The primary origin of coliforms in water supplies is

• A. Bacterial contamination
• B. Natural algae growth
• C. Industrial solvents
• D. Acid rain

Answer: (A)

Coliforms are indicator bacteria used to gauge the sanitary quality of a water supply. Their presence mainly signals bacterial contamination from fecal sources—such as sewage leaks, animal waste, or runoff carrying waste into the water. Because coliforms originate from the intestinal tracts of warm-blooded animals, finding them suggests possible intrusion by enteric pathogens, which is why their appearance is a red flag for contamination. The other options don’t fit because natural algae growth, industrial solvents, and acid rain are not sources of coliform bacteria; they relate to different types of contaminants (biological vs. chemical or environmental factors) and do not explain why coliforms would be present in the water.