Research by hydrogeologists at The University of Texas at Austin, which appears in the May 11 edition of the journal Nature Geoscience, shows for the first time that virtually every drop of water coursing through 311,000 miles (500,000 kilometers) of waterways in the Mississippi River network goes through a natural filtering process as it flows to the Gulf of Mexico.
The analysis found that 99.6 percent of the water in the network passes through filtering sediment along the banks of creeks, streams and rivers.
Such a high level of chemical filtration might sound positive, but the unfortunate implication is that the river’s natural filtration systems for nitrates appear to be operating at or very close to full capacity.
Although further research is needed, this would make it unlikely that natural systems can accommodate the high levels of nitrates that have made their way from farmland and other sources into the river network’s waterways.
As a result of its filtration systems being overwhelmed, the river system operates less as a buffer and more as a conveyor belt, transporting nitrates to the Gulf of Mexico. The amount of nitrates flowing into the gulf from the Mississippi has already created the world’s second biggest dead zone, an oxygen-depleted area where fish and other aquatic life can’t survive.
http://www.utexas.edu/news/2014/05/12/research-mississippi-dead-zone/