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Radon (222Rn) is a natural gas produced by the radioactive decay of uranium-bearing rocks and soils in the Earth?s crust. Geology is one of the main factors controlling source and distribution of radon. Most rocks and soils possess small amount of uranium, but there are particular types of bedrocks and unconsolidated deposits characterized by higher than average uranium contents. These lithological types include some granites, light-colored volcanic rocks, phosphatic rocks, ironstones, limestones and shales rich i...

The Ogosta Valley downstream the town of Chiprovtsi, NW Bulgaria, was subject to long-lasting contamination with arsenic and other toxic elements due to mining and ore-processing activities in the second half of the last century. Although the exploitation of lead-silver and iron ores was ceased in the end of the 90`s, the soil in particular sections of the Ogosta?s river floodplain remained highly contaminated with arsenic, lead and other heavy metals. Arsenic mobilization and transport in the vadose zone is highl...