The effect of grain size, sorting, and emanation on measurement of radon in soil: A comparison of two measurement techniques

Soil surveys of radon were conducted in the Coastal Plain of New Jersey, Alabama and Tennessee. Also along each transect, permeability, soil grain size, uranium, thorium, and radium were measured. Soil-gas radon concentration was measured using two techniques: MK-II Radon/Permeability Sampler (RPS) and the Reimer grab sampling technique (RG). In well sorted, medium- to coarse-grained sands the two gas sampling techniques yield similar radon concentrations, usually within 10% of each other. In poorly sorted sands the RG obtains consistently higher concentrations than the RPS. Moisture produces secondary effects on sampling and can determine whether a sample can be obtained at all. Under high moisture and saturated conditions, the RPS cannot obtain a radon sample whereas the RG will obtain a sample 50% of the time. Permeability data show no apparent correlation with radon concentration. Radon in soil gas from the Coastal Plain sediments averaged between 700-1,000 pCi/L with a maximum of 16,200 pCi/L measured in the Nevesink Formation in NJ. Uranium and radium analyzed from soil at one meter depth average 1.3 ppm and 0.7 pCi/g, respectively and do not appear to be in disequilibrium. Permeability varies from 1.2x10-5 cm2 in coarse-grained sands to impermeable in clay. High soil-gas radon (>1,000 pCi/L) is associated with the occurrence of glauconite, phosphate, and organic material in the sediments. Equivalent uranium from gamma spectrometry at the surface and uranium and radium measured chemically in the soil have been compared with the soil-gas radon concentrations. A positive correlation exists. However, relatively anomalous amounts of soil-gas radon with respect to its parent radionuclides are found in a variety of geologic settings. The common factor causing these anomalous concentrations is the occurrence of metal oxides with U and RA, coating grains and fractures, and dramatically enhancing the emanation of the rocks and soils