Volume 3, Issue 7 (July 2006)
Disposal of Dredged Material in a Local Confined Disposal Facility: Budgeting and Accounting of Contaminant Transport
Disposal of contaminated dredged material in local confined disposal facilities (CDF) has the advantages of reduced costs and reduced need for transport of the dredged material compared to many other disposal alternatives. CDFs can often be combined with land reclamation which can add considerable value to a dredging project. A local CDF has been constructed for Hg and HCB contaminated dredged material on a contaminated seabed close to one of Norway's largest industrial areas, Herøya. The construction has been completed with permeable barriers allowing transport of water from dewatering of dredged material and consolidation of sediment below the CDF. However, this design will also allow some transport of contaminants with the released pore water. The efficiency of the CDF was evaluated by making a budget of estimated contaminant emission from the disposal area before, during, and after construction and filling of the CDF. Laboratory tests and an analytical model were utilized for calculation of advective and diffusive transport of contaminants both from the dredged material and from the original contaminated seabed sediment at the disposal site. Our estimation of contaminant transport predicted that the leakage of contaminants from dredged material in the disposal facility would be orders of magnitude less than what was released from the seabed sediments at the disposal site before the realization of the CDF. Monitoring of contaminant transport, after realization of the CDF, has so far shown that the actual transport is much lower than the conservative estimate presented in our budget of contaminant transport. These results show that there can be a large and positive cost benefit from disposal of dredged material in CDFs constructed with permeable barriers in already contaminated areas. Sensitivity of calculated remediation efficiency to uncertainties in the estimated transport was found to be small in the case presented here. Our results also show that budgeting and accounting of contaminant transport can be a powerful tool both for impact assessments before realization of a project in contaminated sediments and for interpretation of monitoring data during and after the realization. Budgeting of contaminant transport also helps identifying the most critical transport mechanisms during the different phases of the project.