Measuring dissolved He diffusion coefficients and 3He and 4He fractionation during diffusion through sandstone

Abstract

The He concentration in groundwater has been used as a tracer for groundwater age in numerous studies and this requires an understanding of He provenance and accumulation in the target aquifer. Few studies of dissolved He diffusion through rocks have been performed even though He diffusion into or out of adjacent aquitards can strongly affect the He concentration in groundwater. Additionally, little information about 3He and 4He fractionation during diffusion is available. Information about 3He fractionation may improve our understanding of the migration history (diffusion and/or advection) of He in groundwater, but this will require our understanding of 3He and 4He fractionation during diffusion processes to be improved. A new system for performing diffusion experiments was developed in this study. Dissolved He in a downstream cell was separated from the bulk solution using a silicon tube coil and measurements were made using a sample loop directly connected to a gas chromatograph. The system minimizes He degassing and contamination by air because sampling and analysis are performed while keeping the system closed. Dissolved He diffusion coefficients (DHe) for three types of sandstone (Tako, Kimachi, and Izumi sandstone) were determined, and the values were 18 × 10−11, 10 × 10−11, and 1.6 × 10−11 m2/s, respectively. Cl− diffusion coefficients (DCl) and D2O diffusion coefficients (DD2O) were also determined and compared with the DHe values. Formation factors were calculated from the coefficients for diffusion through the rocks and in free water. The He and D2O formation factors agreed well for all three sandstones, indicating that dissolved He and water diffused through similar rock pore paths. The Cl− formation factors were lower than the He and D2O formation factors, possibly because of anion exclusion. It was inferred that DHe for sandstone can be roughly predicted from DD2O. 3He and 4He fractionation was assessed by comparing the 3He/4He ratios before and after diffusion through rock pores. The D3He to D4He ratios for Tako, Kimachi, and Izumi sandstone were 1.137, 1.139, and 1.179, respectively. The Tako and Kimachi sandstone D3He/D4He ratios were similar to the D3He/D4He ratio for free water. The D3He/D4He ratio was markedly higher for Izumi sandstone than free water, indicating that collisions with pore walls may affect 3He and 4He fractionation during diffusion through some rock pores.