Radon-222 monitoring of soil diffusivity

The temporal evolution of activity profiles of the radon isotope Rn-222 in the top 50 cm of soil is used to quantify diffusive gas transport in the ai r-filled pore space. Air is continuously collected from gas-permeable, hydr ophobic membrane tubes placed at various depths under the surface and circu lated through a high-sensitivity Rn detector. With a micro-processor contro lled inlet system an automatic monitoring of several levels is possible for extended periods of time with a typical time resolution of one hour. In ad dition, a new dynamic approach to measure the in-situ soil diffusivity D-s (m(2)/s) around the subsurface tubes is presented where the recovery from a ir injection is evaluated every 3 hours. Within a test period of 3 weeks D- s dropped from 10(-6) m(2)/s to 3 10(-7) m(2)/s at a depth of z = 50 cm due to increasing water content of the soil.