Radon-222 concentrations in ground water and soil gas on Indian reservations in Wisconsin

The weighted average radon-222 concentration of indoor air in homes located on Wisconsin Indian Reservations is 5.8 picocuries per liter, which exceeds the U.S. Environmental Protection Agency action limit of 4 picocuries per liter. Ground water is the principle source of drinking water on Wisconsin Indian Reservations and generally accounts for about 5 percent of the total indoor air radon-222 concentrations found in homes. To determine the distribution of radon-222, ground water from 29 private and community Wisconsin Indian Reservation wells and soil gas at a depth of about 3 feet below land surface adjacent to the wells were sampled. Sites with wells were distributed among the 11 Wisconsin Indian Reservations so that each Reservation contained at least 2 sites. The remaining seven sites were divided among the Reservation by acreage held by each tribe.

Ground-water samples were collected using a syringe technique after the wells had been purged sufficiently to reach chemical stability. Samples were then sent by overnight mail to the U.S. Geological Survey, National Water Quality Lab for analysis by liquid scintillation counting. Soil gas was collected in Lucas cells, using a small-diameter soil probe and peristaltic pump. The cells were then analyzed by Lucas cell alpha scintillation counting with a portable radon detector.

The highest radon-222 concentrations in ground water and soil gas are from sites with wells finished in the crystalline bedrock aquifer. Radon-222 concentrations in 29 ground-water samples collected from the sand and gravel and sedimentary and crystalline bedrock aquifers range from 260 to 22,000 picocuries per liter with a median concentration of 560 picocuries per liter. Only 2 of the 29 ground-water samples have radon-222 concentrations less than the U.S. Environmental Protection Agency proposed standard of 300 picocuries per liter. The highest radon-222 concentrations were found in ground water from wells in Shawano, Menominee, Forest, and Marathon counties. Radon-222 concentrations in soil gas range from 130 to 7,810 picocuries per liter with a median concentration of 560 picocuries per liter.

For sites with wells finished in the sand and gravel aquifer, the coefficient of determination (R2) of the regression of concentration of radon-222 in ground water as a function of well depth is 0.003 and the significance level is 0.32, which indicates that there is not a statistically significant relation between radon-222 concentrations in ground water and well depth. The coefficient of determination of the regression of radon-222 in ground water and soil gas is 0.19 and the root mean square error of the regression line is 271 picocuries per liter. Even though the significance level (0.036) indicates a statistical relation, the root mean square error of the regression is so large that the regression equation would not give reliable predictions. Because of an inadequate number of samples, similar statistical analyses could not be performed for sites with wells finished in the crystalline and sedimentary bedrock aquifers.