ALBERT

Notes: The compatibility of dense nonaqueous phase liquids (DNAPLs). triehloroelhylene (TCE), methylene chloride (MC), and creosote with commercially available sodium bentonite pellets was evaluated using stainless steel, double ring, falling head permeameters. The hydraulic conductivity (K) of the bentonite clay was measured under three experimental conditions: (1) water soluble constituents of the DNAPLs were used to hydrate and permeate the bentonite: (2) bentonite pellets were submersed in DNAPL prior to hydration and permeation with water: and (3) DNAPLs were pooled on water-hydrated bentonite. Further, the effect of hydraulic head (2.5. 145, and 710 cm) on water-hydrated bentonite permeated with TCE and the effects of TCE exposure time to mixtures of bentonite grout and sand were measured. Solubility concentrations of DNAPL constituents did not effect the hydraulic conductivity of the bentonite pellets relative to baseline (water only) measurements; a competent hydraulic barrier was formed (K 〈 5 × 10−9 cm/s). Bentonite pellets submersed in DNAPLs retained their rigid shape, did not swell, and did not perform as a hydraulic barrier. However, when the DNAPL was removed and replaced with water, the DNAPL-wetted pellets imbibed water to swell and form an adequate hydraulic barrier (K 〈 6 × 10−9 cm/s). Competent hydraulic barriers constructed with bentonite pellets and hydrated with water were subsequently permeated with TCE, MC, or creosote DNAPLs developing desiccation cracks up to 5 mm wide. The intrinsic permeability of water-hydrated bentonite was 46 to 2640 limes greater to DNAPLs relative to water, indicating that the desiccation cracks predominantly facilitated preferential DNAPL transport. The propagation rate of cracks was positively correlated to the hydraulic gradient, bin the formation of cracks is chemically dependent on the contact lime of DNAPL. Silica sand is expansively inert, yet 95, 90, 83, 75, and 50% (wt sand/wt bentonite) mixtures with bentonite grout were insufficient to prevent desiccation cracks and hydraulic failure. The incompatibility between DNAPLs and bentonite observed in this study is important due id the numerous exploratory borings in DNAPL/ones, the increasing dependency on hydraulic containment as a remedial alternative, and the fact that such remedial systems are often designed to be functional for many years.