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April Showers Bring . . . Pollution?

Watershed pollutants contribute to summer “dead zone” in Gulf of Mexico, SPEA study finds

Expert perspective: Winter floods and spring downpours are washing large amounts of nutrients off Midwestern farms, according to new research led by Todd Royer, an assistant professor at the Indiana University School of Public and Environmental Affairs. These nutrients end up in streams feeding the Mississippi River and ultimately in the Gulf of Mexico, where they contribute to its oxygen-starved “dead zone”.

The new study, featured in Environmental Science & Technology, focuses on three Illinois watersheds and found that more than 97 percent of the year’s total runoff of dissolved nitrogen and more than 98 percent of the year’s dissolved phosphorus runoff occurs when stream flow is greater than the yearly median. These high flows occur mainly from January through June, according to data from more than 2000 water samples collected from October 1993 through September 2005. The geographic size of the study and the large number of data points make this work a valuable addition to a growing body of research showing that nutrient releases peak during high river flow.

The work by Royer and his colleagues validates other research showing that water flow in May at the mouth of the Mississippi is the best predictor of the size of the Gulf's dead zone. The mass of nutrients, especially nitrogen, dumped in the Gulf in May feed the growth of excess algae that die, sink to the bottom, and ultimately absorb oxygen in late summer.

Most of these nutrients originate in upper Midwestern farms, particularly those with underground tile drainage systems, Royer says. “Roughly 20 million hectares of farmland in the Mississippi River basin are drained by tile systems,” he says. Practices such as fertilizing bare fields in the fall, before the next summer’s plants have a chance to absorb the nutrients, leave nitrogen and phosphorus free to be washed off fields during winter and spring rains and floods, he adds.

Currently, the nutrient-laden floods that take place outside the growing season receive little regulatory scrutiny. The federal Total Maximum Daily Load (TMDL) program, which determines the amount of pollution that can be tolerated by healthy water bodies, tends to focus on summers, when excess algae can foul local streams, Royer says. Because Midwest summers tend to be relatively dry, that is the time of year when the smallest amount of nutrients ran off from farms into the Mississippi.

The federal Hypoxia Action Plan sets a goal of reducing nitrogen loads in the Mississippi River by 30 percent by 2015. Reducing nitrogen loads in streams by 25 percent during periods of high flow could cut total annual export of nitrogen by nearly 21 percent, Royer estimates. This could be accomplished by eliminating fall fertilization and planting cover crops, he says.

Royer's study, “Timing of Riverine Export of Nitrate and Phosphorus from Agricultural Watersheds in Illinois: Implications for Reducing Nutrient Loading to the Mississippi River,” appears in the current issue of Environmental Science & Technology.

The SPEA Toolkit: Todd Royer is an assistant professor at SPEA and aquatic ecologist. His work focuses on water resources, nutrient and carbon cycling in streams and rivers, water quality, and nutrient standards.

Click here to read more about Todd Royer.

Click here to read the full watershed report.