BRAINERD, Minn. -- The U.S. Environmental Protection Agency has given a green light to changes to Minnesota’s water quality rules that advocates say will weaken protections for the state’s lakes and rivers.

The Minnesota Pollution Control Agency proposed amending the Class 3 and Class 4 water quality standards, which are intended to protect water for use by industry, agriculture, livestock and wildlife.

The MPCA said the old standards were based on outdated science, without a lot of supporting evidence behind the pollution limits. But water advocates worry that the amended rules could result in more salty pollutants ending up in Minnesota’s lakes and rivers.

Melissa Lorentz, staff attorney with the Minnesota Center for Environmental Advocacy, one of the groups that opposed the changes, said they are “disappointed” with the EPA’s approval and are considering options, including a possible appeal.

“We're concerned because this rulemaking actually removes standards that were in place to protect our waters from salt pollution,” Lorentz said. “And that's pretty unusual under the Clean Water Act, to actually be removing standards that are in place.”

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Under the federal Clean Water Act, the state sets standards based on how the water from that lake, stream or river might be used, such as for drinking, for fishing and swimming, or to support aquatic life.

A water body can have more than one of these beneficial uses. Most Minnesota lakes and rivers are protected for aquatic life, industrial and agricultural use.

State regulators use the standards to determine how much of a particular pollutant a lake or river can handle when issuing permits to anyone who discharges a large amount of wastewater into that water body, such as industries, mines or sewage treatment plants.

The MPCA’s changes, which were in the works for nearly a decade, eliminated some numeric limits on certain pollutants, including those affecting salinity, and replaced them with narrative statements that describe what the water quality should be.

When proposing the changes, state regulators said there wasn't a lot of supporting scientific evidence behind some of the existing numeric limits.

"What we were really trying to do was to build in some flexibility to look at Minnesota-specific conditions, and to make sure we had water quality standards that were really tailored to the conditions that we have in Minnesota,” Catherine Neuschler, who manages the MPCA’s water assessment work, said in March.

But water advocates argue that the narrative standards will be more difficult to enforce. They are especially concerned that the changes could lead to more chloride, bicarbonates and other salts being discharged into lakes and rivers, where they can harm aquatic life.

“There is research showing that our lakes are becoming saltier in Minnesota, and that the impacts of that pollution is really long term.” Lorentz said. “So it's something that we need to put a stop to before it starts. It's not something that we can clean out of our water.”

Several of Minnesota’s tribal nations also opposed the changes, arguing that they would lead to degraded water quality in lakes and rivers that support fish and wild rice.

The EPA issued a letter on Oct. 8 approving the amended rules, saying it had determined that they met the requirements of the Clean Water Act and will protect aquatic life.