Broad-Scale Analysis of Factors Influencing Inputs of Domestic Wastewater Constituents from Onsite Wastewater Treatment Systems to Streams

This study provides new field-based evidence of the physical and socioeconomic watershed factors and streamflow conditions that influence effluent inputs to streams from onsite wastewater treatment systems (OWTSs), including potential differences between inputs via slow (groundwater) and more rapid (subsurface preferential, overland, direct pipe) transport pathways. Stream sampling data were compiled for 46 watersheds in Ontario, Canada, with analyses including a conservative chemical tracer (acesulfame) representing all (slow and rapid) pathways and a nonconservative human fecal bacteria tracer (HF183) representing only rapid pathways. Acesulfame stream concentrations ranged from tens to over 1000 ng/L, indicating OWTS effluent inputs to streams are widespread. Additionally, HF183 was detected in >20% of stream samples, indicating the prevalence of rapid pathways. Linear mixed-effects models indicate that the percentage of OWTS effluent reaching streams, based on acesulfame data, was higher under high flow conditions and in watersheds with older houses, more houses within 200 m of a stream, and a lower topographic wetness index. Higher human fecal contamination, based on HF183 detections, was observed in streams that drain watersheds with high OWTS density, more houses within 200 m of a stream, and a higher topographic wetness index. These findings support improved pollutant load predictions and better targeting of watersheds for OWTS management.