In groundwater systems that disinfect and have deficiencies, 4-log inactivation of viruses must be demonstrated with adequate C x T.

• A. Cryptosporidium, that coliforms are absent in 95% or more of all samples collected for sampling period
• B. Cryptosporidium, adequate C x T
• C. viruses, adequate C x T
• D. viruses, that coliforms are absent in 95% or more of all samples collected for that sampling period

Answer: (C)

Disinfection performance is evaluated using the CT concept (concentration times contact time) to achieve a specific log reduction for a given pathogen. For groundwater systems that disinfect and have deficiencies, the requirement is to demonstrate four-log inactivation of viruses with adequate C x T. This means the combination of disinfectant concentration and contact time must be sufficient to reduce viral particles by 10,000-fold under the expected conditions.

The emphasis is on viruses, not Cryptosporidium, and not on the absence of coliforms. Coliform absence is a general sanitary indicator, but it does not guarantee the required level of viral inactivation. Cryptosporidium is known to be very resistant to disinfection, so the standard target here focuses on viruses with an appropriate CT value to achieve the 4-log reduction.