A Des Moines utility's plan to sue three northwest counties for polluting central Iowa's drinking water supply may have broad ramifications for state and U.S. farmers, who environmentalists complain have been too slow to embrace meaningful conservation practices.

It's too soon to say exactly how Des Moines Water Works' threatened lawsuit could play out in farm fields across Iowa and the nation.

But agriculture experts and environmentalists are closely watching the case, which they say could bring to a head a decades-long national fight over who is responsible for water pollution that originates from cropland that often is hundreds of miles away.

They say the outcome could, for the first time, indirectly require farmers to meet federal clean-water regulations that limit nutrients such as nitrates and phosphorus that enter U.S. waterways.

"The ruling, however it comes down, could be precedent-setting," said Neil Hamilton, an agricultural law professor at Drake University.

The federal government now considers water from farmlands as surface runoff and exempts it from oversight.

But the Des Moines agency contends the underground tiling widely used by farmers bypasses the natural filtering soil provides, acting as "a continuous mechanism for transporting nitrates to streams."