New evidence from University of Texas at Austin researchers posit that the great Mississippi’s natural ability to chemically filter out nitrates is being overwhelmed. UT’s hydrologists demonstrate the enormity of the filtering process for almost every drop of water that enters into the 311,000-mile long course ending in the Gulf of Mexico.The study found that 99.6% of the water is filtered through bank sediments in the watershed’s creeks, streams and rivers. While the intensity of chemical filtration seems like a good thing, the reality is that the river’s natural filtration systems for nitrates, which rob the water of oxygen, resulting in algal blooms creating dead zones, appear to be operating at or very close to full capacity. Researchers believe that it is unlikely that the river’s natural system can accommodate the high levels of nitrates that have made their way from the contributing farmlands and communities into the watershed.
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.
The research, conducted by hydrologist Bayani Cardenas and Brian Kiel, a Ph.D. candidate in geology, provides valuable information to those who manage water quality efforts, including the tracking of nitrogen fertilizers used to grow crops in the Midwest, in the Mississippi River watershed.
Aaron Packman, Civil and Environmental Engineering professor at Northwestern University says, “There’s been a lot of work to understand surface-groundwater exchange; this is the first work putting together a physics-based estimate on the scale of one of these big rivers, looking at the net effect of nitrate removal in big river systems.”
The Mississippi River network includes the Ohio River watershed to the east, the Missouri River watershed to the west and the Mississippi watershed in the middle.
Cardenas and Kiel analyzed the waterways for sinuosity (how much they bend and curve); waterway texture materials; the time spent in the sediment (known as the hyporheic zone); and water flow rates through the sediment with the help of ground level data from the USGS and the EPA.
Operating as a chemical filter, microbes in the sand, gravel and mud gobble up compounds such as oxygen and nitrates from the water before the water discharging it back. The more time water spends in the sediment, the more some of these compounds are transformed.
Read more at the University of Texas at Austin.
The Little Missouri River, contributing river to the Mississippi River, image via Shutterstock.