Diffusion coefficients of nitrate, chloride, sulphate and water in cracked and uncracked Chalk

SUMMARY The rates of diffusion of chloride, sulphate and water, labelled respectively with 36 Cl, 35 S and 3 H, and unlabelled nitrate, were measured in small cylindrical Chalk monoliths. Using a simple mathematical model, diffusion coefficients were calculated to provide a basis for comparing the movement of these substances through Chalk rock in the absence of hydrostatic pressure gradients. The diffusion coefficients of chloride and nitrate were similar, with a range of values (0.52–3.23 × 10 −6 cm 2 s −1 ) and (0.53–3.20 × 10 −6 cm 2 s −1 ) respectively. These were slightly less than for tritiated water (0.60–3.51 × 10 −6 cm 2 s −1 ), while the coefficient for sulphate was about half that of the others (0.28–1.47 × 10 −6 cm 2 s −1 ). The coefficients indicate the absence of any interaction with the Chalk surfaces.