Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Fred Longstaffe

Abstract

The Old Ausable River Channel (OARC) is located parallel to the shoreline of Lake Huron between the towns of Grand Bend and Port Franks, Ontario, Canada. Largely isolated from other water bodies, OARC water levels are mostly maintained by shallow groundwater. It is periodically in a state of eutrophication; a state commonly attributed to local anthropogenic nitrate loading. The primary goal of this work is to identify and quantify the sources of nitrate in the OARC watershed by measuring the oxygen and nitrogen isotope compositions of nitrate.

We have developed a dual method approach to measure the δ15N, δ18O and Δ17O of nitrate at low concentrations. This approach is necessary to fully correct for mass interferences, scale compression, and oxygen isotope exchange with water.

We measured the spatial and temporal variations of nitrate isotopic compositions and concentrations within, and contributing to, the OARC watershed waters and precipitation in a multi-year monthly sampling and analysis program. We use these data and Bayesian modeling techniques to quantify nitrate source contributions to the OARC and the adjacent ground waters. We show that the Northeastern portion of the river channel is contaminated by septic effluent. Nitrate produced in the atmosphere is the single largest source of nitrate in the central portion of the OARC; these results highlight the potential importance of regional transport of anthropogenic nitrate. The Southwestern portion of the OARC exhibits elevated atmospheric nitrate loading but receives a periodic influx from the Ausable River Cut water that is contaminated by agricultural run-off. These results can be used to guide watershed management, in particular, of wastewater effluent in Grand Bend and wastewater treatment in Pinery Provincial Park.

We compared nitrate isotopes in precipitation from Pinery Provincial Park and London, Ontario. Nitrate nitrogen isotope compositions at both sites overlapped known ranges for vehicular-emitted NOx. For London, oscillations in nitrate nitrogen isotope compositions are attributable to seasonal fluctuations in soil and anthropogenic emissions. Pinery Provincial Park precipitation exhibits irregular nitrogen isotope oscillations that may reflect its location distal from upwind anthropogenic emissions. The oxygen isotope compositions of nitrate in precipitation at Pinery Provincial Park and London, Ontario, exhibit oscillations that arise from seasonal variations in atmospheric oxidation pathways.

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