Where did that aberrant E.coli result come from?

It is perhaps not surprising that a positive Escherichia coli result in a drinking-water sample from a well treated supply that usually complies may be regarded with suspicion. After all, if the treatment plant is working well and there are no other indications of a problem, and perhaps the water even has an FAC residual, how can the water really still contain E. coli? It is tempting to think of the result as an aberration or “false positive” caused by contamination of the sample. However, there are a number of possible explanations of such a result: sample contamination, laboratory error, false positive E. coli, inadequate disinfection, post-treatment contamination, and the erratic distribution of microbes in water.

The various causes of drinking-water E. coli transgressions are discussed as well as how they may be investigated and prevented.