How did the EPA come up with its 5 picoCuris per litre (pCi/L) limit for Combined Radium?

The lowest measurable level for Combined Radium was 5 pCi/L in 1976. By 1991, studies showed that a more appropriate limit might be 20-30 pCi/L (in which case the city would be in compliance). Unfortunately, the EPA's Safe Drinking Water Rule has a "non-backsliding" clause, which restricts the EPA's ability to "relax" limits once established.

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1. Is our water safe?
2. What are radionuclides?
3. How did the EPA come up with its 5 picoCuris per litre (pCi/L) limit for Combined Radium?
4. How do you resolve the radionuclide problem?
5. What efforts has the city made in the past to address radionuclides in the groundwater?
6. What are we doing about the radionuclides now?
7. When will we start construction?
8. Even if we reduce radionuclides now, is there a risk from radionuclides in the water pipes?