Quantification of septic system contribution to nutrient loads in surface waters
Abstract
Freshwater systems worldwide are threatened by excessive nutrient (nitrogen and phosphorus) loading. This study evaluated the contribution of septic systems to stream nutrient loads in nine subwatersheds. Stream sampling was conducted during low and high discharge conditions, with samples analyzed for total phosphorus (TP), soluble reactive phosphorus (SRP), nitrate (NO3-N), and acesulfame (ACE; wastewater tracer). Higher septic effluent reached the subwatershed outlets during high discharge conditions. Subwatersheds with newer households had a lower percentage of septic effluent reaching the stream compared with subwatersheds with older households. Seasonal and event-based ACE concentration-discharge relationships revealed that the hydrological pathways delivering septic effluent to the outlets are different between the subwatersheds. Finally, the contribution of septic systems to TP, SRP, and NO3-N stream loads were greater during low discharge conditions, with the percentage contributions greater in more urban subwatersheds with higher septic density. The study provides new information needed to guide septic system management strategies.