Radon is a natural radioactive gas produced by the decay of uranium and thorium in rocks and soils. High surface radon concentrations are commonly associated with high levels of uranium in underlying rocks. High radon levels can also occur in groundwater, even in rocks with normal crustal uranium concentrations. The discharge of groundwater via faults and fractures can produce radon anomalies unrelated to the distribution of uranium. The surface distribution of radon is therefore influenced by the distribution of uranium and of transport pathways along faults and fractures.
The principal radon isotope (Rn-222, half-life 3.825 days) is capable of migrating significant distances from its source, both as a gas and in solution. In the gaseous form radon can migrate into houses where, in high concentrations, it can represent a health hazard.
The distribution of radon in streams in several areas of southwest England has been investigated. The results show a clear association of high radon levels with the uranium-enriched granite of the region, although this may simply reflect the high fracture permeability of the granite rather than its uranium enrichment with a strong influence of major fracture systems. With regard to radon as a potential health hazard, both source and transport pathways must be identified if high-risk areas are to be located. Surveys of radon in surface waters may provide a useful and speedy technique for the broad identification of such areas.