356-9 Drainage infrastructure as a pathway for delivering contaminants from onsite wastewater treatment systems to surface waters

Abstract:

Human wastewater contains a wide range of contaminants including fecal pathogens, nutrients, and emerging contaminants that pose risks to human and ecosystem health if released to surface waters. Onsite wastewater treatment systems (OWTSs) are widely used globally for the treatment and dispersal of domestic wastewater, serving approximately 14% of Canadian households and 20% of households in the United States. Although well-functioning OWTSs can provide substantial contaminant attenuation within and below the soil drainage bed and during transport through groundwater, poor design, installation, siting, or maintenance can reduce treatment effectiveness, potentially increasing contaminant inputs to groundwater and nearby surface waters. Preliminary evidence suggests that shallow drainage infrastructure may provide an important pathway for the rapid delivery of OWTS contaminants to surface waters. However, the prevalence and significance of this pathway remain unclear, including the physical and socioeconomic conditions under which it is most important. This study addressed this uncertainty by conducting widespread sampling at more than 100 residential, agricultural, and storm sewer drains discharging to streams across the Canadian Lake Erie Basin. Samples were analyzed for human-specific wastewater tracers including artificial sweeteners (acesulfame, saccharin, sucralose) and the microbial DNA marker HF183, as well as phosphorus, nitrate, E. coli, and chloride. Data revealed elevated wastewater contaminants in all types of drains, suggesting that each type may be important conduits for short-circuiting OWTS wastewater contaminants to streams. Statistical analyses were conducted to examine associations between the prevalence of drains with notable OWTS contaminant signatures and physical (e.g., soil permeability, topographic slope) and socioeconomic (e.g., household age and income) variables. The findings highlight the need to consider subsurface drainage infrastructure in designing, siting and reinspection programs for OWTSs, and in predicting inputs of OWTS wastewater contaminants to surface waters.

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