⇦ Back to Water Resource Management Home
¶ Introduction
Bacterial contamination of private water supplies is actually fairly common. Studies have shown that over 40 percent of private water supplies are contaminated with coliform bacteria. The simple presence of bacteria does not necessarily mean the water is unsafe to drink, but may require some precautions.
¶ Bacterial Sources
Bacteria of greatest concern in drinking water are those that originate from the gut of warm-blooded animals. Sources include wildlife, pets, and livestock. Human sources are a particular concern as they include bacteria of human origin and may include human pathogens.
Contamination problems often arise from improperly designed, failing, or overloaded waste water treatment systems, including septic systems from private homes, and leaking sanitary sewer pipes.
Surface waters typically contain bacteria. Flood waters often have very high bacterial levels from diverse sources.
Ground water should naturally be free of bacteria from animal sources. Bacteria are be filtered out by soil particles during natural recharge as rain water percolates downward. Proper well design and construction should protect coliform-free ground water from contamination by surface waters or other sources.
¶ Potential Health Effects
¶ Total coliform:
“Coliform” refers to a group of different kinds of bacteria commonly found in the environment, including soil, vegetation and untreated surface water. Total coliform bacteria are generally not harmful. Only disease-causing bacteria, known as “pathogens”, lead to disease.
¶ Fecal coliform:
These bacteria are a subgroup of total coliform that exist in the intestines and feces of humans and animals. The presence of fecal coliform bacteria (“E. coli”) in drinking water is a strong indication of recent contamination by sewage or animal waste. This indicates a greater risk that pathogens are present.
Other non-coliform microbes in these wastes may cause short-term effects (diarrhea, cramps, nausea, etc.) headaches or other symptoms, as well as potentially pose long-term health effects. They may pose a special health risk for infants, young children, some of the elderly and people with severely compromised immune systems.
¶ Heterotrophic bacteria:
This is a term for the most abundant bacteria in nature. These bacteria use carbon compounds for food and are found in soil, water, and food. They may be beneficial or harmful to humans. Presence of heterotrophic bacteria in water does not pose a human health risk, but a high population indicates ideal conditions for bacterial growth.
¶ Figure 1. Color Change Test for Coliform
¶ Testing for Bacteria
Public water supplies are regulated by the Environmental Protection Agency (EPA). EPA requires all public water supplies to regularly test for bacterial contamination.
Testing for all potential pathogens would be impractical and expensive. Total coliform testing is relatively easy and inexpensive, so is used as an indicator of microbiological water quality. Coliform bacteria react in the natural environment and to water treatment similar to many pathogens.
The EPA standard for total coliform bacteria in drinking water is “zero” total coliform per 100 milliliters of water. In routine water testing using the color change method, results are reported as simply “present” or “absent” to indicate whether or not bacteria were detected (see Figure 1). More extensive tests use plate counts or other methods to quantify coliform populations.
A water sample testing positive for total coliform is not necessarily unsafe to consume because pathogens are not necessarily present. A positive test does however indicate that there is a high potential for pathogens to be present.
A positive test for fecal coliform bacteria indicates recent fecal contamination. This suggests there is a very high risk that pathogens are present.
¶ Treating Bacterial Contaminations
The EPA requires that all public water suppliers provide coliform-free water. A positive coliform test requires public notification. Private water supplies are themselves responsible for providing coliform-free and pathogen-free drinking water.
A sample tests positive for the presences of total coliform, use an alternative water supply or disinfect the water supply for drinking and food preparation. Continue until further coliform testing is negative.
Untreated water can generally be used for showering and bathing as long as the water is not swallowed.
¶ Boiling:
Bring water to a vigorous rolling boil for one minute for short-term disinfection of drinking water. Boiling any longer can concentrate other chemical contaminants that may be present. Since water boils at a lower temperature as elevation increases, boil for three minutes at altitudes above 6,600 feet (2,000 meters).
¶ Chlorination:
Household chlorine bleach without fragrance or other additives and with 4 to 6 percent sodium hypochlorite is an option for disinfecting small amounts of water used for dish washing.
For clear water, add six drops per gallon of water using a medicine dropper. Strain very cloudy water through a clean cloth, and add a larger dose of 16 drops per gallon.
Stir the water and let it stand covered for 30 minutes. For adequate disinfection, the water should have a light chlorine odor to it after the 30-minute waiting period. If this odor is not present after 30 minutes, repeat the dose and let it stand covered another 15 minutes.
¶ Shock Chlorination:
If total coliform and/or E. coli bacteria are detected in a private water supply, an immediate effort should be made to identify and eliminate the source of contamination. After addressing the contamination source, disinfect the entire water system, using shock chlorination. Table 1 has a short list of selected publications with shock chlorination instructions.
After shock chlorination, submit another water sample for testing. The water should test negative before use.
¶ Other Methods:
If the source of bacterial contamination or well construction errors cannot be identified and eliminated, continuous disinfection of the water supply may be necessary. Options include continuous chlorination, ultraviolet radiation, distillation, and ozone treatment.
¶ Table 1. Shock chlorination references |
| ServiTech Laboratories. 2015. Crop File 5.02.202, “Treating Bacterial Infestations in Water Supply Wells”. 2 pages |
| Powell, et. al. 2005. Publication MF-911 revised. “Shock Chlorination for Private Water Systems”. Kansas State Univ. Cooperative Extension, Manhattan KS. 4 pages. |
| Saha, et.al. 2012. Circular 858-4 revised. “Disinfecting Your Well Water: Shock Chlorination”. University of Georgia Cooperative Extension. 4 pages. |
| Benham & Ling. 2011. Publication 442-663. “Shock Chlorination: Disinfecting Private Household Water Supply Systems”. Virginia Cooperative Extension. 6 pages. |
¶ References
USDA Extension. 2019. Drinking Water Contaminant – Bacteria. accessed 06July2022. https://drinking-water.extension.org/drinking-water-contaminant-bacteria/
Mancel, K.M. 1989. Bull. 795. Bacteria in Drinking Water. Ohio Coop. Ext. Svc. 11 pg.