Crossing the Border with SENSEmap: Estimating Point and Nonpoint Nutrient Inputs Across Ontario and Quebec

Abstract:

While nitrogen and phosphorus are essential nutrients for life, anthropogenic inputs can push excessive loads into aquatic ecosystems. These loads fuel algal blooms and hypoxia, which can wreak havoc on these ecosystems, impact human health, and cause economic toil. In order to manage nutrient loading, nutrient inputs to the landscape must be understood. Unfortunately, nutrient inputs delivered from nonpoint sources, though often greater in collective mass than point sources, are not documented as thoroughly or precisely as point sources. By identifying and quantifying nonpoint nutrient inputs, we can better understand the source of aquatic nutrient loads and more effectively manage watersheds. Here, we present methods for quantifying point and nonpoint nutrient inputs at 30 meter resolution for seven sources in Ontario and Quebec, including septic systems, manure, and chemical fertilizers, as part of a previously US-based nutrient input mapping project, the Spatially Explicit Nutrient Source Estimate map (SENSEmap). For example, septic systems, a nonpoint source commonly found in rural areas, discharge nutrients directly to groundwater but are rarely mapped or monitored. We estimated locations and nutrient inputs from septic tanks using available data, such as location and effluent flow data for WWTPs, population census data, and maps of roads and water bodies. SENSEmap is complete for the United States Great Lakes Basin (US-GLB), but due to differences in available data, unique methods were developed and updated for Canada. One critical update to how we estimate agricultural sources includes delineating field boundaries to more realistically apply chemical fertilizers and manure as compared to our pixel-based approach in the US-GLB. With SENSEmap completed across the transnational boundaries of the entirety of the Great Lakes Basin, watershed managers and regulators will have access to a seamless, consistent, high resolution, source specific view of the nutrient landscape, allowing improved management of these critical ecosystems. Too often nutrient management efforts do not extend beyond national boundaries, a critical shortcoming that SENSEmap hopes to help managers in both nations overcome.