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open ocean, but “special care" measures enhance the effects of these condi- 
tions on sequestering of mercury and mercury compounds and some, such as 
capping, will preclude contact of benthos with the polluted material. It 
should be mentioned also that hydrous iron oxide, prevalent in some sedi- 
ments, scavenges dissolved mercury in the water column. 
Methyl mercury is, of course, highly toxic, but in Japan where, as is well 
known, mercury contamination in some sediments has been a problem, methyl 
mercury was found to enter benthos from water and not from sediments 
(Fujiki, Hirota, and Yamaguchi, 1977). 
Both capping of mounded dredged material and that disposed in borrow pits 
(these are special care measures mentioned in this report hereinafter) 
should be desirable disposal methods with low potential for adverse envir- 
onmental effects. 
Cadmium 
Although cadmium is a highly toxic metal the disposal of cadmium-polluted 
sediments in the marine environment poses relatively low risks to the pela- 
gic environment within or adjacent to the dumpsite (Gambrell et al., 1978). 
Moreover, even though it appears that benthic organisms can accumulate cad- 
mium from some contaminated bottom sediments, it is espoused here that the 
physicochemical conditions that accompany some "special care" disposal 
methods will effectively sequester the metal from these organisms (Chen et 
al., 1976). Cadmium is immobilized by (a) reducing conditions, in which it 
1s bound as insoluble organic and sulfide forms, and (b) by alkaline pH. 
High concentration of sulfides and/or ogranic material in reducing bottom 
sediments will precipitate and complex cadmium in immobile forms with lit- 
tle or no diffusion of soluble cadmium to the overlying waters. Some or 
all of these conditions will be met by "special care" disposal methods. 
Chlorinated Hydrocarbons 
The chlorinated hydrocarbons of principal concern in dredged material are 
