Radon‐222 monitoring of soil diffusivity

The temporal evolution of activity profiles of the radon isotope 222 Rn in the top 50 cm of soil is used to quantify diffusive gas transport in the air‐filled pore space. Air is continuously collected from gas‐permeable, hydrophobic membrane tubes placed at various depths under the surface and circulated through a high‐sensitivity Rn detector. With a micro‐processor controlled inlet system an automatic monitoring of several levels is possible for extended periods of time with a typical time resolution of one hour. In addition, a new dynamic approach to measure the in‐situ soil diffusivity D s (m²/s) around the subsurface tubes is presented where the recovery from air injection is evaluated every 3 hours. Within a test period of 3 weeks D s dropped from 10 −6 m²/s to 3 10 −7 m²/s at a depth of z=50 cm due to increasing water content of the soil.