Despite decades of effort to reduce pollution in the Mississippi River Basin, nitrate levels haven’t improved since 1980, according to a study from the U.S. Geological Survey (Environ. Sci. Technol., DOI:10.1021/es201221s).
Nitrogen and phosphorus fertilizer washes into groundwater and rivers that make up the Mississippi River Basin. Eventually this pollution makes its way into the Gulf of Mexico, where it feeds a dead zone, an area of low-oxygen water where many organisms cannot survive.
But evaluating the effectiveness of policies to shrink the zone is difficult, says Lori A. Sprague, a USGS researcher. Weather variations, such as rainfall, cause year-to-year fluctuations in the amount of pollutants flowing into rivers, complicating analysis of runoff trends.
Sprague and her colleagues used a statistical method to control for weather variations. The researchers analyzed data collected by USGS between 1980 and 2008 from eight sites on the Mississippi and its tributaries.
The researchers found that at sites on three tributaries, nitrate pollution had not changed. Meanwhile, pollution increased by as much as 75% at five sites in the basin. Overall, they determined that the amount of nitrate flowing through the basin into the Gulf increased by 9%.
But the trends they found are disappointing, he says: “We’ve been trying to address this problem for quite some time, and it doesn’t look like we’re making any progress.”