Accumulation of DNAPL waste in subsurface clayey lenses and layers.

Chlorinated solvents like trichloroethylene (TCE) and tetrachloroethylene (PCE) are prevalent groundwater contaminants being detected more than half of the Superfund Sites. They are introduced into the subsurface due to improper disposal of hazardous wastes containing these chlorinated dense non-aqueous phase liquids (DNAPL). DNAPL wastes sink in the subsurface and form pools on clay layers. Further transport into the underlying clay layer is dominated by diffusion due to the low permeability of these soils and results in accumulation of contaminants therein overtime through diffusion. However, field evidence reported that mass storage of solvents in such zones was higher than what can be attributed to simple diffusion. To evaluate this finding, mass storage of TCE in a hypothetical aquitard after 30 years of diffusion was calculated. The diffusion coefficient reported in the field study resulted in mass storage of 137.7 g in this hypothetical aquitard whereas the mass calculated using the measured diffusion coefficient of TCE from pure solvent into water-saturated clayey soil was 25-65% of it (35.0-89.4 g). The calculated mass storage was even lower (8.2 g) if the measured diffusion coefficient of TCE from DNAPL waste into waste-contacted soil was used. So, mass of TCE accumulated through sole diffusion was not able to explain the one observed in the field. This excess mass in the field might be the mass of DNAPL entered into the cracks which were reported to form in clay layers as a result of the direct contact between water saturated clay and DNAPL waste. Mass of TCE in the cracks was calculated using reported average crack size and an assumed crack depth, and it appeared that cracks filled with DNAPL could increase the stored mass up to a minimum of 334 g which could easily account for the enhanced mass storage observed in the field.

Effective diffusion coefficients of DNAPL waste components in saturated low permeability soil materials.

Diffusion is regarded as the dominant transport mechanism into and out of low permeable subsurface lenses and layers in the subsurface. But, some reports of mass storage in such zones are higher than what might be attributable to diffusion, based on estimated diffusion coefficients. Despite the importance of diffusion to efforts to estimate the quantity of residual contamination in the subsurface, relatively few studies present measured diffusion coefficients of organic solutes in saturated low permeability soils. This study reports the diffusion coefficients of a trichloroethylene (TCE), and an anionic surfactant, Aerosol OT (AOT), in water-saturated silt and a silt-montmorillonite (25:75) mixture, obtained using steady-state experiments. The relative diffusivity ranged from 0.11 to 0.17 for all three compounds for the silt and the silt-clay mixture that was allowed to expand. In the case in which the swelling was constrained, the relative diffusivity was about 0.07. In addition, the relative diffusivity of 13C-labeled TCE through a water saturated silt-clay mixture that had contacted a field dense non-aqueous phase liquid (DNAPL) for 18months was measured and equaled 0.001. These experimental results were compared with the estimates generated using common correlations, and it was found that, in all cases, the measured diffusion coefficients were significantly lower than the estimated. Thus, the discrepancy between mass accumulations observed in the field and the mass storage that can attributable to diffusion may be greater than previously believed.

A mechanism of basal spacing reduction in sodium smectitic clay materials in contact with DNAPL wastes.

There has been concern regarding the possible attack of clays in aquitards, slurry walls and landfill liners by dense nonaqueous phase liquid (DNAPL) wastes, resulting in cracking. Despite the fact that a reduction in basal spacing in sodium smectitic clay materials has been linked to cracking, no plausible mechanism by which this reduction occurs in contact with waste DNAPLs has been formulated. To elucidate a mechanism, screening studies were conducted that showed that the combination of an anionic surfactant (AOT), a nonionic surfactant (TritonX-100) and a chlorinated solvent, tetrachloroethylene (PCE), could replicate the basal spacing reduction and cracking behavior of water-saturated bentonite caused by two waste DNAPLs obtained from the field. FTIR measurements of this system showed a displacement of the HOH bending band of water symptomatic of desiccation. Sorption measurements showed that the uptake of AOT by bentonite increased eight fold in the presence of TritonX-100 and PCE. The evidence presented here supports a mechanism of syneresis, involving the extraction of water from the interlayer space of the clay through the synergistic sorption of a nonionic and anionic surfactant mixture. It is speculated that the solvation of water in reverse micellar aggregates is the process driving the syneresis.