With phosphorus levels more than twice the legal limit, Lake Sarah is among the state's most polluted waterways.

The phosphorus levels are not considered toxic, but the popular recreational lake 24 miles west of downtown Minneapolis has so much vegetation in it that fewer swimmers, boaters and water skiers each year have been able to enjoy it.

But with the Minnesota Pollution Control Agency (MPCA) set to complete a plan to clean up the 561-acre lake, a bureaucratic snag has left the agency unable to force the city of Greenfield -- the lake's biggest polluter -- to participate.

"There's no legal mechanism to force any action," said Randy Lehr, senior manager of water resources at Three Rivers Parks District, which is working on the MPCA clean-up plan. "It is a bit unusual, but I'm cautiously optimistic."

Phosphorus, essential for plant nutrition, is used in fertilizers. It works so well that chemical traces carried to the lake by water runoff have spurred tremendous plant growth, notably Eurasian milfoil and curly leaf pondweed.

Environmental officials believe that farmland runoff in Greenfield, which has more acreage on the lake than any other municipality, is the biggest source of the phosphorus.

But Greenfield, with about 3,000 residents, lacks the population size or density to require a sewer system to control discharge into the lake, as mandated by the federal Clean Water Act. As a result, Greenfield falls outside the purview of state and federal water discharge requirements.

"We have a big challenge ahead," said Barb Peichel, the MPCA's Lake Sarah project manager, who acknowledges that the clean-up will be slowed considerably if Greenfield refuses to take part.

The MPCA hopes by late spring to have a final plan to reduce the amount of phosphorus entering the lake by about 1,100 pounds a year.

The agency has notified the five municipalities on the lake that each will be responsible for preventing a fixed number of pounds of phosphorus from reaching Lake Sarah.

The biggest number belongs to Greenfield, which is responsible for more than half of the pollution entering the lake, officials estimate. The MPCA wants the city to eliminate about 560 pounds of phosphorus a year on its own.

Greenfield's status puts into question whether it will comply with the MPCA plan. Failing to participate could significantly retard clean-up efforts.

"It's frustrating," said Brad Spencer, a City Council member in Independence. "The MPCA is really powerless. ... There is no way to persuade Greenfield to participate."

A lack of cooperation?

Compounding the problem is governmental instability in Greenfield, which last year openly talked about disbanding because of problems caused by political infighting.

"It created a long delay," said Loren Harff, a longtime Greenfield City Council member, in reference to the political turmoil.

The topic of cleaning up Lake Sarah has been raised various times in the past year in Greenfield, but the City Council has not reached a decision.

"Greenfield's had an interesting political past," Spencer said. "If there is no way to encourage them to participate, they might choose not to."

This winter a new mayor was sworn in, raising expectations that the city would be more amenable to participating, said Sylvia Walsh, a former Greenfield City Council member and city representative to the Pioneer Creek Watershed Commission.

Mitigation costs could top $1 million if more elaborate options are decided upon by other cities on the lake, including Independence, Loretto, Medina and Corcoran.

At the moment, said Lehr and MPCA officials, there appears to be enough government grant money to offset the cost of the clean-up.

That might not be the case in four to five years, or whenever Greenfield gets big enough to require a sewer system and is forced to participate.

"It's a chunk of money -- I've heard millions," Harff said of the clean-up costs. "We can't afford to pass up the grant money. If we don't take the money now then we are going to have to come up with it. And we can't afford it. It's as simple as that."

Heron Marquez Estrada • 612-673-4280