MORRISON, Wis.  All it took was an early thaw for the drinking water here to become unsafe.

There are 41,000 dairy cows in Brown County, which includes Morrison, and they produce more than 260 million gallons of manure each year, much of which is spread on nearby grain fields. Other farmers receive fees to cover their land with slaughterhouse waste and treated sewage.

In measured amounts, that waste acts as fertilizer. But if the amounts are excessive, bacteria and chemicals can flow into the ground and contaminate residents’ tap water.

In Morrison, more than 100 wells were polluted by agricultural runoff within a few months, according to local officials. As parasites and bacteria seeped into drinking water, residents suffered from chronic diarrhea, stomach illnesses and severe ear infections.

“Sometimes it smells like a barn coming out of the faucet,” said Lisa Barnard, who lives a few towns over, and just 15 miles from the city of Green Bay.