Private Well With High Water Radon and Elevated Basement Air
An owner of a rural home with a private drilled well calls after a 48-h basement CRM test reads 14.8 pCi/L; a first-floor 48-h test reads 3.1 pCi/L. The family showers, washes laundry, and uses the well for all domestic water. You collect a water sample in a proper container, with headspace minimized, and submit it to a certified lab; the result is 22,000 pCi/L. The owner has no water treatment and asks whether a point-of-use carbon filter in the kitchen will solve the problem. The basement is unfinished with a sump and visible block-wall cracks; the first floor is occupied daily. The owner wants to know whether to fix water or air first and what re-testing is needed.
1. Interpret the water result and its relationship to the air results.
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The water result of 22,000 pCi/L is well above the 10,000 pCi/L level at which treatment is normally considered for a private well. A rough rule of thumb is that 10,000 pCi/L in water can contribute about 1 pCi/L to indoor air, so 22,000 pCi/L might contribute roughly 2 pCi/L. That does not explain the 14.8 pCi/L basement result; soil gas is the dominant source there, especially with a sump and block-wall cracks. The first-floor result of 3.1 pCi/L is lower, which is consistent with soil gas entering mainly at the basement. Both the air and water pathways need attention, but they are not the same problem.
2. What treatment order and type would you recommend, and why is a kitchen point-of-use carbon filter inadequate?
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Address the air first because 14.8 pCi/L is the immediate inhalation exposure and is driven mainly by soil gas: recommend ASD and seal major soil-gas entry routes such as the sump and wall cracks. For water, a point-of-use carbon filter in the kitchen does not protect the family during showers, laundry, or other whole-house water use; radon is released throughout the house. A whole-house aeration system is generally preferred for high water concentrations because it strips radon before use and avoids the disposal and buildup issues associated with granular activated carbon. A whole-house carbon system may be considered in some cases, but it requires maintenance and radioactivity considerations. The water and air systems should be coordinated, not substituted for each other.
3. What re-testing should be done after treatment?
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After the ASD system is installed and operating normally for at least 24 h, run a new 48-h post-mitigation air test under closed-house conditions in the basement, and consider a concurrent test on the first floor. Do not use the pre-mitigation 14.8 pCi/L as the post-mitigation result. After the water treatment system is installed and operating, collect a new water sample from the treated supply using a certified lab and proper sampling technique. Re-test the air after water treatment as well, because the water contribution should drop. Compare the new results with the treatment goals and document both the air and water pathways; if the air remains at or above 4 pCi/L, further diagnostic work is needed.