Helium in Porewater and Rocks of Crystalline Bedrock from the Fennoscandian Shield, Olkiluoto (Finland)

Abstract Dissolved noble gases in matrix porewaters of low-permeable crystalline rocks are useful tracers for investigating porewater migration and interactions between porewater and groundwater. Samples from the 1160m deep OL-KR56 borehole, drilled at the Olkiluoto site (southwestern Finland), have been analysed for concentrations and isotope signatures of helium in porewater, groundwater and in the rock matrix. These are the first 3 He/ 4 He ratios combined with 4 He concentrations in porewaters and rocks from the Fennoscandian Shield. High 4 He porewater concentrations suggest long porewater residence times, while concentration differences of one order of magnitude between porewater and groundwater indicate a transient state between these reservoirs. Concentrations of 4 He in porewater show small variations with depth, which are mimicked in the concentrations of conservative chemical tracers as well as in 4 He concentrations of groundwaters from various regional boreholes. 3 He/ 4 He ratios in porewaters are higher than in the rock matrix, requiring either fractionation during redistribution of He between these reservoirs or the addition of an isotopically light component.