Radon and rain

[jtjat@pipeline.com]

My home in New Jersey is on a hill and at one time contained radon levels of
15 pCi/L in the basement, which required remediation through ventilation of
a pre-existing French drain system. A stream runs by the side of the house,
built in 1929. 

After 6 inches of rain fell in 24 hours on July 24, during which time all
windows in the house were closed, I recorded about 250 cpm throughout the
basement with a 2 inch GM thin-window pancake, and about half that on the
upper floors (normal background 40 cpm). This was little plating out of
radon daughters on the detector window, because when I went outside the
count rate fell to near normal. The ventilation pump was working. The effect
was transient: the count rate in the basement was at the background level 24
hours later.

In winter when the humidity is low I have observed accumulation of radon
daughters on fabrics and other static-prone surfaces up to 250 cpm, but I
have never found GM-detectable radon daughters in air before. I assume the
massive rainfall produced a sudden increase in the groundwater level that
somehow forced large amounts of radon into the house. Is this a well-known
phenomenon? Is it possible to estimate the radon concentration from the GM
count rate?

Jonathan Robert
jtjat@pipeline.com