Contamination in fractured clay systems

Aquifers are often protected by a clay layer, which until recently was believed to be an adequate protection against contamination due to its low hydraulic conductivity. However, fractures in the clay have been observed on several locations. These fractures may increase the hydraulic conductivity of the clay aquitard. The aim of this study is to examine transport and biodegradation of selected creosote compounds in fractured clay, i.e. both abiotic and biotic processes. Different approaches are applied. One is to do experiments in large intact fractured clay columns (0.5 m in height, and 0.5 m in diameter). The columns are then installed in the laboratory (at The Geotechnical Institute, Lyngby) at pressure and temperature close to the natural field conditions. Another approach is a field experiment where the same processes are studied on a much larger scale. Both kinds of experiments seem to reveal that transport through fractures is a very fast process, even for organic compounds. Despite this fact, biological degradation does occur to a certain extend for several important compounds, in particular under aerobic conditions. The abiotic processes, diffusion and sorption, are studied separately in clay columns sampled from the fulle scale test- site. Sorption studies are done with dispersed clay to determine the sorption capacity of specific compounds in complex mixtures. Transport into the matrix is a slow process. However, remediation by the reverse out-diffusion process is also very slow, thereby preventing short term clean-up. The research has shown that efficient natural biodegradation takes place for the biodegradable creosote compounds.