Abstract

An environmental geophysical survey was conducted to assess the radon-hazard potential of the southern St. George Basin. Levels of uranium in soil, and radon in soil gas, were measured.

Uranium (²³⁸U) levels measured by gamma-ray spectrometry were highest (up to 6.7 parts per million [ppm]) in shale and residual soils on Triassic bedrock, and were also high (up to 3.4 ppm) in granular soils of the Virgin River flood plain and fine-grained soils of the Washington Fields area. Shale in the Petrified Forest Member of the Triassic Chinle Formation is bentonitic and derived from the alteration of volcanic ash. Flood-plain soils are derived in part from Miocene granitic rocks eroded and transported from the Virgin River drainage basin in the Pine Valley Mountains to the north. Soils of the Washington Fields area are both of flood-plain arid residual origin.

The levels of radon (²²²Rn) in soil gas were measured by radon emanometry, and were found to be highest, up to 878 picocuries per liter (pCi/L) (3.25 × 10⁴ Becquerels per cubic meter [Bq/m³]), in areas of high uranium levels. Shallow ground water and low soil permeability, however, decrease soil-gas radon levels. In areas of shallow ground water, radon dissolves and is transported with the water rather than in soil gas. Areas of low soil permeability inhibit soil-gas transport.

Linear regression of radon-uranium data pairs shows that concentrations are related by the formula Rn = 52.8 U + 7.5, where Rn is the concentration of radon in pCi/L and U is the concentration of uranium in ppm. The sample size is 82. At the 99 percent confidence level, the correlation coefficient of 0.415 exceeds the threshold value of 0.283.

Uranium concentrations, ground-water levels, and soil permeability were combined to derive a map showing the relative potential for elevated indoor-radon levels in the southern St. George Basin. The radon-hazard potential is highest south of St. George. Here, uranium levels are relatively high, and either ground water is deep, soil is permeable, or both. The radon-hazard potential is lowest in the northern part of the study area where uranium levels are relatively low, and either ground water is shallow, soil is impermeable, or both.

Levels of radon in soil gas were used to verify the accuracy of the hazard-potential map, but were not used to assess the hazard potential. Variations in hazard potential approximate variations in soil-radon levels, but differences are the result of the sampling grid, map scale, and the influence of physical phenomena not considered in the hazard evaluation because of difficulty in quantification. Such physical phenomena include meteorological effects; changes in ground-water pressure, temperature, fluid solubility, and gas content; and the effect of grain size on radon emanation. These physical phenomena ultimately influence indoor-radon levels, which are also affected by such non-geologic factors as foundation condition, building ventilation, construction material, and life styles.