Dissolved Organic Matter in the Anthropogenically Impacted Grand River and Natural Burnt River Watersheds
Hutchins, Ryan H. S.
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Dissolved organic carbon (DOM) is one of the largest cycled organic carbon pools on Earth and an important biogeochemical factor in aquatic systems. DOM can act as an energy source for microorganisms, alter the depth of the photic zone for photosynthesis, absorb harmful ultraviolet radiation, as well as alter the transport and toxicity of contaminants. The purpose of this research project was to characterize DOM in the Grand River watershed in Ontario, Canada using a wide range of qualitative and quantitative techniques and determine the impact of anthropogenic activities as well as seasonal and longitudinal changes on DOM processes. To reach the study objectives, historical data was analyzed to determine the seasonal cycle in the Grand River watershed. Intensive longitudinal sampling surveys were undertaken to evaluate the DOM characteristics and processes in the Grand River. Surveys of the less impacted Burnt River watershed were used as a comparison watershed to the Grand River to evaluate allochthonous and autochthonous indicators of DOM source and human impacts on DOM processes. Drinking water surveillance data was used to evaluate the effect of DOM in the Grand River on formation of disinfection by-products (DBPs). Different trends were seen in the Grand River in terms of longitudinal area and season. The headwaters of the river showed more autochthonous DOM in the spring and winter compared to the fall and summer. The lower-central river peaked in autochthonous DOM in the summer and was more allochthonous in the winter. DOM generally became more autochthonous downstream in the Grand River and was most autochthonous below the large sewage treatment plants (STPs) in the central portion. Protein content, measured as protein-like fluorescence normalized to DOC concentration, was strongly related to δ15N of DON; both are associated with autochthonous DOM in the Grand River and show the effects of the major STPs. The increase in autochthonous DOM below the STPs is likely associated with nutrient enrichment stimulating primary production and macrophyte growth. Based on the comparison of the Burnt River with the more impacted Grand River, the effect of lakes and photodegradation can make discrimination of autochthonous and allochthonous DOM more difficult. The ratio of DOC/DON and protein-like fluorescence proved to be robust indicators despite photodegradation. Human impacts on the Grand River watershed result in a greater seasonal cycle, high primary production in the summer and a downstream trend of increasing autochthonous DOM compared to the Burnt River. Based on drinking water surveillance data and literature review, autochthonous DOM caused greater DBPs in the drinking waters fed by the Grand River. This is currently a threat to human health and DBPs in sewage treatment plant effluent may be a threat to ecosystem health.