Geospatial tool to identify locations of septic systems within the Canadian Lake Erie and Lake Simcoe Basin

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.