HASTINGS - Debbie Carlson can laugh at the irony: She's the wife of a well digger who can't find good water for his own family.

Like one out of three wells in Dakota County, hers is so contaminated with nitrates she won't let anyone drink from it -- especially her 8-year-old granddaughter. Most likely it comes from nitrogen used as fertilizer on the cornfields surrounding her home. "Nitrogen was a great thing for the family farm," Carlson said. "But I am paying the price."

Thanks to a combination of geology and some of the country's richest farmland, thousands of Minnesotans face elevated levels of nitrates in their drinking water. It's a health risk -- mostly for infants and pregnant women -- and a significant economic burden. Hastings is one of nearly a dozen Minnesota communities that has spent millions to clean the toxin from drinking water. Well owners like the Carlsons have three choices: Drink it, which some do. Pay thousands for a new well. Or install expensive treatment systems.

The prairie that once protected groundwater is long gone from Dakota County and from most of Minnesota and the Midwest. That loss lays bare what one leading agricultural economist calls the "wicked problem" of global nitrogen pollution.

By converting grasslands and pastures to fertilizer-intensive crops, modern agriculture has produced an extraordinary bounty of corn, sugar beets and potatoes for a growing global population. But it also has eliminated the valuable "ecological services'' that native landscapes provide -- such as filtering groundwater -- at great cost.

Now, through an emerging statewide strategy, the Minnesota Department of Agriculture is devising a range of fixes, including more water monitoring and guidance on how communities can restore some of the lost prairie landscape.

In the process, officials and farmers will tackle two thorny questions: How will government use its power to regulate nitrogen use in contaminated areas? And even if every landowner follows the best guidance science can provide, when will they know if it works?

Striking the right balance is crucial because the current approach, said Jill Trescott, Dakota County's groundwater-protection supervisor, imposes a cost shift from agriculture to taxpayers and homeowners that is "just not fair."

Says Carlson: "I think water is one of our most precious resources. What are our grandchildren going to be left with?"

Corn and geology

In Minnesota, three-fourths of people get their drinking water from groundwater. On average, 6 percent of private wells are contaminated with nitrates. About a dozen community water systems have "pretty severe nitrate problems," said Bruce Montgomery, manager of the Agriculture Department's fertilizer and pesticide division. Health officials say that once or twice a year another community hits the limit.

The problem is concentrated in several regions: Dakota and Washington counties; the 14 counties that make up the Central Sands region in the middle of Minnesota; the southeastern "karst'' region, where the cracked limestone geology sends water straight down to the aquifers, and southwestern Minnesota, where a shortage of water in general aggravates the nitrate problem.

In Dakota County, the first place in Minnesota to trace nitrates directly to agriculture, the problem is partly an accident of geology. West of Hwy. 52 a thick layer of till -- clay, gravel and sediment left behind by the last glacier -- lies beneath the rich soil, so that water percolates slowly down from the surface.

But on the county's eastern side, the melting glaciers left behind sand on top of bedrock, and water rushes through it like a sieve -- down to the aquifers or into the Vermillion River and eventually the Mississippi River, said Tim Cowdery, a U.S. Geological Service hydrogeologist who has studied it. Carlson's husband, she said, describes it as "young water."

Geology wasn't as much of a problem back in the day when farmers planted more varieties of crops, many of which required less nitrogen. But in Dakota County, like much of Minnesota, corn and soybeans are now the primary crop. Soybeans pull nitrogen out of the air and fix it in soil, where it can leach into the water. And corn, more than most any other crop, demands fertilizer to produce the yields that have climbed steadily for decades.

Since the late 1980s, corn yields per acre have nearly doubled. The number of acres devoted to corn in Minnesota has grown by a third. In short, nitrogen is money for corn farmers, said Greg Buzicky, head of the nitrogen-management plan for the state Agriculture Department. "It drives their profit and their income," he said.

Farmers actually have cut their nitrogen use drastically -- the volume of corn produced has climbed seven times faster than fertilizer use.