Another round of testing has found more evidence of human and animal waste in wells in three southwest Wisconsin counties.
Scientists with the Southwest Wisconsin Groundwater and Geology Study sampled 35 wells randomly selected from a larger group of contaminated wells.
Tests found fecal contamination in 60% of those wells based on the presence of viruses and bacteria associated with human, bovine and swine waste. Just under half the wells tested positive for pathogens associated with stomach illness.
State geologist Ken Bradbury said the goal was to identify the species of pathogens in the water.
Researchers have not calculated the health risks, which they say depends on multiple factors, including the specific pathogen, its concentration and the health of someone drinking the water.
This is the fourth and final round of testing in the two-year study of private wells in Grant, Iowa and Lafayette counties.
Between 73% and 91% of wells tested in each of the previous three rounds had traces of some combination of human and animal waste. Each set of wells were randomly selected from a pool of wells that were previously shown to have coliform bacteria or nitrate levels above the drinking water standard
Like other parts of Wisconsin, the southwestern part of the state has areas where porous bedrock means livestock manure spread on farm fields and human waste from private septic systems can seep into the aquifers tapped for drinking water. In Grant, Iowa and Lafayette counties, some 44% of people get their water from private wells. Across the state, some 25% of residents rely on tap water that comes from private wells.