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Testing Your Well Water for Bacteria

Coliform bacteria are found in the environment and in the feces of humans and animals. Most coliform bacteria do not cause illness, but they can indicate that other disease-causing organisms are present in the water. Waterborne illness from these other organisms can cause nausea, vomiting, fever and diarrhea.

See our Water Testing Fees and Services page for additional water tests available!

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How often should I test my well water for bacteria?

You should test your well every year for bacteria. In addition, test for bacteria if:

  • You notice a sudden change in your water's taste, appearance, or odor.
  • The water turns cloudy after rainfall or the top of the well was flooded.
  • You suspect a contamination source (sewage system, barnyard, etc.) is within 50 feet of your well.
  • Family members are experiencing unexplained flu-like symptoms.
  • Your water supply system has been serviced or you have made changes to the system.

How do I test my well water for bacteria?

  • Obtain sample bottles from the Environmental Health office.
  • When you are ready to sample, remove the aerator from the faucet, if possible. (If ithe aerator is difficult to remove, just leave it on). Dip the tip of the faucet into a small container of bleach for 30 seconds. Leave the bleach on the faucet for 5 minutes to kill any bacteria present.
  • After disinfection, turn on the cold water full stream and let it run for 5 minutes. While the water is running, remove the lid from the sample bottle. DO NOT touch the interior of the bottle or lid. Also, leave any pill or powder in the bottle. Turn the faucet down to a small stream and collect the sample. Fill the bottle up to the neck of the bottle (but don't overflow!) and replace the lid.
  • Put the aerator back on the faucet.
  • Return the sample to the Environmental Health office. Samples must be delivered to our office by 3:00 p.m. Monday through Friday. It is best to collect the sample just before delivery to our office. Samples should be refrigerated or chilled while stored or transported. On dates prior to holidays, no samples will be accepted.
  • Our office will call you if bacteria is present in your well. Otherwise, you will receive a copy of your results in the mail.
  • If you receive a notice that your sample is positive for bacteria, you may want to disinfect your well.

Total coliform, fecal coliform, and E. coli – what’s the difference?

Total coliform, fecal coliform, and E. coli are all indicators of drinking water quality:

  • Total coliform bacteria are found in the environment (soil or vegetation) and are usually harmless. If only total coliform bacteria are found, the source is probably environmental. If environmental contamination can enter the system, there may be a way for other pathogens to also enter the system.
  • Fecal coliform bacteria are a sub-group of the total coliform group. They are found in the intestines and feces of warm-blooded animals. Fecal coliform in a drinking water sample often indicates recent fecal contamination, meaning there is a greater risk that other pathogens are present.
  • E. coli is a subgroup of the fecal coliform group. They are also found in the intestines of people and warm-blooded animals. Most E. coli are harmless but some strains may cause serious illness. E. coli in a drinking water sample indicates recent fecal contamination, meaning other pathogens are likely present. Most outbreaks of E. coli are due to food contamination caused by a specific strain known as E. coli 0157:H7, which can cause serious illness and death. When a drinking water sample is reported as “E. coli present” it does not mean this strain. However, it does indicate recent fecal contamination and should be addressed.


How can bacteria get into drinking water?


Coliform bacteria do not occur naturally in groundwater. However, coliform bacteria can live within slime formed by naturally occurring ground water microorganisms. The slime (or biofilm) clings to the well’s screen, casing, drop pipe, and pump. Disturbances during well construction, pumping or maintenance can cause the slime to dislodge, releasing the coliform bacteria into the water. The following can also lead to contamination:

  • Missing/defective well cap - seals around wires, pipes or where the cap meets the casing may be cracked
  • Cracks or holes in the well casing - allow water that has not been filtered through the soil to enter the well (common in wells made of concrete, clay tile, or brick)
  • Many older wells were not sealed with grout when constructed - allows contaminant to seep into well
  • Well flooding - common problem for wellheads below ground in frost pits that flood during wet weather
  • Close proximity of a well to septic tanks, drainfields, sewers, drains, privies, barnyards, animal feedlots, abandoned wells and surface water - contamination can enter the well
  • Cross-connections with wastewater plumbing - wastewater can mix with the well water

What do the results mean?

If coliform bacteria are present in your drinking water, your risk of contracting a water-borne illness is increased. A positive total coliform sample should be considered an indication of pollution in your well.   Positive fecal coliform results, especially positive E. Coli results, should be considered indication of fecal pollution in your well.

What should I do if coliform bacteria are detected in my well?

Be concerned but do not panic if coliform bacteria are detected. Resample immediately if you have a positive sample before you treat, repair or replace the well. If you receive a second positive sample for total coliforms, or if the initial sample is positive for fecal coliforms, do not drink the water. Bring the water to a rolling boil for three minutes to kill the bacteria. You may also want to use bottled water as a temporary water source.

How can I eliminate coliform bacteria from my well water?

If coliform bacteria are present, the source of the problem should be identified. Resampling from several locations within the water system may be helpful. The entire water system may need to be thoroughly flushed and disinfected before a negative bacteria sample can be obtained. A well drilling contractor or a Washtenaw County Sanitarian can help identify structural defects in the system. After the defects are corrected, the system should be disinfected and the water retested before drinking.

For more information, please contact:

  • Your Area Sanitarian (based on your Township)
  • Washtenaw County Environmental Health Division - (734) 222-3800


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