Particle Migration Through GCL Systems Used in Marine Dredge Disposal Applications

Dredging of marine harbours can generate significant quantities of fined-grained contaminated sediment that must be managed. Dioxins and furans (PCDD/Fs) contaminated sediments are present across Canada, with over 163 federally registered contaminated sites with PCDD/Fs and/or polychlorinated biphenyls (PCBs). PCDD/F fine-grained sediments can make for challenging dredging and disposal due to their persistence in the environment and their known risk to mobilize via movement of the fine-grained organic matter portion of the sediment. The containment of these fine-grained sediments (i.e. particulate matter) on land-based near shore (i.e. "upland") containment structures is a common approach in which engineered liner systems such as geosynthetic clay liners (GCLs) are used. GCLs represent a practical and cost-effective engineered liner system that can be installed quickly and economically for remote sites (i.e. most marine harbours) yet provide adequate containment for the sediment and associated contaminants. For dissolved contaminants, defects in the GCL itself and the overlaps between GCL rolls have been shown to play a role in potential contaminant migration. Since allowable limits of PCDD/Fs are very low for remediation objectives, even "micro" amounts of PCDD/Fs transport from containment cells could be viewed as significant. Little research can be found regarding PCDD/F transport through damaged GCLs and/or GCL seams via particulate matter. Initial research in this program will focus on performing laboratory experiments to identify the dominant mechanisms responsible for particulate matter transport through damaged GCL systems (holes and thinned bentonite) using fluorescent microspheres as a surrogate for fine-grained sediment particles. Experiments will be performed using fluids which represent the range of ionic strengths and ionic constituents found in upland containment cells. The role of various geotextiles used in GCLs in the filtration of particles will be explored and supplemented with lab tests performed using GCLs with "thinned" bentonite. These tests will attempt to simulate small holes in GCLs that fully or partially heal due to bentonite swelling or GCLs on slopes that have undergone bentonite erosion. As a secondary objective, laboratory experiments with fluorescent microspheres will be performed to identify the extent of particulate matter through various types of GCL overlaps. Experiments will be performed in the laboratory with large-scale steel flow boxes (1m x 1m) capable of providing adequate scale to the flow effects of GCL overlaps. Variables to be examined during testing will include GCL overlap width, degree of bentonite application to the overlaps, different permeating fluids and microsphere sizes. Modelling of the microsphere movement through the GCL overlaps will be performed with HYDRUS. This work will improve the knowledge base for containment of PCCD/F contaminated sediment in with GCL lined facilities.