Connecting nutrient sources to water quality condition in US rivers and streams
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Numerous studies have quantified the inputs of the nutrients nitrogen (N) and phosphorus (P) to landscapes at a variety of scales, and the connection to nutrient concentrations reflects the water quality response to these inputs. EPA recently assembled comprehensive spatial information from the National Nutrient Inventory about nutrient inputs to the contiguous US landscape, illustrating the magnitude of point and non-point nutrient pollution from anthropogenic sources including agricultural, residential, and atmospheric inputs. Agricultural inputs, consisting of livestock waste, synthetic fertilizers, and cultivated crop biological N fixation, account for 60-64% of N and 90% of P inputs. Outside of major agricultural and urban regions, atmospheric N deposition is the largest N input in sub-basins with mixed land use or natural land cover. Only a small number of sub-basins have urban associated inputs as their largest source of nutrients for both N and P, including several major cities. Probability-based surveys of river and stream water conducted by the states and EPA between 2000 and 2014 show that over 43% of the length of all flowing water exhibits poor condition for total nitrogen (TN) and 60% for total phosphorus (TP), based on ecoregional nutrient concentration benchmarks. Among the watersheds where streams are categorized as ‘poor condition’, agricultural non-point sources are the largest source in 67% and 66% of stream length for TN and TP, respectively. Fertilizer is the largest source for N across the watersheds of US streams, while livestock waste is the largest source of P across all US stream length. There are striking ecoregional patterns with western streams generally having low inputs and atmospheric sources, the northern and central plains streams dominated by fertilizer inputs, eastern streams having a variety of sources, and southern plains dominated by livestock N and P sources. Many streams in good condition had agricultural or human waste inputs as the leading source, providing examples to further study how inputs may be mediated by conservation or other factors in the watershed. Understanding the connection between nutrient source and condition is an important step in providing quantitative links between the nature of human-driven pollution and efforts to improve instream water quality.