Inside the plant that supplies St. Peter's drinking water are several arrays of 20-foot long white plastic pipes -- an industrial-scale version of a water cleansing system routinely used in homes.
Using reverse osmosis, pumps force well water through the pipes at high pressure. Filters catch impurities, including nitrates, a by-product of nitrogen fertilizer, and a major reason for the system.
Nearly every Minnesota corn farmer applies the fertilizer to the state's biggest crop. Its use is so prevalent that at least two dozen Minnesota communities have unhealthy levels of fertilizer by-products in their drinking water supplies.
When nitrates seep into drinking water supplies they pose a health risk to infants, pregnant women and others. Take them out and the health risk drops -- but water bills are certain to rise.
"Without a doubt, because it's an additional level of treatment that you might not otherwise undertake, St. Peter City Administrator Todd Prafke said. "If you're doing more stuff there's an additional cost related to that more stuff that you're doing."
Crop researchers say there is a proven solution: perennial crops such as alfalfa, which absorb nitrates. But not many farmers have adopted it. As a result, farming communities must bear the cost in other ways.
St. Peter officials haven't analyzed exactly how much nitrate removal costs an average resident. But state agriculture officials say the process can add as much as $200 a year to a typical homeowner's water bill.
That's one of the factors that pushed St. Peter into the top third of municipal water rates in Minnesota, Prafke said.
"Certainly our rates are higher today and we're not happy about that," he said. "But we know that's the case."
St. Peter and other communities have tried to lower nitrate levels in their water aquifers by establishing a wellhead protection zone aimed at keeping pollutants from reaching groundwater. In St. Peter, that affects about 10 square miles of largely agricultural land.
"The program is to protect community water supplies from various contaminants such as nitrate/nitrogen, which frequently comes from our agricultural cropping systems," said Bruce Montgomery, manager of the fertilizer management section for the state Department of Agriculture. "The growers are usually quite receptive to doing those types of things."
But Montgomery said adjusting fertilizer practices can only partially help protect water supplies. Despite farmer cooperation, nitrate levels in one St. Peter city well are still 60 percent above recommended levels.
"[Alfalfa is] like a very good Hoover vacuum cleaner... It does a really, really good job of removing those nitrates."
To significantly lower nitrate levels, he said, farmers near the city well field will have to change what types of crops they grow. That's where alfalfa, a voracious feeder, comes in. Its root system grows as much as eight feet a year.
"What the science strongly suggests is the use of perennial cropping systems such as alfalfa strategically placed throughout the wellhead area may have the most significant impact," Montgomery said. "It's like a very good Hoover vacuum cleaner," Montgomery said. "It does a really, really good job of removing those nitrates."
Montgomery said alfalfa can reduce the amount of nitrogen fertilizer reaching aquifers by up to 90 percent. But he said farmers are reluctant to plant alfalfa because they don't think it will bring make them as much profit as corn.
That's not necessarily true, said Mike McDuffy, an extension economist at Iowa State University. "I know several people that have made and are making quite a bit of money," Duffy said.
This year, for example, alfalfa could be quite profitable, in part because the crop is in short supply due to drought. According to Iowa State University data, alfalfa farmers this year could clear as much as $300 an acre. Corn, on the other hand, is fetching lower prices than recent years, and corn farmers may see profits of less than $200 an acre this year.
Duffy said farmers can do well financially growing alfalfa for several years, but then rotating it with other crops. That would still keep a good share of the environmental benefits in place, while also increasing profitability, he said.