Impact of Clay-DNAPL Interactions on the Transport of Chlorinated Solvents in Low Permeability Subsurface Zones.

Abstract

Chlorinated solvents are often disposed of in such a manner that they form pools on subsurface clay layers. There they slowly dissolve and migrate into the clay layers, accumulating therein over time. Due to the low permeability of these layers, it is assumed that the migration occurs by diffusion. However, field evidence suggests that more solvent may be stored in such layers than can be accounted for through simple diffusion. Since there are few reported measurements of the diffusion coefficient in clayey soils for contaminants of interest, measurements were made in silt and silt-clay mixtures. The diffusion coefficient for trichloroethylene in a silt-clay mixture was at least two to four fold smaller than predictions used in field studies. Calculations based on the measurements obtained in this research suggest that there is an even greater discrepancy between the amount of mass storage in low permeability layers and that which can be attributed to diffusion. To account for this enhanced transport, it was postulated that direct contact between the waste and these layers altered the structure of the clay, and consequently the transport properties. Measurements using X-ray diffraction showed that contact with chlorinated field wastes decreased the basal spacing of water-saturated smectites from 19 Å to 15 Å, accompanied by cracks with apertures as large as 1 mm, within weeks. Calculations showed that even minimal cracking could easily account for the enhanced mass storage observed in the field. To investigate the mechanism of basal spacing decrease, a set of screening experiments were performed, which identified a nonionic surfactant, an anionic surfactant, and a chlorinated solvent, as the minimum waste components necessary. Sorption measurements showed enhanced synergistic sorption of the surfactants in the presence of the chlorinated solvent, while Fourier transform infrared (FTIR) spectroscopy suggested a partial displacement of water from the interlayer space. Based on all the accumulated evidence, it was hypothesized that the nonionic surfactant sorbs in the interlayer space, displacing some of the interlayer water. The anionic surfactant interacts with the nonionic surfactant through their hydrophobic moieties and enhances the dehydration of the interlayer space via its anhydrous nature.