Fracture Distribution and Occurrence of DNAPL in a Clayey Lodgement Till

Abstract

This study examines the spatial fracture network in a clayey lodgement till, and the preferential flow pattern of DNAPL in the till. The study was conducted on a former gasworks site contaminated with coal tar. Fracture analysis was carried out and four fracture systems were recognised. Apart from fractures, burrows and root channels were recorded in the near surface sediments. Field investigation shows that two fracture systems were formed subglacially by loading of a glacier and horizontal shear within the lodgement till. Two other fracture systems were formed subsequently by desiccation and unloading/freeze-thaw processes in the unsaturated zone. Spatial distribution of free-phase DNAPL in the lodgement till is controled by the fracture network. The migration path of the DNAPL through the fracture systems is predominantly vertical in the upper 2 m and horizontal between 2 and 3.5 m. b.s. Below this depth the migration pathway is vertical. DNAPL was visually observed to a depth of 9 m.b.s. within fractures which continued downwards to unknown depth. Upscaling of the fracture properties was performed and a regional fracture model is suggested as a tool in future remidiation.