Bacteria are an enormous kingdom of single-cell organisms that are found almost everywhere on Earth, including inside our bodies, where they do important work for us.

Most bacteria found in well water are not harmful to human health. Occasionally, bacteria that is harmful to health (“pathogenic”) finds its way into a well, either through leaching from locally polluted soil, polluted surface water intrusion from heavy rain or flooding, dirty well-drilling equipment, or breakdowns in the sanitary design of the household plumbing system, e.g. back-flow from the house, or broken well casings.

Coliform bacteria are a large subset of bacteria, only some of which are bad for you. Testing for Coliform bacteria is done not to evaluate the health risk of the water, but to look for changes in water quality or failures of parts of treatment systems in public water supplies. In other words, a positive test for Coliform doesn't necessarily mean the water is unsafe to drink; it just means the water system is not completely bacteria-free and unhealthy bacteria, protozoa, and viruses are potentially present.

Escherichia coli (aka E. coli) and other Fecal Coliform bacteria are two of the pathogenic bacteria that will trip a positive Coliform test, and they can be quite sickening. While most E.coli strains live in happy harmony with the guts of warm-blooded animals, some E.coli strains can kill you. A "positive" for Fecal Coliform may suggest an animal source of bacterial pollution, while a Positive for E. coli may suggest a human sewage source of pollution.

So what should I test for, Coliform, or E.coli, or Fecal Coliform?

Public Water Systems test for Coliforms on a routine basis to comply with State of Maine safety regulations. Well-owners should do the same.

Testing for Coliform is relatively easy and is used an an indication that other, nasty bacteria may be present. Ideally, your test results should show ZERO Coliform bacteria in your water. Anecdotally, we hear from some homeowners, that they have been drinking water that is positive for Coliform for years with no adverse effect. The human body is amazingly adaptive. This may be why a camp owner’s visit “upta camp” results in good memories, while their guest’s memories focus on trips to the outhouse and doses of Cipro.

So, start with Coliform, of which E.coli and Fecal Coliforms are types. If the result is Absent, you know that you have no E. coli or fecal coliform. Lucky you!

If you get a "Present" for Coliform, you have several options. The first is simply to proceed to treatment, which is to bleach clean your well and household plumbing system then re-test until you get an "Absent" result. This is a popular choice, as the cost of bleaching the system is generally lower than the cost of additional testing to narrow down which specific type of bacteria you have. [See HOW TO BLEACH YOUR WELL.]

If bleaching doesn’t kill the bacteria in your water, and you continue to get "Positive" Coliform results, it means you either missed bleaching some part of the well/plumbing system, or the bacteria is re-entering your system on an ongoing basis. This is when further testing is needed to determine where the pollution might be becoming from. A "positive" for Fecal Coliform may suggest an animal source of bacterial pollution, while a Positive for E. coli may suggest a human sewage source of pollution.

What about my well water and Giardia, Cryptosporidium, Legionnaire’s?

Other water-born organisms can cause disease, but are of rarer concern in drilled wells because their large size means they are typically filtered out by soils before entering the well.

These organisms are viruses like polio and protozoa like Giardia and Cryptosporidium. Surface water sources of drinking water should be treated to remove these organisms, which come from human and animal fecal pollution. Cholera, typhoid, dysentery and polio are all transmissible through untreated fresh water sources.

Legionnaire’s disease is from the bacteria Legionella transmitted through contaminated water mists, such as might be found in large air conditioner units or hot tubs, and is more likely to develop in people with weakened immune systems. While it is possible that Legionella bacteria is present in a water well, it cannot cause disease unless it in inhaled as a mist.

What is Iron Bacteria/Red Slime?

Oily sheen created by naturally occurring iron bacteria in Maine.

Oily sheen created by naturally occurring iron bacteria in Maine. From The Maine Field Guide to Aquatic Phenomenon (Schmitt, 2005).

Iron bacteria can occur in wells and spread into the rest of the plumbing system. It is not harmful to human health, but it can cause problems for the water system and create unpleasant smells. Such smells have been described as “fuel oil, cucumbers, or sewage,” and may only be apparent after the system hasn’t been used in a while.

Iron bacteria is a bacteria that binds with naturally occurring iron in the water. It creates a red slime that can cause staining of plumbing fittings and laundry, can provide a place in wells for other potentially harmful bacteria to live, can increase corrosion of pipes, and can cause a crusty build-up on well screens and pumps. In addition, iron bacteria’s “bio-film” slime can be hard for chemical treatments like bleach or muriatic acid to penetrate and kill off entirely.

Manganese in the water can also be leveraged by certain bacteria to create a brown or clear stinky slime.

What is Sulfur Bacteria?

Sulfur bacteria, like iron bacteria, are a slime-forming bacteria that thrive in a sulfur-rich aquatic environment. They are less commonly seen than iron bacteria simply because sulfur is less common than iron in well water.

Read more about Sulfur and Iron Bacteria and what to do about it.

Treatment Options for Bacteria (from

  • Disinfection, which kills the bacteria (i.e. chlorination or disinfection via chemicals, ozone, bromine)
  • Filtration, which traps the bacteria
  • Percolation which removes and can kill the bacteria
  • Ultraviolet irradiation, which kills bacteria.


How to Sample for Bacteria (1 pg. .pdf)

State of Maine -Maine CDC Coliform Fact Sheet (website)

VERY GOOD - Bacteria and Private Wells Cliff Treyens, National Groundwater Association (4 pg. .pdf)

EXCELLENT, IN DEPTH - Public Information Pamphlet #10 - Bacteria and Water Wells American Groundwater Trust (webpage)

How to Take a Water Sample for Bacteria:

BacteriaSamplingTechnique-05 (.pdf, 1 pg.)

E. coli Bacteria (16598492368)

E. coli Bacteria