In 1999 - 2000, contaminated private well water caused 26% of the drinking water outbreaks that made people sick.  The U.S. Environmental Protection Agency's (EPA) rules that protect public drinking water systems do not apply to privately owned wells.

Coliform bacteria are microorganisms found in the intestines of warm-blooded animals as well as in soil, on vegetation, and in surface water runoff. Finding coliform bacteria in a water supply is an indication that there is a potential for disease-producing organisms to be present also.

Coliform bacteria washed into the ground by rain or melting snow are usually filtered out as water percolates through the soil. However, poorly-constructed or unsealed wells, fractured rock outcroppings, sinkholes, and quarries may provide a path for coliform bacteria to enter groundwater.

Once in the groundwater, bacteria can easily pollute drinking water used by private well owners. Other bacteria, viruses and parasites, which can cause illness, can be in water containing coliform bacteria.