What does E. coli mean in well water results, and what is the maximum allowed in lab results?

• A. Environmental contamination by organic matter; 5 CFU/100 ml
• B. Fecal contamination from animals or humans, 0 CFU/100 ml
• C. Industrial pollutants, 0 CFU/100 ml
• D. No contamination allowed, 100 CFU/100 ml

Answer: (B)

E. coli is used as a fecal indicator in well water testing. Its presence means fecal material from humans or animals has contaminated the water, which raises the risk that pathogens could be present. Because of this, the typical standard for drinking water is that there should be no detectable E. coli in a 100 mL sample, i.e., 0 CFU per 100 mL. The other options misinterpret what E. coli indicates or set an allowable level that would imply unsafe water; E. coli is not a marker for environmental organic matter or industrial pollutants, and allowing up to 100 CFU/100 mL would indicate substantial contamination.