Wells are routinely tested for coliform bacteria, which come from soil or vegetation, and in the intestinal tract of warm-blooded animals (fecal coliform bacteria). The many sources of bacterial pollution include runoff from woodlands, pastures, and feedlots; septic tanks and sewage plants; and animals (wild or domestic).
Most coliforms are harmless residents of soil and will not make people sick. Some strains of E. coli, the most common fecal coliform bacterium, may be pathogens. Some E.coli found in food have been lethal, so their presence should be taken very seriously. Testing for coliform bacteria is inexpensive and their presence indicate that harmful, pathogenic bacteria could possibly enter or exist in the well. So, if a well tests positive for coliform bacteria, follow-up testing for E.coli sometimes is recommended depending on the specific lab test results. Some coliform tests can provide results for both total coliform bacteria (the indicator) and E. coli, the sign of fecal contamination and potential pathogens.
The federal standard for bacteria in public water systems is zero presence. While private, household wells are not regulated by the federal government, public health authorities recommend that private well owners also keep their well water free of all coliform bacteria. A sanitary well water is not entirely sterile (free of bacteria) but may contain naturally occurring, harmless to health, bacteria in low numbers. Appearance of slime (biofouling) buildup in water system components indicates a need for cleaning to prevent potential sanitary issues.