Origin of the high radon concentration of Csaloka spring in Northwestern Hungary
Á. Freiler 1, Á. Horváth 1, K. Z. Szabó 2, L. Sajo-Bohus 3*
1 Department of Atomic Physics, Eötvös Loránd University, Pázmány Peter sétány 1/A, H-1117 Budapest, Hungary
2 Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös Loránd University, Pázmány Peter sétány 1/C, H-1117 Budapest, Hungary
3 Nuclear Physics Laboratory, Universidad Simon Bolivar, Caracas 1080A, Venezuela
The Sopron Hills is an interesting area in the north-western corner of Hungary from the radon point of view since its geological background. The rocks here are metamorphic type created from granite and sedimentary rocks. After the metamorphism the main rock of this hill is gneiss. At several sites the gneiss itself went through other processes like milonit creation or magnesium metasomatism. The basically high uranium content of the granite and the processes after its original appearance make this site interesting from the radon point of view.
The area is interesting from dosimetric point of view also. There is an old city at the hill where high indoor radon levels were determined in homes. The first investigation of radon in this geological formation was a survey for the radon in the springs, which showed a high water radon content in the so called Csaloka Spring (220 Bq/l).
In this study we tried to determine whether the soil and the rocks near to surface can be the origin or one of the considerable origins of this radon. First we measured the water radon level during a year 13 times to determine the time dependence of the concentration. On the other hand we took several soil samples and rocks samples from 0 – 30 cm depths to investigate them in laboratory. The specific radium activity was determined by gamma-spectroscopy, and the specific radon exhalation to air was measured using a radon-chamber and RAD7 radon monitor after 3 weeks equilibration time. Small amount of the soil samples were put into glass vials with pure water and Optifluor O scintillation material and the radon exhalation to the water was determined.
Our results show that the time dependence of the radon concentration in the water was less than 20%, and a small trend was demonstrable. We explain this small change by the changes in yearly precipitation values. The radon exhalation of the soil to water and air were found about the same: M=9.1 +/- 1 Bq/kg. The radium content was 136 +/-10 Bq/kg. For the rock samples we cut them to a cylindrical shape and the average specific exhalation was 9.8 +/- 0.5 Bq/kg, the radium content was 77 +/- 5 Bq/kg. The exhalation coefficient of this rocks were surprisingly high, 13 +/- 2 %.
We estimated the radon concentration from this exhalation values due to the known porosity and density using the cmax= RM/p. Here R is the soil mass volume or the average density and p is the porosity. Another important factor of the evolving radon level of the water is the time that it spends in the exhalation material (soil or rock). If we assume that this time is longer than 3 weeks and therefore the radon concentration in the water and the exhalation are in equilibrium, we get the potential water radon level that this material can produce is cmax. The soil showed about 52 Bq/l potential water radon level, and the rock did 480 Bq/l due to smaller porosity and higher density. According to this result the soil itself can be the origin of the 25% water radon concentration, but the rock explains the total amount. The high radon level in the water occurs due to the high exhalation coefficient of the rocks.
Geological study of these rocks determined that they are milonit type deformed gneiss and this deformation at high pressure causes the high exhalation coefficient.