This article uses information from a Fact Sheet provided by the Minnesota Department of Health.
Contaminated drinking water may contain harmful bacteria, viruses, or other microorganisms that can make you sick. Disinfection is one process of killing or inactivating microorganisms to make water safe for drinking. Disinfection can also eliminate nuisance bacteria that can cause unpleasant tastes and odors.
A well or water system may become contaminated with harmful bacteria and other organisms when the well or plumbing is open to the environment during construction, repair, or routine maintenance. A well may also become contaminated if the well casing is deteriorated or damaged, or if the well is flooded. Contamination problems can also be caused by improper plumbing connections between water treatment devices and wastewater piping, between the potable water plumbing and heating/cooling systems, or other cross-connections.
And there can be a problem with the water source itself.
Whatever the cause, the water should be checked again after the well is disinfected.
Typically, a well should be tested once a year for coliform bacteria and other possible contaminants, or whenever there are changes in the water’s taste, odor, or appearance. Many organisms that can be in your well water are not actually harmful, but a few are. Their presence indicates that surface contamination has found its way into the well, and disease organisms may also be present. The presence of E. coli or fecal coliform represents a more serious health risk and any water use should be strictly limited to non-potable uses (not used for drinking, cooking, or human contact).
For additional information, see our complete guide to shocking your well.