Determining the bacterial quality of drinking water is the single most important water quality test. Why? Because one glass of water containing just a few disease organisms can cause illness. When minimal exposure creates an immediate health risk that factors is known as an acute health risk. In contrast, a meaningful health risk from chemical contaminants such as arsenic, radon, or benzene, to name only a few, requires a long period of exposure, typically over many years.

The total coliform test is the basic yardstick for determining a water supply's biological quality. This test is performed frequently because of the risk that disease causing organisms pose to health. The test is easy to perform, inexpensive, and errs on the side of caution.

Total Coliform As An Indicator Organism

The organisms in the total coliform group are called indicator organisms. That is, if present, they indicate that there is a possibility, but not a certainty, that disease organisms may also be present in the water. When absent there is a very low probability of disease organisms being present in the water. The ability of the total coliform test to reliably predict the bacterial safety of water relative to the hundreds of possible diseases that might be present is critical since it impossible, in a practical sense, to check separately for every disease organism directly on a monthly or quarterly basis.

Recently however, public health experts have recognized that certain protozoa which cause disease, such as giardia and cryptosporidium, can be present in surface water even when the total coliform test shows absence. Although an important exception, the total coliform test remains the standard for determining the bacterial quality of drinking water in the US and the world.

Members of the Total Coliform Family

There are a number of subgroups within the overall coliform family as shown in the diagram on the right. The presence of bacteria from each progressively smaller subgroup heightens the concern that disease-causing organisms may be present in the water. These groups and their relative risk implications are discussed below.

Total Coliform. These organisms are very prolific in the soil and their presence does not necessarily imply poor wastewater disposal or other sanitation based health risks. The presence of only total coliform generally does not imply an imminent health risk but does require an analysis of all water systems facilities and their operation to determine how these organisms entered the water system. "Public notice" to public water system users is required since a properly constructed and properly maintained water system should not have total coliform present.

Fecal Coliform. This is a much smaller group within the total coliform family. Fecal coliform generally originate in the intestines of mammals. They have a relatively short life span compared to more general coliforms. Their presence could be related to improper disposal of sanitary waste. Immediate "public notice" to public water system users is required due to the higher perceived risk of disease organisms also being presence in water.

Escherichia Coli (e-coli). This is a specific species (subgroup) within the coliform family. They originate only in the intestines of animals and humans. Like fecal coliform they have a relatively short life span compared to more general coliform. Their presence indicates a strong likelihood that human or animal wastes are entering the water system. Immediate "public notice" is required due to higher perceived risk of disease organism also being presence in the water.

Multiple tests can be used to determine the presence of coliforms. The membrane filter test produces a test results for non-coliform organisms in the water. This value has importance in the two areas:

When the number of non-coliform organisms is high, their presence may inhibit the growth of other more important organisms in the total coliform family. That action will invalidate that total coliform test.

It is believed that the number of non-coliform organisms in a well should be small. Thus when numbers are high, there is concern that the water in the well is not being sufficiently filtered. Reasons for a lack of adequate filtration include:

a. The well is not properly constructed; or

b. The soil / rock is not properly filtering the rainfall water that is percolating down from above.

For More Information

For more information concerning total coliforms please call DES's Water Supply Engineering Bureau at (603) 271-3139 or the DES laboratory at 603-271-3445. For more information concerning giardiasis and cryptosporidium see fact sheets WD-WSEB-4-6 and 4-7 respectively.

For an overall listing of water supply related fact sheets, please request WD-WSEB-15-2. We would appreciate your suggestions concerning this fact sheet.

Drinking water fact sheets are available through the DES web site at: http://www.des.state.nh.us/ws.htm then select: publications, fact sheets, water division, water supply. 8/98