Minnesota's plans to reduce nitrates in drinking water are moving closer to completion.

On Tuesday, Gov. Mark Dayton and agriculture commissioner Dave Frederickson outlined a proposal years in the making: By creating a system of voluntary and mandatory measures on farmland and in public water supplies, the state hopes to safeguard and improve water quality across the state.

More than a dozen public water supplies across the state have been forced to take steps to reduce nitrates in their drinking water, at a cost of millions of dollars.

"While the vast majority of Minnesota households have access to safe drinking water supplies, wells in some areas may have high nitrate levels which can pose serious health problems for humans," Frederickson said Tuesday.

So far, about 20,000 drinking water wells across the state have been tested for nitrates, and 10 percent were found to have unsafe levels.

Excessive levels of nitrates can be toxic, particularly to bottle-fed babies under 6 months old, because they can affect how blood carries oxygen. They can cause a life-threatening disorder known as blue baby syndrome.

In adults, nitrates can cause a variety of problems, including headaches and cramps.

There's also been some research associating high nitrate levels in drinking water with elevated risks for certain cancers. High levels can harm fish and other aquatic life.

Nitrates can come from a variety of sources, including some naturally occuring ones. But it's believed that farm fertilizer runoff causes most of the problem.

The state's plan to reduce nitrate contamination includes provisions that could require some farmers to clean up their runoff.

"One of the ways in which we're protecting water quality in Minnesota is by asking farmers to look twice at their practice of spreading nitrate ... on their land in the fall," Dayton said at Tuesday's news conference.

Random acts of conservation: Water quality depends on farmers' willingness, not regulation

But the practice increases the possibility that nitrates in those fertilizers and manure will seep into the groundwater and create runoff into water systems when winter snows begin to melt.

In the past, most efforts have been voluntary. The state has encouraged farmers to take steps to reduce nitrate runoff, but so far those measures haven't been required.

"Minnesota and most of the country have been relying on purely voluntary farm conservation practices for 75 plus years," said Trevor Russell, water program director at the nonprofit environmental group Friends of the Mississippi River.

"And we are seeing nitrate levels increase, we are seeing pollution levels increase. Purely voluntary approaches aren't necessarily working," he said.

And it's likely the enforcement side of the governor's plan would be long and complicated.

At first, a farmer would be encouraged to adopt new land management practices to reduce the amounts of nitrates leaving the land and entering the groundwater. They might try reducing the amounts of fertilizer they use, or plant cover crops to soak up excess fertilizer.

If those steps fail, a farmer could be subjected to mandatory measures. But it would be a lengthy process to get to the strongest level of enforcement.

"The way the rule is structured, it could be fully 10 years before any kind of mandatory action would be required of landowners," said Russell. "And in the interim communities and their drinking water supplies are still compromised."

Frederickson said the process to reduce nitrates that's in place now is decades old. He was a member of the Minnesota Senate in 1989 when legislation authorizing the state's first nitrate reduction efforts was passed.

"It has taken literally nearly 30 years to actually move forward with some form of approach that will assist us in reducing the amounts of nitrates in groudwater," he said.

The final plan is expected to be in place by December.

A 30-day public comment period will begin in mid-May, and the Minnesota Department of Agriculture will hold more public hearings sometime this summer.

The Associated Press contributed to this report.