Minnesota and Wisconsin are contributing an increasing share of the Mississippi River pollution that is killing a wide swath of the Gulf of Mexico.

According to a new federal study, nitrogen flowing into the river from the two states has increased 75 percent over the past two decades and is a major reason why nitrogen levels at the mouth of the river in Louisiana have increased 10 percent over the same period.

The gulf's dead zone is one of the largest such polluted areas in the world, according to researchers from the U.S. Geological Survey (USGS) who published the study this week.

More importantly, they said, it proves that despite decades of efforts to slow agricultural runoff and clean up wastewater, pollution in the river has not gotten better.

"It's disappointing," said Deborah Swackhamer, head of the Water Resources Center at the University of Minnesota, who was not involved with the research. "But this is hard-core science that backs up what a lot of people have thought."

Overall, the USGS found that between 1980 and 2008, the levels of nitrates - nitrogen dissolved in water - held steady at six of the eight spots along the length of the river they studied. But they increased between 60 and 75 percent at two sites - one in Missouri and the other in Clinton, Iowa, the most northern study site. The water at the Iowa site comes primarily from the major tributaries in Wisconsin and Minnesota, including the Minnesota and St. Croix rivers.

Lori Sprague, the lead USGS researcher, said it was not known why levels increased at such a dramatic rate at those two spots. But the potential factors include everything from weather to farming practices to groundwater. Nitrogen occurs naturally in the environment, but in high concentrations it feeds algae growths and sucks oxygen out of the water, killing fish, plants and other forms of life.

It comes from a variety of sources, including wastewater treatment plants, yards, forests and the atmosphere. But 66 percent of the nitrogen pollution in the Gulf of Mexico, which now has a dead zone the size of Connecticut, comes from fertilizer used on corn and soybeans, according to earlier federal studies.

The Mississippi is especially high in nitrogen and phosphorus, another fertilizer, because the river runs through the heart of the nation's corn and soybean belt.

The increasing size of the dead zone in the gulf has raised an outcry from the fishing industry and environmentalists. Since the Clean Water Act was passed in 1972, many forms of pollution are controlled by state and federal regulations, but farmers are exempt from that law. Many environmentalists now say that the voluntary efforts by farmers to manage the use of fertilizers and control water runoff and soil erosion are not working.

"We don't have any regulatory hammer for dealing with nitrate discharges from agriculture," Swackhamer said.

Warren Formo, executive director of an industry trade group, the Minnesota Agricultural Water Resources Coalition, said that farmers are learning which techniques work best to keep nitrogen fertilizer out of the water. That includes more efficient use of nitrogen so there's less runoff, and using wetlands to filter pollutants.

But wastewater treatment plants, which also discharge nitrogen into the water, will have to do better as well, he said.

"I would expect that within the next 10 years, the wastewater industry will have a technological breakthrough, and the same is true on the agricultural front. But it's not going to happen overnight," he said.

In the meantime, the problem may be getting worse, environmentalists say. The number of acres devoted to corn has increased substantially in recent years, driven by demand for ethanol and higher commodity prices. In the past decade, corn acreage in Minnesota has increased from 6.2 million to 8 million. The use of nitrogen fertilizer, however, has not increased at the same rate, because of improvements in fertilizer and more careful use by farmers, experts said.

Still, farmers have a long way to go in changing how they manage the land to protect the water, said Wayne Anderson, an expert with the Minnesota Pollution Control Agency.

"We have to move their whole culture," he said. "It's good economically, and it's good for the environment."

Josephine Marcotty • 612-673-7394