Canada's nitrogen legacy: Combining modeling & isotope approaches for drinking water quality and aquatic ecosystem health of rivers

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Nitrogen (N), an essential nutrient for plant growth, is added in excess of requirements to agricultural watersheds as fertilizer and manure to maximize crop yields and to dispose of animal wastes. Excess application of N over time is termed the N legacy. In many cities, wastewater treatment plants (WWTPs) discharge effluent with high loads of N directly into rivers. Population growth in Canada is increasing the pressure on freshwater systems for two essential but diametrically opposed services; the provision of drinking water and the assimilation of wastes. In streams and rivers in southern Canada, nitrate (NO3-) in rivers is increasing with time and in some areas approaches drinking water limits, especially in winter and during storm events. In many rivers, both ammonia (NH3) and NO3- are above guidelines designed to protect aquatic ecosystem health. The quantity, time trajectory for release and the ultimate fate of this N legacy are unknown. We will focus on the largest Canadian watershed feeding Lake Erie, the Grand River, where several agencies and regulators require information for science-based management. The watershed is home to 950,000 people, and the expected population increase is over 40% within 20 years. Currently, more than 500,000 people rely on the river for drinking water while there are 30 WWTPs along the river. Non-point source nutrient loads are also extremely high, as agriculture (including large livestock operations) is the dominant land use. To accommodate this new growth, billion dollar decisions for wastewater and drinking water treatment and agricultural land management have been taken. In 2012, two major wastewater treatment plants were modified to change the form of nitrogen (N) released to the river. As a consequence, river NO3- levels have risen to levels of concern for large cities that use the river for drinking water. This research project capitalizes on expertise in both biogeochemistry and landscape scale modelling that has been established very recently at the University of Waterloo. A comprehensive, watershed-scale N model will be developed that will be transferable to other agricultural watersheds in Canada. We will use year round field sampling, new natural isotopic techniques developed by our research group, historical reconstructions of N application and water quality in the watershed and new modelling approaches to examine the key N cycling processes and the consequences of N legacy and to separate agricultural from WWTP contributions to high levels of nitrate and ammonia in rivers so that management actions can be directed to where they will have the greatest impact.