The presence of bacteria and pathogenic (disease-causing) organisms is a concern when considering the safety of drinking water.

Human and animal wastes are a primary source of bacteria in water. These sources of bacterial contamination include runoff from feedlots, pastures, dog runs, and other land areas where animal wastes are deposited. Additional sources include seepage or discharge from septic tanks, sewage treatment facilities, and natural soil/plant bacteria.   Bacteria from these sources can enter wells that are either open at the land surface or do not have water-tight casings or caps.

Insects, rodents or animals entering the well are other sources of contamination. Old wells were dug by hand and lined (cased) with rocks or bricks. These wells usually have large openings and casings that often are not well-sealed. This makes it easy for insects, rodents, or animals to enter the well.

Another way bacteria can enter a water supply is through inundation or infiltration by flood waters or by surface runoff. Flood waters commonly contain high levels of bacteria. Small depressions filled with flood water provide an excellent breeding ground for bacteria. Whenever a well is inundated by flood waters or surface runoff, bacterial contamination is likely.  Shallow wells and wells that do not have water-tight casings can be contaminated by bacteria infiltrating with the water through the soil near the well, especially in coarse-textured soils.

Older water systems, especially, dug wells, spring-fed systems, and cistern-type systems are most vulnerable to bacterial contamination. Any system with casings or caps that are not water-tight are vulnerable. This is particularly true if the well is located so surface runoff might be able to enter the well.

Indications of Bacteria

Bacterial contamination cannot be detected by sight, smell or taste. The only way to know if a water supply contains bacteria is to have it tested. There is no requirement to have private water wells, springs or other sources tested, it is up to the individual homeowner.

Owners of private water supplies are responsible for having their water supply tested to ensure it is safe from bacterial contamination. Generally, private water supplies should be tested for bacterial safety as follows:

  • at least once a year;
  • when a new well is constructed;
  • when an existing well is returned to service;
  • any time a component of the water system is opened for repair -- the water system includes the well, pump, pressure tank, piping, and any other components the water will contact;
  • whenever the well is inundated by flood waters or surface runoff;
  • whenever bacterial contamination is suspected, as might be indicated by continuing illness;
  • when a laboratory test indicates high nitrate and human or livestock waste is suspected.


Often, lending agencies require private water supplies be tested before home loans will be approved, including refinancing a loan.