grab samples

SECTION 7
CHEMICAL TESTS WHICH MAY AID IN INTERPRETATIONS
UNDER SCHEDULE III
Several tests have been devised to assess such items as reactivity of sediments with seawater, oxygen
stripping of the water column by either chemical or biological processes, and the relative ease with which
sched uled items might be released to the water column during disposal of dredge spoils. The tests are artificial at
best, but do constitute a first step beyond the determination of what is present and in what quantities.
7.1 Seawater Reactivity
There has been considerable debate among marine geochemists as to the best method of determining
the degree of interaction between sediments and seawater. Depending on the size characteristics of the
dumped sediment and the degree of dispersion upon dumping, the ratio of interaction of the sediment with
the water column and the degree of desorption from or adsorption of pollutants to the sediments can be
highly variable. The original concept of the elutriate test was an effort to quantify the changes that might
result when dredge sediment is dumped at a marine disposal site. However, the test is only a simulation of
actual conditions. The emphasis throughout the test has been on keeping the seawater/sediment mixture
well oxygenated so that all trace meta ls remain relatively insoluble. In the natura l environment these highly
oxidizing conditions may not exist, and there may be some trace metal dissolution into the water column.
The dumping of heavily polluted material from an anoxic sediment area may create a high chemical
demand for oxygen from the overlying waters and neighbouring sediments, as the new chemical
equilibrum is approached. This type of chemical oxygen demand is very hard to quantify or even define.
The basic concept behind leaching tests, elutriate tests or seawater reactivity tests is the determination
of how readily the pollutants in the sediments move back into solution in the overlying waters. For
example, the preferred method to date for metal determinations has been total dissolution of the sediments.
However, this method includes metals adsorbed onto the sediments, organometallic complexes adsorbed
onto the sediments, metals contained within day structures, and metals forming part of the basic crystal
lattice structure of the sediment minerals. A more reasonable value for" pollutant" metal concentrations
might be determined by leaching techniques (Malo 1977). However, much more research is needed to
determine what met-a l fraction truly represents the" poIlu ting fraction."
Much of the reactivity of sediment with seawater is determined by reactions at the seawater/sediment
interface, and, therefore, pollutant concentrations on the sediment particle surface are important when
considering seawater reactivity. Recent investigations have shown that bulk chemical concentrations
predict very different chemical reactions and equilibria than those actually found (MacKnight 1976).
In conclusion, it is suggested that" relatively short-term" experimen ts be carried out where seawater is
interacted with the sediment under conditions simulating the natural environment as closely as possible.
The design of the experiment should allow for longer time periods of interaction than those specified in the
elutriate test. The results of such seawater interaction experiments would probably yield a better assessment
of the-pollution factor of heavy metals and perhaps other pollutants.
References
MACKNIGHT, S. D . 1976. Phosphorite solubility behavior in seawater. Ph.D. diss. Dalhousie University, Halifax, N.S. t 60 p.
MALO, B. A. 1977. Partial extraction of metals from aquatic sediments. Environ. Sci. Tech. It: 277 - 282.
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