Conceptual model for the origin of high radon levels in spring waters - the example of the St. Placidus spring, Grisons, Swiss Alps

A variety of geological, hydrochemical and isotopic techniques were applied to explain the origin of exceptionally high radon levels in the St.Placidus spring near the city of Disentis in the Swiss Alps, where an average of 650 Bq/(LRn)-Rn-222 was measured. Rn-222 is a radioactive noble gas with a half-life of 4 days, which results from the disintegration of radium (Ra-226). The high radon levels can neither be explained by generally increased radium content in the fractured aquifer rock (orthogneiss), nor by the radium concentration in the spring water. It was possible to show that there must be a productive radium reservoir inside the aquifer but very near to the spring. This reservoir mainly consists of iron and manganese oxides and hydroxides, which precipitate in a zone where reduced, iron-rich groundwaters mix occasionally with oxygen-rich, freshly infiltrated rainwater or meltwater. The iron, as well as the reduced and slightly acid conditions, can be attributed to pyrite oxidation in the recharge area of the spring. Radium cations strongly adsorb and accumulate on such deposits, and generate radon, which is then quickly transported to the spring with the flowing groundwater.