AQUITARD INTEGRITY AND DNAPLS

Aquitards are expected to strongly protect underlying aquifers from contamination, however, chlorinated solvents such as tetrachloroethene (PCE), trichloroethene (TCE), chloroform and carbon tetrachloride are commonly found in these buried aquifers. These chlorinated solvents are dense, non-aqueous phase liquids (DNAPLs) and are typified by low viscosities and interfacial tensions, which cause them to have much stronger propensity to move through fractured aquitards than dissolved-phase contaminants. Studies were directed at developing understanding of the integrity of clayey aquitards in the DNAPL context and involved: i) laboratory experiments using large, cylindrical aquitard columns, ii) field experiments in which solvent DNAPLs were released into a thick clayey aquitard and into a surficial sand aquifer overlying a 8m thick clayey aquitard and iii) field investigations of contaminated sites where large volume solvent releases occurred several decades ago. The laboratory column experiments showed TCE DNAPL entry and flow in extremely small natural fractures (5-20 μm) that could not be detected hydraulically. These results are consistent with the field experiments in which small, natural fractures of contraction-origin provided DNAPL pathways. The lab and field experiments relied on contaminant analyses of thousands of closely spaced samples collected from aquitard cores to delineate fracture pathways by identifying diffusion haloes along the fractures. These core sampling procedures and other methods were applied at several industrial sites. The conclusion drawn from these studies is that aquitards generally have poor integrity with respect to DNAPL and this should be assumed until site-specific data are acquired indicating otherwise. Although aquitards commonly have fractures allowing DNAPL entry, it has been found that some provide strong protection to the underlying aquifers, either because there is a thin zone without fractures, fractures are insufficiently connected to the overlying fracture network, or sand lenses provide sufficient storage capacity to distribute DNAPL and dissipate the potential causing downward DNAPL flow. At some sites strong attenuation occurs due to diffusion, sorption and degradation that retards or prevents the downward mass flux.