What are bacteria?
Bacteria, microscopic in size, are the smallest single-celled organisms that can live completely on their own because they have the ability to ingest food and transform it into energy. The vast majority of bacteria found in water do not cause disease; however, some can. These are called pathogens.
What kind of bacteria can be found in well water?
Coliforms are bacteria naturally present in the environment and used as an indicator that other, potentially harmful, bacteria may be present. Fecal coliform and E. coli are bacteria whose presence indicates that water may be contaminated by human or animal wastes and harmful to human health.
What are sources of bacteria and how do they enter a private water system?
Potential sources or pathways of bacteria include:

  • Runoff from woodlands, pastures, and feedlots; septic tanks and sewage plants; and animals (wild or domestic).
  • Backflow from contaminated sources such as a sink-top carbon filter or bucket of water.
  • Reduced pressure or suction in water lines that draws in soil water at the pipe joints.
  • Through faulty sanitary seals in the well system such as when the well cap, grout around the well casing, or pitless adapter that connects an underground water distribution pipe to the well are compromised.

Many types of bacteria are native or adapted to saturated sediments and rock, and are present in significant numbers in aquifers.

What are the health risks associated with bacteria?
While most coliforms are not pathogens, they serve as indicators of the microbial quality of water. Pathogens—the bacteria, protozoa, and viruses that make people sick—can be rare and difficult to detect even if they are present in the water. Total coliforms are indicators and are more common and easy to grow. Testing for them provides a margin of safety.

Pathogens may not be present if coliforms are, but some strains of E. coli have been lethal, so their presence should be taken very seriously. Other health effects from pathogenic bacteria can include diarrhea, cramps, nausea, headaches, or other symptoms. They may pose a special health risk for infants, young children, and people with severely compromised immune systems.

People become accustomed to the natural bacteria in their water while guests may experience some discomfort or diarrhea.

Is my private well at risk?
The best way to assess your well system’s risk for bacteria is to 1) test the water annually for bacteria, and 2) to periodically get an inspection to make sure there are no maintenance problems that might allow bacteria to infiltrate the well system.

Well inspections should be done by a qualified water well system professional and include visual inspect of the wellhead, well system components, and other related equipment; physical inspection of well system components; and documentation of the entire inspection.

What is the measurement of bacteria?
The U.S. Environmental Protection Agency Maximum Contaminant Level Goal for coliform bacteria, fecal coliforms, and E. coli is zero.
What types of treatment solutions are available to private well owners?
Treatment methods include:

  • 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.

Sometimes conditions may dictate using a combination of methods to effectively deal with bacterial contamination of well water. All of these methods require proper maintenance by a qualified professional.

Where can I get more information?
In addition to the information on, you can call the toll-free Private Well Owner Hotline at 855-420-9355 or visit the National Ground Water Association Website, You can learn more about appropriate treatment technologies for bacteria by visiting the Websites of two organizations that certify home drinking water treatment systems: The National Sanitation Foundation at and the Water Quality Association at