In October 1983 PIANC established a working group to investigate whether the disposal of dredged materials at sea would be acceptable as alternative to land disposal. The world”s large ports are indispensable points in transport routes and of major socio-economic. importance. Dredging is essential to keep them accessible.
Rules and regulations concerning the disposal of dredged materials at sea are very much influenced by the London Dumping Convention (LDC). But this Convention is not implemented in the same manner by ditTerent member countries. Some modes of implementation may lead to unnecessary increases in cost or to undue delay. By far the largest part of dredged material is not coniaminated and therefore constraints should only apply to the small contaminated part. This report concentrates on the management of these contaminated materials during the coming ten to twenty years, because in the long run a solution should be reached by the elimination of pollution at its source.
Relationship between environmental requirements and cost considerations
The regulations and controls embodied in the London Dumping Convention were primarly developed with a vieuw to the disposal of industrial waste and sewage sludge. Thus, the necessary differentiation between industrial waste and dredged materia] was not made and the substances from the Annexes wcre equated with “”wastes and other matter””. The discussions regarding the disposal of dredged materials at sea concentrate on the interpretation of the clauses referring to “”trace contaminants””, “”significant amounts”” and “”rapidly rendered harmless””. Only in exceptional cases are the proscribed substances present in dredged material in such proportions that they should not come under the first two clauses.
It is PIANC”s opinion that in those cases special-care measures will generally render these substances rapidly harmless. The rules should provide for this solution.
Up to now implementation of State and local regulations in the USA led to cost increases of two to five times the amounts which were already imputed to the Federal evaluation process and to extra delays of three to five years.
The choice of the best method of disposal, if made on the basis of an impartial environmental assessment, will in PIANC”s opinion, lead to the conclusion that with adequate boundary conditions, disposal at sea is in most cases a sound solution.
Effects of disposal at sea.
The impact of ocean disposal of dredged materials is mainly physical and of a temporary nature. There are a few occasions, however, with persistent irreversible or cumulative effects. If pollutants are released they are usually nutrientsthe release of toxic metals and hydrocarbons is negligible. Biochemical interactions are infrequent and show no clear trends. The uptake of toxic metals and of hydrocarbons is usually negligible.
Land-based and near-shore disposal methods appear to offer less protection against adverse impact on human life than does ocean disposal and are often excessively costly.
Land-based alternatives often drastically change the geochemical qualities of dredged material with a subsequent enhanced potential for the release of chemical constituents. Land sites are usually located in or adjacent to highly productive near-shore areas or possibly in contact with groundwater aquifers.
Even highly contaminated dredged materials can be disposed of in ocean locations if sufficient care is exercised in site selection to ensure that the material will remain isolated from the biotic zone of the marine system. This approach involves disposal site management using capping techniques or locating disposal in areas where the biosystem is not sensitive. For disposal in the marine environment dredged material should be regarded as a highly manageable material.
Management of disposal.
The various options for disposal at sea are described and evaluated. The resulting rccommendations are based on the concept of limiting the dispersion of ontaminants as far as is necessary.
The ocean environment may be divided into four zones: the deep ocean, the open shelf, the near-shore and the coastal zone, adjacent to inlets, rivers and estuaries.
