ST. PAUL, Minn. (AP) — Minnesota has added another 500 lakes and stretches of river to its list of impaired waters, raising the state's total to more than 3,600.

Impaired waters are defined as having excess nitrogen, phosphorus, mercury, bacteria or other pollutants, hurting their ability to support swimming or fishing, or to provide healthy habitats for fish and wildlife.

The federal Clean Water Act requires states to update their list of impaired waters every two years. Minnesota is one-fifth of the way through surveying its nearly 12,000 lakes and nearly 70,000 miles of rivers and streams. So far, researchers say, about 40 percent of Minnesota's waters are impaired.

While the survey is a first step in addressing problems, some critics say the state isn't doing enough to reduce the pollution because it relies on voluntary measures that haven't proven effective and isn't strict enough with the state's farm sector.

Once waters are on the list, the state works with local governments and groups to design cleanup plans, but it's a slow process. Minnesota Public Radio reported Friday (http://bit.ly/x9d1lm) that while about 900 cleanup plans have been approved or are being developed, they've resulted in only 15 bodies of water coming off the list.

The process is slow, acknowledged Glenn Skuta, the Minnesota Pollution Control Agency's water monitoring manager.

"We have to be cautious and remember, these water bodies did not become impaired overnight and they're not going to be cleaned up overnight either," Skuta said.

Regulators have already addressed the easier pollution sources to fix. Factories and wastewater treatment plants have to obtain permits, so they can be held to stricter standards.

The main problem is with other kinds of pollution, the "nonpoint sources" such as runoff from suburban lawns and farm fields, said Gene Merriam, a former Department of Natural Resources commissioner and state senator who now heads the Freshwater Society.

"With the nonpoint sources, they are in large measure dependent on voluntary action," Merriam said. "The question is, why do we expect that's going to work?"

Voluntary actions tried so far include buffer strips along waterways to filter run-off. Farmers can recoup some of the costs from federal and state conservation programs.

Craig Johnson, a lobbyist with the League of Minnesota Cities, points to a cleanup plan for the Mississippi River that is expected to be released in the next few weeks. He said it will require cities to invest money to clean up storm water systems and wastewater treatment plants, but it will impose no requirements on the agricultural sector.

"Cities contribute about 5 to 6 percent of the load and the rest comes from other sources," Johnson said. "We'll be required to spend $843 million, and ag will be given a goal of reducing their load but no requirements."

But Skuta is optimistic. Because his agency decided to tackle the problem by focusing on entire watersheds, rather than individual lakes and stretches of river, the research has helped identify where cleanup work should be targeted.

"If we can focus our resources, our time, our money, our people, on the areas where we can get the most bang for the buck, we'll be quicker and more efficient about actually seeing improvements," Skuta said.

The agency has ramped up its survey efforts with new money from the Legacy Amendment, which raised the state's sales tax in 2009 by three-eighths of 1 percent to pay for environmental, outdoors and cultural projects. By 2018 the MPCA expects to complete studies on all of Minnesota's lakes and rivers.

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Information from: Minnesota Public Radio News, http://www.mpr.org

Copyright 2012 The Associated Press.