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

Abstract

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/L 222 Rn was measured. 222 Rn is a radioactive noble gas with a half-life of 4 days, which results from the disintegration of radium ( 226 Ra). 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.

Diffuse surface emanations as indicator of structural permeability in fault-controlled geothermal systems

“…The measured radon and thoron activity concentration at the surface depends on the uranium and thorium concentration in the subsurface. Having both values measured, migration time from the reservoir to the surface can be estimated (Gainon et al, 2007).…”

Review: Thermal water resources in carbonate rock aquifers

“…Mixing of reducing water from deep flow systems, including dissolved Fe 2+ and Mn 2+ with oxygen-rich water from shallower flow systems, can cause precipitation of iron and manganese oxides and hydroxides in discharge zones. These mineral deposits accumulate 226 Ra, the mother isotope of 222 Rn, which can often be found at high levels in spring waters where mixing processes take place (Gainon et al 2007). Microbial mats are often involved in such precipitation and accumulation processes, as shown for the Misasa hot springs in Japan (e.g.…”

Temperature and radon-222 as tracer of groundwater flow: application to El Hamma geothermal aquifer system, southeastern Tunisia

“…The difference between the activities of radon deep aquifer and the water surface was controlled by lithology and petrophysical characteristics of the rock matrix. The radon anomalies show characteristic features which may be structurally controlled (King et al 1996;Gainon et al 2007). Thus, CI aquifer comprises sandstone (less than 10 % clay) clay and sandstones (10 to 50 % clay) (ERESS 1972).…”