Their presence in your well is usually the result of sewage contamination from a nearby source. How to interpret laboratory results . Do not attempt to apply these interpretations to surface water used for swimming. Maintain regular testing. Repeat samples may not show exactly the same results because bacteria are not distributed uniformly in water.
Contamination tends to enter intermittently.
Common with new wells before disinfection and shallow dug wells which are not properly scaled. The water is contaminated and should not be used for drinking under any circumstances. The contamination is likely due to wastes from human or animal origin. Number has been estimated due to some interference with the test. Exact number is not really critical, especially if in excess of limits shown above, for judging safety.