Aquatic ecologist Patrick Mulholland of the Oak Ridge National Laboratory reports that streams are losing their ability to filter excess nitrates from fertilizers and sewage. They released an unusual isotope of nitrogen into 72 different streams — from urban waterways to pristine rivulets — to find out how much made it downstream.
Typically, bacteria remove excess fertilizer from water through a chemical process known as denitrification, which enables them to convert nitrate to nitrogen that is then released into the atmosphere as a gas. The team found, however, that bacteria in the streams they studied only eliminated an average of 16 percent of the nitrogen pollution… not the normal 46%.
What is clear is that a significant portion of such fertilizer is still making its way through the soil and water to the sea. As a result, algae and other microorganisms take up the nitrogen, bloom and, after they die, suck the oxygen out of coastal waters. Such “dead zones” have appeared seasonally near most major river mouths, including those emptying into Maryland’s Chesapeake Bay as well as the Gulf of Mexico, where lifeless waters now cover more than 7,700 square miles (20,000 square kilometers) during the summer months.
The bulk of this nitrate comes from fertilizer running off agricultural fields. A boom in crops such as corn for biofuel will only make matters worse.
Simon Donner of the University of British Columbia and atmospheric scientist Christopher Kucharik of the University of Wisconsin–Madison predict that nitrogen pollution from the Mississippi River Basin—the nation’s largest watershed—will increase as much as 34 percent by 2022 if corn kernels continue to be the source of a growing proportion of ethanol fuel that U.S. energy legislation mandates. That would also make it almost impossible to reduce the New Jersey-size dead zone at the Mississippi’s delta.