LABORATORY STUDY OF WATER TABLE DEPTH AND SOIL-GAS RADON EMANATION

Measurements of radon emanation from soil have been obtained with a tall column of soil, apparatus to vary the water table depth, and a continuous radon monitor placed on the top of the soil column. In the laboratory setting, a rising water table causes an increase in radon emanation at the top of the water column. A falling water table causes a decrease in radon emeanation from the soil column. We speculate that the water, enriched in radon due to the high solubility of radon in water, brings "evaporating" radon toward the surface as the water table rises, and when this radon-rich water table falls, the radon "evaporating" from the water is spread throughout a greater volume of soil between the water table and the surface, resulting in a greater dilution of the radon gas in the soil (causing a decrease in the radon measurement at the top of the water column). These results may explain the seasonal differences noted in the radon concentration of homes with basements. In the winter, as precipitation increases, and radon "evaporating" from the rising water table can cause the observed winter increases of radon concentrations in basements. In the summer, dryer conditions cause the water table to fall, perhaps causing the commmonly observed decrease in basement indoor radon concentrations.

It was also found that when the water table remains constant at a shallow depth, radon emanation from the top of the soil column is less than when the water table remains constant at a greater depth. These results presumably occur because radon emanation off the grains of soil in water-free pores above the water table determine the quantity of radon gas that can rise to the surface of the soil column. That is, under stable conditions, a deeper water table produces a greater thickness of water-free soil grains, which results in greater concentrations of radon at the top of the soil column. These results may explain other seasonal differences in basement indoor radon concentrations. That is, in the fall, before precipitation increases, the water table is low and basement indoor radon is relatively high. In the spring, after the winter precipitation, the water table is high and basement indoor radon is commonly lower than in the fall.