Onsite wastewater treatment systems (OWTSs) are widely used globally for wastewater treatment and dispersal, including by approximately 14% of Canadian households. Many wastewater contaminants can impair water quality, including fecal contaminants, nutrients, and emerging contaminants (e.g., pharmaceuticals, microplastics, per- and polyfluoroalkylated substances [PFAS]). While well-functioning OWTSs can provide high contaminant attenuation, many OWTSs may be under-performing, leading to higher contaminant inputs to groundwater and surface waters. In some cases, particularly when OWTSs are located in areas with a high water table, wastewater-impacted groundwater plumes may be short-circuited to nearby surface waters by shallow drainage infrastructure. OWTSs may also be directly connected (illegally) to drainage infrastructure. The objective of this study is to investigate the importance of shallow drainage infrastructure in rural areas in facilitating the rapid delivery of OWTS contaminants to streams. This objective was addressed by sampling and measuring flows in 105 drains including residential, agricultural, storm sewer and relic (old or of unknown use) discharging to streams across the Canadian Lake Erie Basin. Samples were analyzed for wastewater constituents including soluble reactive phosphorus, nitrate, E. coli and chloride, and human-specific wastewater tracers including artificial sweeteners and the microbial DNA marker HF183. Data revealed elevated concentrations and loads of human wastewater tracers across all drain types, suggesting that all types may act as important pathways for delivering wastewater contaminants to streams. The study findings highlight the need to consider subsurface drainage infrastructure in designing, siting and reinspecting OWTSs, and in predicting inputs of OWTS-derived wastewater contaminants to surface waters.