A comprehensive new study pinpoints agriculture -- specifically, half a century of artificial field drainage -- as the primary force behind the massive runoff of sediment that is adding pollution to the Mississippi River and threatening the future of Lake Pepin.

The study, presented Wednesday at a conference in St. Paul, identifies with new precision the sources of sediment that is slowly filling in Lake Pepin, one of the state's recreational jewels, and coursing down the Mississippi to the Gulf of Mexico, where it contributes to a massive "dead zone'' that cannot sustain aquatic life.

Scientists said it's the latest in a growing body of evidence indicating that transformation of the land from prairie and wetlands to corn and soybeans -- not, as some have argued, more rain and natural erosion -- has accelerated the rate of sedimentation.

"It's the weight of the evidence," said Peter Wilcock, a geography professor from Johns Hopkins University.

He was not involved the study but attended the University of Minnesota's annual Water Resources Center conference, where it was presented.

The issue is controversial because it lands squarely on farmers and the economic choices they face, especially at a time of high prices received for corn and soybeans. Tile drainage has helped make fields along the Minnesota River valley some of the most productive land in the country.

Some say the study leaves unanswered questions about the forces that are in play.

"I don't think they've proven their point," said George Rehm, a retired University of Minnesota soil scientist who now leads research for an industry group, the Minnesota Agricultural Water Resource Center.

70 years of data

The new research, conducted by scientists at the St. Croix Watershed Research Station and the University of Minnesota, included examination of rainfall, flow and land use changes along the 21 tributaries to the Minnesota River.

Researchers analyzed more than 70 years' worth of precipitation and other data from the river basin's 21 watersheds. They found that field drainage and loss of wetlands across the landscape is adding enormous volumes of water to the state's second-largest river. That added volume scours the fragile, sandy banks, sending millions of tons of sediment downstream to the Mississippi, where it settles out in Lake Pepin.

Jason Ulrich, the university research fellow who presented the findings, said that rainfall increased between 6 and 15 percent in eight of the 21 watersheds. Flow -- the volume and speed of water -- in the Minnesota increased by about the same amount, he said.

But since much of the rain is held on the land or absorbed by plants, it does not account for all the added volume in the river.

"Sixty to 90 percent is unaccounted for," he said, leaving field drainage as the primary contributor.

Dan Engstrom, a scientist with the St. Croix Research station, said much of the water that now ends up in groundwater and in rivers used to lie across the surface of the land and slowly evaporate. That process is part of what's been lost, he said.

Rehm said, however, that drainage may actually slow erosion of the river banks because it takes water out of the ground quickly, which prevents the river banks from sloughing away after heavy rainfalls.

The contentiousness of the debate among scientists, farmers and agricultural interests is similar to the debate about climate change, Wilcock said. The science has become entangled in advocacy, he said. That interferes with getting to the more important research --whether sedimentation can be slowed or reversed, and what that will cost.

"Do we really need to answer the question to take action?" he asked.

Josephine Marcotty • 612-673-7394