Diffusion of volatile organic compounds in natural clay deposits: Laboratory tests

Diffusion coefficients for four volatile organic compounds of environmental concern, benzene, trichloroethylene, toluene and chlorobenzene, were measured on cores of saturated, unweathered glaciolacustrine clay obtained from boreholes at a hazardous waste disposal site near Sarnia, Ontario. The core material was transferred to stainless-steel cylinders constructed such that a closed solution reservoir was in contact with the upper surface of the core. The decline in concentration of the organic compounds in the reservoir was monitored over time and diffusion coefficients were determined by fitting a solution of the diffusion equation to the resulting data. The measured diffusion coefficients ranged from 0.3·10^-7 to 1.1·10^-7 cm² s^-1 for the four compounds, and were in reasonable agreement with values determined from concentration profiles in the clay material beneath the disposal pits. Diffusion coefficients were also calculated on the basis of distribution coefficients (K_d) determined from laboratory batch tests and from correlations with the organic carbon content of the clay material (f_oc). Values calculated on the basis of batch tests were substantially lower than the measured values, and those calculated on the basis of f_occ exceeded the measured values by almost an order of magnitude. The results indicate that reasonable estimates of diffusion coefficients for hydrophobic organic contaminants, for the purpose of predicting diffusive transport in natural fine-grained geologic materials can be obtained using the diffusion-cell procedure described in this paper. The poor estimate of the diffusion coefficients provided by the f_oc correlation procedure is consistent with significant sorption onto the mineral phase. Though not investigated, sample disturbance may have contributed to the low values calculated on the basis of batch K_d's.