Levels of PFOS, a chemical manufactured by 3M Co. for a variety of commercial uses until about 10 years ago, have improved significantly in the Mississippi River between Hastings and St. Paul -- except for the area around the company's Cottage Grove plant, where they have worsened.

The Minnesota Pollution Control Agency (MPCA) on Monday released results of its 2012 monitoring report for levels of PFOS -- perfluorooctane sulfonate -- part of a family of compounds called perfluorinated chemicals (PFCs) at the heart of the state's lawsuit filed against 3M late in 2010 and expected to go to trial this year.

"Overall, this is a success story," said Katrina Kessler, manager of the MPCA's water assessment and environmental information section. The data show strong strides have been made to rid the river and its aquatic ecosystems of PFOS, although more work needs to be done.

The report could have a bearing next year on whether the 32-mile stretch of the Mississippi called Pool 2, between Lock & Dam 2 in Hastings and the Ford Dam in St. Paul, is removed from the MPCA's list of "impaired" waterways, Kessler said. Both 3M and the Metropolitan Council, which operates wastewater-treatment plants in St. Paul, Cottage Grove and Hastings along the river, are seeking to lift the impaired label -- in place since 2008 -- which would reduce the cost of treating water containing PFOS that is discharged into the Mississippi.

Both 3M and the Metropolitan Council (which has joined the state's suit against the company) have invested heavily in efforts to reduce PFOS contamination in the river, Kessler said. 3M installed a carbon-filtration system as a condition of approval for a recently renewed wastewater-treatment permit and dredged 17,000 cubic yards of contaminated soil from the river near the plant and replaced it with clean material.

"We're very gratified" with the efforts to remove PFOS from the river, said Trevor Russell, watershed program director for Friends of the Mississippi River, but there are other significant issues confronting Pool 2, including sediment, increased flow, nitrate levels and Asian carp.

As for the pool's impaired status, Russell said the MPCA should continue doing its work without being influenced by lobbying from 3M, the Met Council or any other entity to have the section removed from the list. "This needs to be a science-first-and-science-only process," he said.

The MPCA's analysis replicated a monitoring program in 2009 in Pool 2. At 12 monitoring stations in four sections of the pool, samples of invertebrates (such as insects, crayfish and frogs at the bottom of the food chain), fish, river sediment and water were tested for PFOS levels.

In the three sections upstream from the 3M plant, PFOS levels in the water dropped significantly over the three years. At testing sites in St. Paul Park and Newport, levels dropped by about half.

In the lower reach of Pool 2, however, the area of river along which the 3M plant is located, the numbers were markedly higher. At two testing sites downstream from the 3M plant, the closest monitor showed PFOS concentration rose from an average of 90 parts per trillion (ppt) to 149 ppt. The next site to the south rose from 15.2 to 24.4 ppt.

It's unclear why levels haven't improved downstream, according to the MPCA. PFOS is a resilient chemical that lingers in the environment for years and accumulates in people and animals.

The Minnesota Department of Health's health-risk level for PFOS is 0.3 parts per billion.

3M began manufacturing PFOS at Cottage Grove in the 1950s, and products containing the chemical were deeply ingrained in consumer culture, used in Scotchgard, floor polish, milk cartons, microwave popcorn bags, metal plating and much more. 3M stopped making PFOS in 2002, but had legally dumped the chemical at its plant and at sites in Woodbury, Oakdale and Lake Elmo, which subsequently prompted a massive cleanup effort and dispute leading to the lawsuit.

Calls to 3M seeking comment were not returned.

Jim Anderson • 651-925-5039 Twitter: @StribJAnderson