Anomalously high concentrations of uranium, radium and radon in water from drilled wells in the Helsinki region

The concentrations of uranium, 226Ra and 222Rn were determined in 308 drilled and 58 dug wells in the Helsinki region. The study area was about 400 km2 and geologically highly variable, with granites, amphibolites and migmatites the dominant rocks. The radioactivity of water in the dug wells was on a 'normal' level, but in numerous drilled wells it was anomalously high. In 14 drilled wells the concentration of uranium exceeded 1000 μg/l, the highest concentration being 14,870 μg/l. For 222Rn the maximum concentration was 880,000 pCi/l. The 226Ra/228Ra and 230Th/232Th activity ratios showed the isotopes of the uranium series to be dominant in the study area. A state of disequilibrium between 238U and 234U was very common in the samples. The 234U/238U activity ratios varied in the range 1.0 to 4.0 regardless of the amount of uranium in the water. The conclusion can be drawn from the isotopic data that the high radioactivity of water is in some cases caused by primary uranium mineralizations, but mostly by uranium deposited in fissures of the bedrock. The paper includes a summary of the results of two studies carried out between 1967 and 1977. (author)