rainfall) conditions when the stream flow is high; (iii) the contribution of septic systems to the amount of nutrients in streams is higher in more urbanized areas. The results of this study provide key information needed for development and implementation of effective septic systems management policies, thus protecting and restoring the quality of surface waters.

Available for download on Friday, December 31, 2021