Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare

2nd Supervisor

Roy, James

Affiliation

Environment and Climate Change Canada

Co-Supervisor

Abstract

There is considerable uncertainty regarding the relative importance of septic systems to phosphorus (P) loading to surface waters. Quantifying P loading from septic systems to surface waters is challenging because i) septic systems are highly distributed across landscapes, ii) there is often no formal inventory of the numbers and locations of septic systems across large geographic scales (e.g., > 1000 km2), and iii) it is difficult to predict the amount of P released from septic systems that actually reaches surface waters. The overall objective of this study was to improve estimates of P loads from septic systems to tributaries in the Lake Erie and Lake Simcoe Basins, Ontario, by developing, testing, and applying an improved geospatial tool. The first step of the geospatial tool is using a remote sensing and machine learning approach to locate individual septic systems across the basins. Septic-derived P loads to tributaries are then determined by combining septic system locations with a statistical model that predicts the percentage of septic effluent that reaches tributaries based on subwatershed characteristics and stream flow conditions. Application of the tool indicates that there are 129,402 and 26,572 septic systems in the Ontario Lake Erie and Lake Simcoe Basins, respectively. The calculated septic-derived P loads indicate that only between 0.09% (low flow) to 1.2% (high flow) of the total historical P load to Lake Erie from Ontario tributaries comes from septic systems. The contribution of septic systems to total historical P loads to Lake Simcoe is estimated to be higher, ranging from 0.4% (low flow) to 4.9% (high flow). Finally, an online tool was developed for practitioners to visualize the findings of the geospatial tool across the two basins. The result of this study can be used to improve septic-derived P load estimates to tributaries as needed to inform septic systems and watershed management programs.

Summary for Lay Audience

Septic systems are commonly used to treat household wastewater in areas that are not connected to wastewater treatment plants. Septic systems release partially treated wastewater into the environment, which can lead to high phosphorus (P) inputs to surface waters (lakes, rivers, and streams). High P inputs to fresh surface waters are a concern because they can lead to harmful algal blooms and other adverse impacts, including fish kills. The relative importance of septic systems to P loads to stream compared to other sources, such as agricultural and urban runoff, is poorly understood. It is challenging to determine how much P reaches tributaries from septic systems because i) septic systems are spread out across rural areas, ii) often there is no formal database containing information on septic system locations in an area, and iii) it is difficult to estimate the amount of P that actually reaches a stream from an individual septic system due to the complex movement of P in the environment.

The study first develops and applies a new geospatial tool that identifies the location of individual septic systems across the Ontario Lake Erie and Lake Simcoe Basins. The amount of P that reaches streams from septic systems is then calculated by combining the septic system locations with an equation that calculates how much septic system effluent reaches streams based on stream flow conditions and the physical and demographic characteristics in an area. Application of the geospatial tool indicates that there are 129,402 and 26,572 septic systems in the Ontario Lake Erie and Lake Simcoe Basins, respectively. As expected, the spatial density of septic systems varies widely across the basins. The calculations indicate that less than 1.2% of the total historical P loads to Lake Erie from Ontario tributaries come from septic systems. The contribution of septic systems to total historical P loads to Lake Simcoe is higher and estimated to account for up to 4.9%. While the overall contribution of the septic system to total P loads to the lakes is relatively low, calculations indicate that there is high variability between the various watersheds that drain into the lakes. Finally, an online tool was developed for practitioners to visualize P loads that may enter tributaries from septic systems across the two basins. The results of this study are needed to inform watershed P management planning as well as to inform programs and policies focused on the design, installation, and maintenance of septic systems.

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