Phosphorus from human waste did prove significant. Counties encompassing Chicago and other major metropolitan areas "showed up as hot spots," David says, because most municipalities don't remove phosphorus from the otherwise clean sewage effluent they discharge into streams.
The team further found that about half of the variation in phosphorus loadings was not explained by their models, suggesting that other factors also contribute, such as stream bank erosion and phosphorus deposits in river sediments.
Overall, the findings suggest that reducing phosphorus pollution will require broad adoption of practices that limit nutrient runoff, such as prudent manure and fertilizer applications — including coordination with weather events — cover crops, buffer strips, and incorporation of fertilizers. It will also require limits on phosphorus discharge from cities.
"To me the value of the study is that it helps shift the debate," David says. "The problem is not as simple as two things. It's not as simple as too much fertilizer or manure."