Confined disposal facilities with filter layers have been used for disposal of dredged sediments for decades. Recent assessment of the environmental risk of contaminants in sediments has resulted in the use of lined confined disposal facilities. A case history outlines a contaminated sediment disposal facility designed with geosynthetics. Trends indicate more stringent requirements for confined disposal facilities in the future.
Contaminated sediments are dredged from waterways to maintain navigable depths in the waterways or for environmental remediation. Per Miller (1998) confined disposal facilities (CDFs) is one the most widely used alternatives in the U.S. for placement of contaminated sediments. CDFs are diked areas designed to provide retention and storage of dredged material.
CDFs function as settling basins, in terms of wastewater treatment technology. Typical CDFs were designed to retain greater than 99.9% of the sediment particles disposed. The dredged sediments are placed into the facility either mechanically by a clamshell or hydraulically by pipeline. Coarse sand and gravel sediments typically settle rapidly near the point of disposal while fine grained silt and clay sediments settle more slowly. Supernatant water is discharged from the CDF during dredging disposal operations.
The CDF design is site specific. A CDF may be constructed as an upland site, a nearshore site with one or more sides constructed in the water or as an island containment area. Dikes for in-water CDFs are usually constructed in layers of heavy protective stone on the outside and progressively smaller soil particle size on the inside (Figure 1). Contaminants often bind with the fine sediments as the water percolates through the walls and into the ground. Permeability is reduced over time due to fine particle sediment sealing.
CDF water quality monitoring is typically conducted during the dredging operation and consists of monitoring the effluent and open water sites near the discharge or around the CDF. Some facilities have monitoring wells installed in the dike walls. The Great Lakes Commission (2003) sites results of water quality monitoring have indicated that these CDF designs are highly effective at retaining the sediment solids and moderate concentrations of attached contaminants. The Chicago Area CDF is used to contain sediments removed to maintain navigation. In one biomonitoring study, organisms were collected in and around the Chicago Area CDF to detect evidence of PCB losses. Organisms from immediately outside the CDF were not significantly different from remote stations, indicating no discernable loss of PCBs from the CDF.