How does predicted total nitrogen concentrations in streams compare to U.S. Environmental Protection Agency (USEPA) recommended nutrient criteria?
A regression model was developed on the basis of NAWQA data to estimate the probability of exceeding (or being higher than) a specific total nitrogen concentration for any given stream (Spahr and others, 2010; http://pubs.usgs.gov/sir/2009/5199/ ). Model results show that streams in which total nitrogen concentrations have a high probability (75 percent or greater) of exceeding the USEPA recommended criteria are broadly distributed across the Nation, and represent 83 percent of all stream miles. This proportion is similar to what would be expected considering how the criteria were designed (U.S. Environmental Protection Agency, 2001, fig. 5; http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/rivers_1.pdf).
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