Determining Pathways Via Which Septic System Wastewater Effluent Reaches Tributaries
Abstract
Septic systems have the potential to contribute various wastewater contaminants, including nutrients, to streams. Contaminant inputs to a stream vary depending on the specific pathway delivering septic wastewater effluent to the stream. The objectives of this study were to i) evaluate the relative importance of different pathways in delivering septic effluent to streams under varying hydrologic conditions, and ii) assess the utility of using multiple wastewater tracers and field sampling approaches to distinguish these contributing pathways. To address these objectives, routine stream sampling, high resolution longitudinal stream surveys and high frequency event-based sampling were conducted in four subwatersheds in the Ontario Lake Erie Basin. Stream samples were analyzed for artificial sweeteners, E. coli, human-specific bacterial DNA markers (HF183 and mitochondrial markers) and ammonium. The combined use of artificial sweeteners with the human-specific HF183 marker was found to be valuable for identifying contributing pathways. Data indicated that multiple pathways including groundwater transport, direct continuously flowing pipes and intermittent drains (e.g. field tile drains) all contribute septic effluent to the streams with the relative importance of these pathways varying between low and high flow conditions. While overland runoff may also deliver septic effluent to the stream, there was limited evidence of this pathway using the sampling approaches adopted.