Most of Minnesota's drinking water supply is under fairly tight regulation. Public water systems for towns and cities that provide about 80 percent of the supply and are tested regularly.
But there are only minimal requirements for private well owners to monitor the safety of their own drinking water. And state health officials are concerned dangerous contaminants may be going undetected.
Arsenic contamination is an emerging concern, said Chris Elvrum with the state health department.
About 10 percent of new wells in the state contain unsafe amounts of arsenic, Elvrum said. The element occurs naturally in rocks and soil and can dissolve into groundwater.
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"The risk really is a long-term cancer risk," said Elvrum. "It's stomach cancer, skin cancer."
About a fifth of the state's drinking water comes from private wells. The water in more than 8,000 of those wells is contaminated with nitrate from human sources, primarily farm fertilizer.
Of those contaminated wells, about 1,000, or 12 percent, have nitrate in concentrations that are considered unsafe for consumption.
The heaviest concentration of nitrate-contaminated wells stretches northwest from the Twin Cities into central Minnesota, an area of mostly sandy soils.
Nitrate can cause health problems by reducing the blood's ability to carry oxygen. Infants are at particular risk of suffocation. Removing nitrate from well water isn't cheap.
At his family's home near St. Peter in southern Minnesota, Bryan Buffington spent about $200 to install a reverse-osmosis system that removes nitrate contamination from his drinking water.
But Buffington complains there are ongoing expenses for the water purification.
"What gets me is having to purchase the filters every six months," said Buffington.
That's another $80 a year. But he said those extra bills give him peace of mind that the water's safe for the 52-year-old, his wife and four children.
Nitrate is just one in a long list of contaminants affecting private wells across the state — bacterial contamination is another potential problem.
And as more threats come into view, state health officials fear the drinking water for thousands of homes has never been tested, or is checked too infrequently. Although there's no reliable data about how many illnesses can be traced to private wells, Elvrum said the risk is real.
It's only been in the past decade that newly drilled wells are tested for contaminants, and huge gaps in the safety net remain. But there's no testing requirement after the initial sample is drawn.
Elvrum said many older wells have never been checked.
"We recommend that well owners test their wells every one to two years for bacteria and nitrate," said Elvrum, "and at least once for arsenic."
Generally, certified laboratories will test a sample sent by mail for around $50. No one knows how many well owners test their water. But David Henrich, president-elect of the National Ground Water Association board, believes it may be a small number. Henrich is also vice president at Twin Cities based well contractor Bergerson Caswell.
"People take for granted that the water comes out of the faucet, goes into the glass, they drink it, and that's good enough for them," said Henrich. "They don't spend enough time on proper maintenance. And I would say there's a lack of testing."