Protocol for the Collection, Transfer and ANSI/AARST MW-RN 2020 Measurement of Radon in Water 18 SECTION 7.0 ACTIONS BASED ON TEST RESULTS 7.1 Action level guidance While there is currently no federally mandated or published action level for radon in water, several states have current or proposed maximum concentration limits for radon in potable water. It is advised to check with the state and local health departments on additional guidance for maximum concentration limits for radon in potable water. Recommended Action Level • 4,000 pCi/L or greater ( ≥ 150 Bq/L) or per local health department requirements - Take remedial action to reduce radon concentrations in the water supply for potable water to below the action level. • Below 4,000 pCi/L (< 150 Bq/L) or per local health department requirements - If greater than half the action level: As with radon contained in soil gas, there is no absolutely safe level for radon in a building’s water supply. Consider remedial action to reduce radon in the water supply for potable water if test results indicate radon concentrations greater than half the action level. - Seasonal verification: Because radon concentrations in groundwater vary by season, it is recommended that the water supply for potable water be measured at multiple times (seasons) during the year to determine the range of seasonal variability in radon concentrations. If seasonal variability is known to be acceptable, as a result of seasonal or semi-annual testing, water retesting should be repeated every 5 years to ensure against unexpected changes to radon concentrations in the water supply. 7.2 Consideration for retesting when two test results disagree 7.2.1 Acceptable When two water samples were collected the same time and location, the average of the two test results is to be used for determining needs for mitigation if: a) both test results are above the action level, or b) both test results are below the action level. Note—Some variation between duplicate sample results is typical. However, if the variation between duplicate results is unusually large, it might indicate problems in the collection process. 7.2.2 Where test results disagree on exceeding the action level When one test result is above the action level and the other test result is below the action level: a) Acceptable —If the higher result is less than twice the lower result, the average of the test results is to be used to determine if this location needs mitigation. b) Not acceptable —If the higher result is twice or more than the lower result: 1. For two co-sampled ( duplicate ) tests conducted at the same time, a repeated collocated sample collection for this location is required to obtain a valid measurement. 2. For two tests conducted in the same location but at different times, obtaining confirmation on whether mitigation is warranted requires additional testing.