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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:
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You should test your well every year for bacteria. In addition, test for bacteria if:
Follow these sampling instructions carefully:
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.
Learn about the meaning of the test results on our Test Result Explanations page.
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 (PDF) can help identify structural defects in the system. After the defects are corrected, the system should be disinfected and the water retested before drinking.