Electronic Thesis and Dissertation Repository


Master of Engineering Science


Civil and Environmental Engineering


Robinson, Clare


Septic systems are often cited as an important source of nutrient loading to surface waters; however, there is limited quantitative evidence supporting this. To address phosphorus (P) load reduction targets for Lake Erie that were set by the Canadian and United States governments, there is a need to quantify the contribution of septic systems to P tributary loads. Doing so is challenging due to the diffuse nature of this nonpoint source, the lack of septic system inventory across Ontario, and the complexity of P transport in the environment. In this study, the artificial sweetener acesulfame, a tracer for human wastewater, is first used to evaluate how much septic effluent is reaching tributaries in three unserviced subwatersheds. Based on the acesulfame stream loads, it is estimated that between 5 and24% of septic effluent generated in the subwatersheds reaches the tributary. Higher acesulfame stream loads during event flow conditions indicates effluent inputs from failing septic systems. Data indicate that septic systems may account for up to 16% of total subwatershed P loads but represent a small percentage (~2%) of nitrate (NO3-N) loads. These findings were then applied in the development of a geospatial model to estimate P loads from septic systems to tributaries in the Ontario Lake Erie Basin. The geospatial model has two components: (i) a module for automated spatial placement of septic systems, and (ii) a module for estimating P loads from septic systems to tributaries. Model results indicate that septic systems may account for only 1-5% of the total P load to Lake Erie from Canadian tributaries. However, considering the slow transport of P between septic systems and the tributary, transient model results suggest that septic systems could account for as much as 21% of the total tributary P loads in the future. The findings of this thesis provide broadly applicable knowledge and methodology needed to quantify the contribution of septic systems to tributary nutrient loads and can be used to inform the development of septic system regulations in the Ontario Lake Erie Basin.