Factors controlling 226Ra, 228Ra and their activity ratio in groundwater – an application in Polish Carpathian mineral waters

Abstract

The influences of aquifer formations and water chemical composition on the occurrence and activity ratio of radium isotopes in groundwater are discussed. Based on the model of desorption/adsorption processes of natural radionuclides in the rock-water system, the concentrations of radium isotopes and their activity ratio in groundwater are evaluated by the numerical Monte Carlo method (MC). In cases where the groundwater is of a similar age, limited flow (up to several meters/year), the physical conditions and the uranium and thorium activity ratios in host water formations are similar, the activity concentrations of radium isotopes ($^{226}$Ra, $^{228}$Ra) and their activity ratio ($^{226}$Ra/$^{228}$Ra) are the highest in the water of high desorption coefficient for chloride sodium water (domination of $Cl^{-}$ , $Na^{+}$ ions), medium in water of moderate desorption (bicarbonate water - $HCO_{3}^{-}$ , $Ca^{2+}$) and the lowest in waters with a low desorption coefficient (sulfate ions prevailing – $SO_{4}^{2-}$, $Ca^{2-}$). The statements are well confirmed in the case of the natural mineral waters from the Polish Outer Carpathians. The total dissolved solids (TDS) of the Polish Carpathians waters varies from several hundred milligrams per liter to several tens of thousands milligrams per liter. The minimum, maximum and average concentrations of $^{226}$Ra, $^{228}$Ra and their activity ratio ($^{226}$Ra/$^{228}$Ra) are 82, 1340, 456 mBq/L, 19, 1240, 354 mBq/L and 0.89, 7.6 and 2.0 for chloride waters, 4, 140, 45.8 mBq/L, 12, 171, 62.7 mBq/L and 0.3, 1.7 and 0.70 for bicarbonate waters and 0.8, 9.3, 3.6 mBq/L, 5.3, 54, 20.1 mBq/L and 0.1, 1.0, 0.3 for sulfate ones, respectively. The desorption coefficients are the highest for the $Cl-Na$, moderate for the $HCO_{3}-Ca$ and the lowest for the $SO_{4}-Ca$ waters (in contrast to the adsorption properties of these waters).