Radioelements, radiogenic helium and age relationships for groundwaters from the granites at Stripa, Sweden

Abstract

The solution of radioelements and radiogenic 4He by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock-water interface. The 234U 238U activity ratio and the dissolved U, Rn and He contents of such groundwaters respond to changes in the flow regime with time. Although 234U 238U activity ratios change with groundwater residence time as a consequence of 234Th-recoil induced solution of 234U, the activity ratio is strongly influenced by the U distribution within fractures, by the extent of the rock-water interface and by the amount of 238U in solution. A model for the quantitative evaluation of these effects is presented. Groundwaters from depths up to 880 m in the Stripa granite have variable dissolved uranium contents and 234U 238U activity ratios. The uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age. Dissolved radiogenic 4He in the groundwaters increases with their depth of origin, and is dependent upon the U content of the granite and upon its fracture porosity. It increases with groundwater residence time but movement of 4He by diffusion and transport processes make the actual groundwater age indeterminate.