Researchers have identified a number of potential factors, including differences in agricultural practices or soil types, that will be explored in the final report, which is due to be published next year, but Borchardt warns they will not be able to say definitively.
They have not calculated the health risks, which they say depend on multiple factors, including the specific pathogen, its concentration and the health of someone drinking the water.
The 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 manure spread on farm fields and human waste from private septic systems can seep into the aquifers tapped for drinking water.
Nitrate — from fertilizer, animal and human waste — is the most common contaminant in Wisconsin groundwater and can be especially hazardous to pregnant women and infants.
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.
SWIGG researchers are now studying the data to find connections between water quality, geology and well construction to guide public policy decisions on things like the density of septic systems in an area or how far livestock facilities should be from a property line.