A dozen years ago, the Minnesota Pollution Control Agency added Wirth Lake to a list of thousands of problem lakes, rivers and streams across the state after finding high levels of nutrients in its waters.

Now, after a collaborative effort by state and local agencies, the lake, which is in Golden Valley but part of the Minneapolis park system, is cleaner, clearer and finally off the list.

“We’re pretty proud of this accomplishment,” said Laura Jester, administrator at the Bassett Creek Watershed Management Commission, one of the agencies that worked to improve the lake’s water quality.

Set in the heart of Theodore Wirth Park, the lake is a popular summer destination for swimming, fishing and nonmotorized boating. A boardwalk arches out from the beach into the water, forming an inner pool that glimmers with Minnesota summer.

On Wednesday afternoon, dozens of children participating in the Loppet Adventure Camp bobbed in the shallow water, the brave ones clambering onto the boardwalk and cannonballing off.

Camp counselors Anna Kleven, 16, and Cia Xiong, 17, sat near the boat house and kept an eye on the water. They described the lake as a cross-section of the city, a place where residents from the nearby North Side mingle with those who treat the park as a recreational destination.

For Kleven, the space around the lake is a place to ski in the winter and canoe in the summer. For Xiong, it’s a hiking path where her mother often takes walks.

“I used to think this beach was pretty gross, just because that’s what I’d heard about it,” Kleven said. “But I’ve really come to like it.”

‘It was dirty’

The lake is certainly cleaner than it once was. Since 1992, its average summer transparency has increased from less than 4 feet to between about 8 and 10 feet, according to a release Tuesday from the Watershed Commission.

The MPCA works with local agencies to monitor and assess bodies of water throughout Minnesota on a rotating cycle, typically hitting about 10 percent of the state’s watersheds each year, said MPCA impaired-waters-list coordinator Miranda Nichols. In even-numbered years, Minnesota, like other states, submits a proposed list of impaired bodies of water to the U.S. Environmental Protection Agency (EPA), as required by the federal Clean Water Act.

Minnesota’s 2014 list — which excludes Wirth Lake — is pending EPA approval.

Bodies of water are often added to the list because they don’t meet certain water quality standards. Those standards — the acceptable amount of mercury in fish, for example — are intended to keep water usable for people and wildlife. Wirth Lake’s problem was that phosphorus levels, which can spur algae growth, were impeding recreational use. Quite simply, Nichols said, “It was dirty.”

When a site is added to the impaired-waters list, the next step is typically to figure out how to solve the problem that put it there — a task that often falls to local agencies, she said.

For Wirth Lake, this involved multiple collaborative projects among the Watershed Commission, the MPCA, the cities of Golden Valley and Minneapolis, the Minneapolis Park and Recreation Board and the Minnesota Board of Water and Soil Resources.

Most recently, the Watershed Commission worked with Golden Valley in 2012 to modify the lake’s outlet, preventing runoff from Bassett Creek that was bringing in phosphorus.

It can take decades for a body of water to be delisted, said Karen Chandler, senior water resources engineer at Barr Engineering, a firm that works for the Bassett Creek Watershed Management Commission. And often delisting happens because of improved data, rather than specific action.

“In this case,” she said, “we were able to do one project that had a really sizable impact on the lake.”