The relationship between He and Cl− as a criterion for the formation of groundwater composition

Abstract

The analysis of the relationship between 1611 parallel determinations of helium and chloride concentrations in the groundwater of crystalline shields and sedimentary basins was performed. The comparison showed that groundwaters have helium and chlorides concentrations in the range of 1.8 × 10−8 to 6.8 mol/m3 and of 5.6 × 10−3 to 6488 mol/m3, respectively, and relatively stable He/rCl ratios equal to 5.5 × 10−5 in sedimentary basins and 7.5 × 10−4 in crystalline shields. Such behavior of helium and chlorides shows that the formation of the groundwater composition is determined primarily by the processes of mixing fresh meteoric and saline sea waters with ancient brines from great depths. During sedimentation, sea water penetrates the subsurface and mixes with ancient brines. On land, fresh meteoric waters dilute the mixtures of seawater and ancient brines. Most of the radiogenic 4He and chlorides comes from the subsurface ancient brine. The radiogenic 4He in the host rocks plays a secondary role. Therefore, the concentration of 4He in saline groundwater reflects the size of the impurities of the ancient brine with a very high content of 4He, rather than the residence time within the subsurface.