RIASSUNTO
ABSTRACT
This paper discusses the planning of harbor dredging when mercury-contaminated sediments are found. The major considerations, in addition to those normally considered, are the extent of mercury contamination and the problem of how to safely dispose of the material on land. It is shown that the oxidation-reduction conditions in the natural sediment strongly influence the binding of mercury in the sediment. When the natural binding is not sufficient to prevent release of the mercury to the overflow or to groundwater, chemical completing agents can be added in small amounts to strengthen the binding. In some cases, the spoils area may require a cover material to prevent the percolation of oxygen-rich surface water.
Work on this project has been partially sponsored by the Environmental Protection Agency
INTRODUCTION
The mercury contamination problem in fish has been widely publicized in both the popular and technical press. A number of freshwater fishing areas have been closed and the swordfish industry virtually decimated. Although the initial hysteria has subsided, many elements of the problem are still with us, including the question of what to do with sediments containing significant quantities of mercury.
The Environmental Protection Agency, in a set of guidelines called ""Criteria for Disposal of Dredge Spoils in U. S. Waters,"" has set an upper limit on the concentration of various pollutants in dredge spoils. For mercury the limit is 1.0 ppm, which means that dredge spoil containing more than this concentration should be disposed of on land by a ""safe"" method. Concentrations of mercury exceeding 1 ppm are frequently encountered in the maintenance dredging of navigable harbors and estuaries. The purpose of this paper is to discuss some of the problems as seriated with land disposal of mercury contaminated dredge spoil, and means for controlling the escape of mercury from land disposal sites.
Under contract with EPA, JBF Scientific has studied the absorption and release of mercury by various natural sediments under reducing and oxidizing conditions. The results show that sediments in their natural reduced state will bind mercury much more effectively than after they have become oxidized by exposure to air. Thus, there is a danger that, when mercury contaminated sediments are exposed to air and to oxygen-rich surface waters in a landfill, the bound mercury will be released to the environment as a solution in the runoff water.
We further find, however, that the retention of mercury by sediments can be greatly increased by the addition of small amounts of mercury-complexion agents. In some cases, the mercury so completed is much more stable to oxidizing conditions than when bound in natural sediments. The application of such complexion agents to dredge spoil is discussed.