Some evidence for surface ion mobility in rock

Electrical conductivity measurements have been used to investigate the effe ct of surface ion mobility in Aspo diorite. Rock tablets were depleted of t heir pore ions by leaching, leaving mostly ions in the electrical double la yer in the pores. One of the tablets had been ion exchanged with Sr2+ befor e leaching, leaving mostly Sr2+ as the cation in ion exchange positions. Th e conductivity through the rock tablets was then measured at increasing por e concentrations of SrCl2 solutions for the ion exchanged piece and SrCl2 o r NaCl solutions for the others. The conductivity measurements show a large contribution from the cation layer to the total conductivity in low salini ty pore waters. For Sr2+, a surface diffusivity, evaluated from the conduct ivity, of about 2 X 10(-13) m(2)/s, was obtained. Dp epsilon was 4 X 10(-14 ) m(2)/s also evaluated from the measurements. In low salinity waters with a K-d(Sr2+) of 0.01 m(3)/kg, as suggested for Swedish rock at low ionic str ength pore water, this means that the overall effective diffusivity would b e more than 100 times higher than if there had only been transport in the f ree pore solution. (C) 1998 Elsevier Science B.V. All rights reserved.