Southeastern Minnesota farmland has been getting a makeover in recent years, and the outcome will degrade private water wells for hundreds of homeowners, according to University of Minnesota researchers.

About one-fourth of the grasslands in an 11-county area were converted to growing more profitable corn and soybeans since 2007, raising the likelihood of more fertilizer entering the water that residents drink.

"We found evidence that recent trends in grassland loss to agriculture between 2007 and 2012 are likely to increase the future number of contaminated wells by 45 percent," said University of Minnesota researcher Bonnie Keeler, "leading to millions of dollars in lost income and remediation costs for private households."

The analysis used federal data about changes in growing patterns in southeastern Minnesota — and publicly available well data and chemistry readings — to predict how the changes on the land will affect underground water.

Southeastern Minnesota is particularly vulnerable to groundwater contamination because its karst geology contains many cracks and fissures in underground rock formations, Keeler said, allowing water on the surface to easily enter the deeper groundwater that residents rely on for drinking.

"Groundwater moves in mysterious ways," she said. "It's not always just who's next door that affects your water because there can sometimes be complex pathways in which water moves underground."

The 11 counties have a population of more than 700,000, and the majority rely on groundwater as their primary drinking water source, according to state data cited in the study.

One of the main concerns is nitrate, a primary component of fertilizer that is highly soluble and mobile in soil. Converting hay and alfalfa fields to corn and soybeans increases the use of fertilizer substantially.

High concentrations of nitrate in drinking water can cause a variety of health problems, including a blood disorder in infants called blue baby syndrome. Keeler's research predicted that the number of wells with nitrate levels exceeding federal drinking water standards will increase from 888 to 1,292 based upon acreage already converted from grassland to crops between 2007 and 2012. And the number of wells at a lower tier of contamination, associated in some studies with higher risks of cancers and birth defects, is estimated to rise from 2,779 to 3,562, a 28 percent increase.

The study does not predict where those wells will be located, but only the higher risk of well contamination because of the additional fertilizer being applied to the landscape. And the analysis does not project exactly when the contamination will occur, since previous studies have shown a lag time of years to decades before nitrate levels bump up enough to be identified.

The additional acreage now used to raise crops "should not be interpreted as an estimate of well contamination in the year 2012, but rather an estimate of potential future contamination" in the region, the study concluded.

The research also focuses only on private wells, and does not include wells used by cities, churches, schools and other institutions.

Keeler published the research in the journal Environmental Research Letters. It was co-authored with Stephen Polasky, from the University of Minnesota's Department of Applied Economics.

The research does not take a stand on whether conversion of grassland to crops is a good thing, and it does not discuss the other potential effects of degraded water quality such as diminished recreation value, lower property values, and human health costs.

It noted that the changes in land use are not unique to southeastern Minnesota, and that "rapid and extensive grassland conversion to row-crop agriculture" has occurred recently across large portions of several Midwestern states.

Private well owners who find excessive nitrate in their water have several options to correct the problem: dig a new well, purchase bottled water, or install a home water purification system. The study estimates that it could cost homeowners up to $12 million cumulatively to address the additional contamination from the 2007-2012 land use changes, depending on how many take action and which options they choose.

That amount of money is likely to be much less than the value of crops grown on the additional farmland, the study estimated, but it demonstrates a "trade-off between a private gain (agricultural production) and degradation of a public good (groundwater quality)."

"So there are benefits that accrue to that landowner or farmer in terms of the additional revenue they receive," Keeler said, "but there's lots of societal costs that are then borne by other individuals."