Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare E.

2nd Supervisor

Roy, James W.

Affiliation

Environment and Climate Change Canada

Co-Supervisor

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.

Summary for Lay Audience

Septic systems are widely used to treat household wastewater in areas that do not have access to a centralized wastewater treatment plant. There is potential for septic systems, particularly poorly performing or improperly installed septic systems, to contribute contaminants of concern to nearby streams and degrade water quality. Contaminants that may be input into streams from septic systems include nutrients (phosphorus and nitrogen), chemicals (pharmaceuticals, artificial sweeteners), and fecal contaminants (E. coli, bacteria, viruses). These contaminants can reach streams through different pathways including slow moving groundwater flow, or rapid pathways such as overland flow, direct pipes, or intermittently flowing drainage systems (e.g., agricultural tile drains). The relative importance of these different pathways in delivering septic system wastewater effluent to streams is currently unclear but is important to understand because more contaminants are delivered to streams via rapid pathways compared to groundwater transport.

This study aimed to address this knowledge gap by conducting detailed stream sampling in four streams in the Ontario Lake Erie Basin, Canada, under dry- and wet-weather conditions. Stream samples were analyzed for multiple microbiological and chemical constituents found in human wastewater including artificial sweeteners, E. coli and human-specific DNA bacterial markers. Results showed that during dry periods septic wastewater effluent was delivered to streams via continuously flowing direct pipes in addition to slow moving groundwater flow. Further, data indicated that additional pathways, in particular drains that flow only when the groundwater table is high and soil is wet (e.g., agricultural tile drains or residential French drains), likely deliver septic effluent to the streams during wet weather conditions. The results from this study provide important information needed to improve the current understanding of the impact of septic systems on stream water quality as well as information needed to improve management and policy on septic system installations, design, and maintenance.

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Available for download on Tuesday, December 31, 2024

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