Some evidence for surface ion mobility in rock

Electrical conductivity measurements have been used to investigate the effect of surface ion mobility in Äspö diorite. Rock tablets were depleted of their pore ions by leaching, leaving mostly ions in the electrical double layer in the pores. One of the tablets had been ion exchanged with Sr2+ before leaching, leaving mostly Sr2+ as the cation in ion exchange positions. The conductivity through the rock tablets was then measured at increasing pore concentrations of SrCl2 solutions for the ion exchanged piece and SrCl2 or NaCl solutions for the others. The conductivity measurements show a large contribution from the cation layer to the total conductivity in low salinity pore waters. For Sr2+, a surface diffusivity, evaluated from the conductivity, of about 2×10−13 m2/s, was obtained. Dp was 4×10−14 m2/s, also evaluated from the measurements. In low salinity waters with a Kd(Sr2+) of 0.01 m3/kg, as suggested for Swedish rock at low ionic strength pore water, this means that the overall effective diffusivity would be more than 100 times higher than if there had only been transport in the free pore solution.