The following information is meant to give you a basis for deciding whether or not to take any action to reduce radon levels in the well water. It is not a black and white issue.
It has never been completely clear what a safe level of radon in well water is. The EPA has equivicated for over 10 years about setting a guideline. Believe it or not, some statements from the EPA still suggest that levels up to 40,000 pci/l do not pose a substantial threat while simulatenousy the agency indicates that a level below 300 would be desirable. In the early 1990s, the Connecticut Department of Health Services (in conjunction with the American Lung Association) published a booklet suggesting treatment for private wells with radon levels measuring above 5,000.
The average radon in well water reading locally is somewhere around a few thousand pci/l. Therefore, good or bad, a certain amount of radon is to be expected in well water. Depending on your sensitivity to the issue, it might be reasonable to accept levels up to 5,000 as “coming with the territory”. In such a case, one should watch for further developments on the issue. Mitigation equipment could be installed sometime in the future, if appropriate.
If the well water tested indicates radon levels above 5,000 but less than 15,000, a charcoal filtering system can effectively be installed to reduce it. The cost is about $1,500+. Above 15,000, effective aeration equipment is available to remove the radon. The present cost of such equipment installed is somewhere around $3,500.
Precedents have been set that radon reduction systems are put in usually at the seller’s expense for high readings. The judgemental benchmark most widely used seems to be the state’s guideline of 5,000 pci/l. As time goes on, the norms may change.
Actual radon levels in well water should not vary as much from day to day as radon in air levels since underground water is insulated from the weather. Nevertheless, since sampling water to test for radon requires much more care than air testing, results may vary.