The number of Minnesota waterways known to be polluted has risen to 1,400, its highest level ever. But the increase has more to do with more extensive testing of rivers and lakes than an overall increase in pollution from farms, cities and factories.

The Minnesota Pollution Control Agency proposed Tuesday to add nearly 300 “impairments” to the state’s inventory of polluted waters. The list, updated every two years, is required by the federal Clean Water Act to identify lakes and river segments in need of cleanup.

For those waterways, the state can impose pollution limits that require farmers, businesses and cities to change their practices.

New on the latest inventory are listings for PFOS, a chemical formerly made by 3M that has been found in fish in 13 metro-area lakes, and acetochlor, a corn herbicide detected in segments of two southeastern Minnesota rivers.

The draft list will be submitted to the Environmental Protection Agency, but a dispute over acetochlor may delay the process. The MPCA had originally listed five rivers as impaired by the herbicide, but dropped three of them after reviewing evidence from the chemical’s manufacturers that a proposed water quality standard for acetochlor was too strict.

But last Thursday, Dow AgroSciences and Monsanto Co., the two leading manufacturers of products containing acetochlor, challenged the inclusion of the two remaining river segments on the list — the Le Sueur River and a short waterway called the Little Beauford Ditch. The firms said that there are “factual disputes” that need to be corrected in a formal two-day hearing before the entire list is sent to the EPA.

Carla Heyl, assistant attorney general for the MPCA, said that the agency will recommend that the chemical firms’ petition be denied for several reasons, and any appeal of the state’s list at this point needs to be filed with the EPA.

The total number of “impairments” listed in Minnesota waterways is 2,575 because some of them have more than one problem pollutant, said MPCA research scientist Howard Markus. Only about 18 percent of the state’s lakes and 14 percent of its rivers have been assessed so far, according to the MPCA. Of those tested, about 40 percent fail and are listed for at least one pollutant.

Most of the pollution problems occur in four areas, Markus said. Some waters contain too much fecal coliform bacteria from animal feedlots or failing septic systems. Others receive excessive phosphorus and nitrogen from farm runoff and sewage plants.

Another problem is mercury from coal-burning power plants, taconite processing and other sources. Mercury is a potent toxin that can damage the brain and nervous systems of people who eat contaminated fish, and its persistence has led to widespread fish consumption advisories in the state.

Turbidity or cloudiness in water is another concern, Markus said, caused by bank erosion, runoff from fields or construction sites, and algae. Murky water has numerous ill effects on otherwise healthy waters, he said, especially if extra sediment restricts aquatic plants, ruins fish spawning areas and smothers insects.

Mike Sandusky, MPCA division director for environmental analysis and outcomes, said that the 2006 Clean Water Legacy Act has provided money that is kicking water quality improvements into a higher gear.

“We have been ramping up recently regarding our monitoring efforts to find the impairments, and likewise the same ramp-up is occurring relative to developing [cleanup] plans and executing them,” he said.