Sampling variability of radon in soil gases

Abstract

Abstract Little information is available on the long-term reproducibility of radon in soil gases, or on the vertical distribution of radon concentrations in soil gas. For five sites in central Pennsylvania, the radon activity in soil gas at depths exceeding about 70 cm varies by factors of 3 to 10 during the year, and shows even larger variability at shallow depths. At depths greater than about 70 cm, values are lower in winter than in summer, apparently because radon is retained in the soil in some form other than the air phase, perhaps in soil moisture or sorbed on solids, as a result of increased moisture or decreased temperature during winter. The low radon values in winter are observed at depths below the frozen surface zone, so they do not result directly from freezing. Values at depths shallower than about 70 cm at one site studied in detail appear to average slightly higher in winter than in summer, but with erratic highs and lows defining a 3-fold annual range. At two other sites, the shallow values show patterns similar to the deep values, with highs in summer. Vertical profiles show reversal and other features differing from previous models for simple diffusive transport. Sampling during the summer period at depths greater than 75 cm shows the best reproducibility for these soils. Several sets of soil-gas measurements along a traverse crossing a fracture trace indicate that elevated radon values tend to occur over the fracture zone, but anomalies also occur outside the fracture trace, and show appreciable variability with time and precise location. The literature indicates that 3- to 10-fold seasonal variations are common. Most other studies show high radon in winter or in wet or frozen soils, attributed to capping by saturated or frozen soil, but clay-rich soils in Sweden have their highest radon in summer and fall. Soils that crack in dry weather can have low radon because of dilution by the atmosphere. These phenomena must be considered in comparing surveys performed during different seasons and interpreting surveys of soils with laterally differing texture or moisture content.