What are key water quality indicators used in public health?

• A. pH and turbidity only.
• B. Lead and mercury levels exclusively.
• C. Temperature and salinity.
• D. Total coliforms (contamination indicator) and E. coli (fecal contamination indicator); free chlorine residual indicates disinfection effectiveness.

Answer: (D)

Key water quality indicators in public health focus on whether water may pose microbial risks and whether disinfection is working. Total coliforms are used as a contamination indicator, signaling that microbes may be present in the water system. E. coli is a more specific indicator of fecal contamination, pointing to possible exposure to enteric pathogens. Free chlorine residual shows how effectively disinfection is being maintained throughout the distribution system, indicating ongoing protection against microbial growth. Together, these indicators provide a practical picture of microbial safety in water.

Other options miss the core idea: pH and turbidity relate to aesthetic and treatment process considerations rather than direct microbial risk; lead and mercury are important chemical contaminants to monitor but do not serve as general indicators of overall microbial safety; temperature and salinity are not standard public health indicators for monitoring microbial quality in drinking water.