Review of investigations into the vadose zone's variable state of saturation in connection with the assessment of radon potential in Bulgaria

Natural radon (222Rn) is a radioactive noble gas that occurs as the immediate decay product of radium (226Ra), part of the 238U family, in the lithosphere. Radon is driven by advection and diffusion with soil gas throughout connected and water-unsaturated pores and/or cracks in permeable rocks and soils. The aim of the present study is to do a review of the existing so far research activities in Bulgaria in connection with the observation and/or evaluation of the degree of water saturation of the near-surface layer, and on that base to distinguish the up-to-date achievements in regards to the radon potential in situ evaluation. Due to this review, the studies in Bulgaria concerning moisture dynamics in the near-surface layers can be divided mainly into two groups. The first one investigates the hydraulic characteristics (parameters) of soils in the vadose zone. Based on that, conclusions or computer simulations for the saturation degree estimation can be drawn. The other group includes in situ observations by sensors on the change of moisture with time. The results of these studies may serve as a base for more precise moisture dynamics assessment at sites with specific radon potential tendencies.