228Ra/ 226Ra/ 224Ra and 87Sr/ 86Sr isotope relationships for determining interactions between ground and river water in the upper Rhine valley

Abstract

Ground and river waters of the upper Rhine valley (Alsace, France) were investigated for chemical composition of the major elements, Sr isotopes and radionuclides from the U and Th series. In particular, the isotope ratios and concentrations of Ra and Sr were used as geochemical tracers to distinguish between different types of water and their interactions. The bulk chemical analyses suggest that the surface waters in the Rhine valley can be described as mixtures between Ca–Na–HCO 3-rich ground water and less mineralized slightly acidic river waters which have migrated through crystalline (mainly granitic) basement rocks of the Vosges mountains. Mixing of these waters yields positive correlation between bulk Sr, U, Ca and HCO 3, indicating that carbonate-rich sediments are the main source of U and (non-radiogenic) Sr in the Rhine valley aquifers. The combination of the Ra and Sr isotope systems ( 228Ra/ 226Ra, 87Sr/ 86Sr) shows, however, that probably three sources contribute to the surface river waters in the upper Rhine valley, i.e. (i) a highly radiogenic crystalline component, (ii) a ground water source and, (iii) a third component from infiltrating Rhine water along the flow path of the parallel running river Ill in the northerly direction. The Sr and Ra isotope systems were also used to calculate small-scale mixing fractions of tributaries along the flow path of the Ill. Mixing ratios of non-pure end-member waters were determined using three isotope diagrams (i.e. 224Ra/ 226Ra vs. 228Ra/ 226Ra) and the results obtained with the Ra isotope system were found to be consistent with the data using Sr isotope relationships (i.e. 87Sr/ 86Sr vs. 1/Sr).