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Dirty Water

A new method of measuring the interaction of surface water and groundwater along the Mississippi River adds further evidence that the network’s natural ability to chemically filter nitrates is being overwhelmed.

Hydrogeologists at the University of Texas at Austin showed 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, which appears in the May 11, 2014, edition of the journal Nature Geoscience, found that 99.6 percent of the water in the network passes through filtering sediment along the banks of creeks, streams and rivers. This finding suggests the river’s natural filtration systems for nitrates—a major component of inorganic fertilizers—are operating at or very close to full capacity.

As a result, 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.

The research, conducted by Bayani Cardenas, associate professor of hydrogeology, and Brian Kiel, a Ph.D. candidate in geology at the university’s Jackson School of Geosciences, provides valuable information to those who manage water quality efforts in the Mississippi River network.

The new model, Cardenas said, can be a first step to enable a wider analysis of the river system.