Department of Health

• Fillmore County — 99 wells tested; 31 wells positive for bacteria (31%) and two of the bacteria-positive wells were flooded.
   
• Houston County — 344 wells tested; 154 wells positive for bacteria (45%) and 15 of the bacteria-positive wells were flooded.
   
• Winona County — 1,475 wells tested; 391 wells positive for bacteria (26.5%).

Flooded wells will usually become contaminated with bacteria. After flood waters subside, properly constructed and structurally sound wells that are bailed out and disinfected will clean up.

Some shallow aquifers in southeast Minnesota have always been vulnerable to bacterial contamination because of the limestone geology. The Minnesota Well Code, created in 1974, has required deeper wells in those areas for that reason. About 70 to 75 percent of the area wells are newer and deeper. Those are being successfully cleaned out and disinfected.

About 25 to 30 percent of the wells were constructed before 1974. Many of the wells unresponsive to disinfection now are old, shallow, in disrepair, and were likely contaminated with bacteria even before the flood. DOH expects that many of these old wells will never clean up.

The chemical contaminant nitrate moves easily through aquifers. When nitrate is found in a well, it is because the aquifer is contaminated with nitrate. Bacteria in a well most often relates to issues with the well itself, such as structural integrity, a missing well cap, or intrusion by animals or insects.