Interpretation of Coliform Lab Results

There are a variety of bacteria, parasites and viruses that cause health problems when humans ingest them in drinking water. Testing water for each of these germs is difficult and expensive. Instead, water quality and public health workers measure coliform levels. The presence of coliforms in drinking water suggests there may be disease-causing agents in the water.

Coliforms are a broad class of bacteria that live in the digestive tracts of humans and many animals. Labs may test for total coliforms, fecal coliforms, or E. coli, any of which indicate microbial contamination. Results are generally reported as no coliforms present, the actual number of organisms detected per 100 ml of water, or as too numerous to count. Some labs may simply report results as bacteriologically safe or unsafe. If your drinking water contains more than 1 total coliform org/100 ml or is reported as unsafe bacteriologically, the well should be disinfected and retested in one to two weeks. If subsequent tests indicate bacteria are still present, the source of the contamination must be identified and eliminated before the water is safe to drink.

Disinfecting Contaminated Wells

There are several options for private water supply disinfection. These include continuous Chlorination, Shock Chlorination, Ultraviolet Radiation (UV), Ozone Treatment, Boiling and Pasteurization. Each of these methods has advantages and limitations, but they are all intended for use on clean, clear water. Water supplies must be sealed and protected from sources of bacterial contamination for disinfection methods to function properly.

Chlorination is the standard method for disinfecting wells because it is highly effective against bacteria. However, the drawbacks include: safety issues in handling concentrated chlorine; the taste it gives the water; the required contact time; its variable effectiveness against other microorganisms.