Continuing research on the amount of nitrate washing down the Mississippi River south of the Minnesota border shows a very significant increase in the yearly loads of the pollutant from 1980 through 2010.
But the latest U.S. Geological Survey analysis of the three decades of water sampling results now shows a reduction in the average annual increase, compared to a similar calculation two years ago.
The analysis relies on modeling that evens out the high nitrate flows in very wet years and the much lower flows in dry years.
Clinton, Iowa, about 160 miles south of the Minnesota border, was the farthest-upstream point where water sampling for the analysis was conducted. The latest modeling estimates that the average nitrate loads at Clinton increased 55 percent – or an average of 1.8 percent per year — between 1980 and 2010.
The earlier modeling, which covered 1980 through 2008, estimated there had been a 67 percent overall increase in the nitrate load, or 2.3 percent per year.
Lori A. Sprague, one of the USGS researchers who did the modeling, said in an interview that the research did not attempt to determine the sources of the big overall nitrate increases over the 30-year period. Nor, she said, could the research provide a reason for the lower total increase calculated after flows from 2009 and 2010 were added to the earlier data.