Investigating the use of the artificial sweetener acesulfame to evaluate septic system inputs and their nutrient loads to streams at the watershed scale

Septic systems are often cited as an important contributor to rural watershed nutrient loads despite limited quantitative evidence regarding their actual contribution. The measurement of the artificial sweetener acesulfame in streams to quantify upstream inputs of septic system effluent has recently been proposed. However, there is a lack of detailed study on this application, including temporal variability, influence of dry and wet weather conditions, and disentangling the contributions from functioning versus failing septic systems. Further, this approach has not been applied to assess the relative contribution of septic systems to nutrient watershed loads. Here we conducted artificial sweetener and nutrient stream measurements in three watersheds with no other identified sources of acesulfame (e.g., no municipal wastewater treatment plants, landfills) over a 10-month period. Measured acesulfame stream concentrations varied from below detection (<2 ng/L) to 87 ng/L with acesulfame stream loads revealing that septic system inputs to streams vary considerably between low and high stream discharge conditions. Positive acesulfame concentration-stream discharge relationships for all three watersheds reveal the potentially important contributions from failing septic systems and surface flow pathways (i.e., effluent breakout from septic drain field) under high discharge conditions. Further they indicate that, contrary to conventional thinking, sampling under low discharge conditions may underestimate septic effluent delivery to streams. The estimated contribution of septic systems to measured total phosphorus stream loads on a given sampling day varied widely (<1–36%, considering attenuation between the septic tank and stream), but typically septic system contributions to nitrate stream loads were low (<2%). The findings provide guidance on the application of this new tool for managing septic effluent contributions of nutrients and other wastewater contaminants to rural watershed streams.