Evaluation of Pathways Delivering Septic System Effluent to Rural Streams Using the Artificial Sweetener Acesulfame

Septic systems can be an important contributor of nutrients and other wastewater contaminants to surface waters in rural watersheds. R ecent studies have shown that the artificial sweetener acesulfame can be a useful tracer for quantifying upstream inputs of septic effluent to streams. However, there is a lack of detailed study on this application including its ability to identify septic effluent inputs to streams from failing septic systems. Septic systems can fail in various ways and understanding the types and rates of septic system failures is critical for predicting the pathways via which septic effluent is delivered to streams, and thus the nutrient loads. Here we conducted artificial sweetener and nutrient stream measurements in twelve subwatersheds in Southern Ontario, Canada with no other identified sources of acesulfame (e.g. no WWTPs, landfills) and with varying surficial geology, septic system density, watershed area, and mean septic system age. The fraction of septic effluent reaching the subwatershed outlets was found to vary between the watersheds with no clear relationships observed with the surficial geology, watershed area, or septic system density. Positive acesulfame concentration-stream discharge (C-Q) relationships were observed for five of the subwatersheds indicating that pathways such as surface runoff may deliver septic effluent from failing septic systems to streams during wet weather conditions (i.e. caused by effluent breakout to surface from septic drain field). Intra-event concentration-stream discharge relationships were examined in five subwatersheds where the changes (hysteresis) in the acesulfame concentrations over individual events were used to disentangle the pathways delivering septic effluent to the subwatersheds. These relationships show that different pathways may dominate for different watersheds and at different times of year. Finally, for all subwatersheds the estimated contribution of septic systems to measured total phosphorus stream loads on a given sampling day varied widely, whereas septic system contributions to N loads were low (<2%). The study findings provide guidance on the application of acesulfame as a tracer for quantifying septic effluent contributions of nutrients and possibly other wastewater contaminants to rural streams.