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ENVIRONMENTAL HEALTH

Coliform Bacteria Contamination - Frequently Asked Questions

What are coliform? 

Coliform bacteria are indicator organisms which are used in water microbiological analysis. Coliforms are a group of bacteria which are readily found in soil, decaying vegetation, animal feces, and raw surface water. They are not normally present in deep groundwater and treated surface water. These indicator organisms may be accompanied by pathogens (i.e., disease-causing organisms), but do not normally cause disease in healthy individuals. However, individuals with compromised immune systems should be considered at risk.  Coliforms, rather than the actual pathogens, are used to assess water quality because their detection is more reliable. Pathogens appear in smaller numbers than coliforms, so are less likely to be isolated.  Drinking water found to contain coliforms is considered biologically contaminated.

How does the laboratory detect coliforms in a water sample? 

Since visual examination of water under the microscope is unreliable, the County Laboratory uses the Colilert presence/absence method. In brief, this method combines two separate examinations. First, the 100 milliliter sample is combined with growth medium, and then incubated for 24 hours at 35.0 degrees Centigrade. After 24 hours, only coliform bacteria will use the growth medium for food, resulting in a change in the water sample's appearance. At the same time, a second examination is performed to determine if the coliform present is from feces. Thus, the test is completed 24 hours after it has begun.

A TOTAL COLIFORM ABSENT report means that there are no coliform bacteria present at the time of sampling and the water may be considered bacteriologically safe to drink.

A TOTAL COLIFORM PRESENT report means that coliform bacteria is present at the time of sampling and the water may not be considered safe to drink. You will be advised to super-chlorinate the water system.

A FECAL COLIFORM (E.COLI) PRESENT report means that fecal coliform bacteria is present at the time of sampling and the water may not be considered safe to drink. You will be advised to super-chlorinate and/or make modifications to the water system.

The California Department of Health Services sets drinking water standards and has determined that the presence of any coliform bacteria in drinking water is a possible health concern. A 100 milliliter sample of water must be absent of coliform bacteria to be considered safe to drink.

How did the water become contaminated? 

Surface waters, such as streams, creeks, and lakes, will almost always contain some degree of contamination. This is due to exposure to animals, humans, aquatic life, etc. Raw surface waters should always be treated prior to domestic use, and must include disinfection and filtration to remove turbidity and parasites that are resistant to simple disinfection. Wells and springs can become contaminated by faulty construction, poor protection from surface activity including rain and flooding, a shallow water table, entrance of unfiltered surface water via rock fractures, close proximity to a sewage disposal system or leaky sewer pipe, and/or contamination during repair or reconstruction. A false, positive coliform count can occur by contaminating the water sample (e.g., putting one's finger into the water sample bottle) or by using a non-approved water sample bottle. A well or spring should be super-chlorinated prior to the coliform test, especially if a repair has been made and the system was not chlorinated at the time.

How do I cure my contamination problem? 

Examine the well, spring, storage tank, etc. to determine that it is adequately sealed to prevent contamination entry. Most of the time, assuming that the well/spring is properly constructed, the contamination can be cured by spot chlorination. This involves pouring chlorine bleach down the well shaft (or into the spring containment box), and allowing to stand for 30 minutes. Next, each tap is opened, one at a time, to bring the chlorine solution into contact with all of the plumbing. Make sure every tap is operated, both inside and outside the house. Once the chlorine odor is noticeable, shut off the tap and proceed through the remainder of the system. When done, let sit overnight (at least 12 hours). The next day, the chlorinated water can be flushed out by opening taps and running water until chlorine odor is undetectable. Once the chlorine is completely gone (can be checked with a swimming pool test kit), the water should be retested for coliform contamination. Normally, the contamination will be eliminated by this temporary disinfection method.

{To estimate the amount of 5.25% chlorine (household type bleach) to use: for a 6-inch diameter well, figure the depth of water in the well and for every 66 feet of water (100 gallons), use 1 quart of chlorine}.

In some cases, if the contamination cannot be eliminated, the well/spring may require reconstruction or another permanent solution.

For information about other forms of Water Contamination - click here.