Wells are routinely tested for coliform bacteria, which come from soil or vegetation, and in the intestinal tract of warm-blooded animals (fecal coliform bacteria). The many sources of bacterial pollution include runoff from woodlands, pastures, and feedlots; septic tanks and sewage plants; and animals (wild or domestic). Most coliforms are harmless residents of soil and will not make people sick. Some strains of E. coli, the most common fecal coliform bacterium, may be pathogens. Some E.coli found in food have been lethal, so their presence should be taken very seriously. Testing for coliform bacteria is inexpensive and their presence indicate that harmful, pathogenic bacteria could possibly enter or exist in the well. So, if a well tests positive for coliform bacteria, follow-up testing for E.coli sometimes is recommended depending on the specific lab test results. Some coliform tests can provide results for both total coliform bacteria (the indicator) and E. coli, the sign of fecal contamination and potential pathogens.

A water well should be disinfected:

1. After construction of the well and before the water is consumed. Water from a newly constructed well should test free of bacteria before the water is used.
2. After an existing well is serviced. Any time a water well system is opened up, it creates the potential for bacteria to enter the well, so disinfection after servicing helps ensure the water is free of bacteria.
3. If there is a visible defect in the well system that could enable bacteria to enter the well. Examples of such defects are a cracked or loose well cap, or damage to the well casing (the vertical pipe that extends above the surface of the ground).