Under the Ground Water Rule, high-risk systems must achieve what log inactivation of viruses?

• A. 4-log, Cryptosporidium
• B. 2-log, Viruses
• C. 4-log, Viruses
• D. 2-log, Cryptosporidium

Answer: (D)

The key idea is that high-risk groundwater systems must achieve a 4-log inactivation of viruses. In practical terms, a 4-log reduction means reducing the viral concentration by a factor of 10,000 (leaving only about 0.01% of the original viruses). This higher standard is used because viruses can be present at low levels and groundwater environments can challenge disinfection effectiveness, so a substantial margin of safety is required to protect public health. Cryptosporidium is addressed separately in regulations and its control is not the target of this particular virus inactivation requirement. So the mandated level is a 4-log inactivation of viruses, not a smaller reduction or a reduction targeting a different organism.