4.3 Discharging of Dredge Spoil and Sludge
Conventionally, dredge spoil from harbors and from shipping channels is dumped
into the sea. The same is done with a variety of earth material, pardy used as land fill
when reclaiming shallow coastal areas for urban or industrial purposes. Waste water
treatment plants release rather clean treated effluents, but have to dispose of the
sludge. Waste water treatment plants close to the sea conventionally dump the sludge
into the sea. In all these cases, the material dumped at sea cannot be classified as
highly toxic, because toxic substances are only in trace concentrations. But because
large quantities of material are concerned, the effect of such disposal upon the coastal
ecosystem has to be carefully evaluated.
Great quantities of sewage sludge from sewage treatment plants in New York and
New Jersey have been dumped onto the Continental Shelf off these two states,
anually up to 4 million m 3 containing 200,000 t of organic solids. This import of
organic matter into the sea is about ten times the amount of organic substance produced by phytoplankton photosynthesis on the shelf off New York. An area of
36 kmz is affected, where, if distribution is even, 4 mm of sludge settle annually.
Anaerobic conditions in the sediment result; poisonous hydrogen sulfide (HzS) form,
and the fauna dies out or is restricted to the few species that are able to resist (compare Fig. 4). In some years anoxic conditions spread from the waste disposal area
southward. Oxygen concentration in the seawater of the disposal area is in general
2-3 mg/llower than in the surroundings; in summer, however, sometimes it is only
about 2 mg/I in a large area of New York Bight, so that a further reduction in oxygen
concentration means anoxic conditions (National Academy of Science 1975a).
In July 1976 dead fish, crustaceans, and molluscs were reported from the sea area
south of the waste disposal locality in New York Bight. Subsequent hydrographical
investigations (Fig. 39) revealed an area of 165 km 2 with less than 2 mg/I of oxygen in
the seawater close to the sea bottom, and some patches with anoxic conditions and
even with hydrogen sulfide in the lower half of the 35 cm deep water. A total of
12,000 km 2 was concerned. It may well be that the even worse conditions in the
dumping area triggered this large-scale catastrophe, where 143,000 t of Spisula clam
died in an area of 6,700 km 2 • However, this assumption is not certain.
The year 1976 was an extreme situation. Spring and summer were very warm, without storm action, therefore a thermocline could develop better than in other years,
and light warm water at the sea surface was separated by a stable transition zone
from the heavy, cold water at the sea bottom. The bottom water, therefore, was cut
off from atmospheric oxygen. Second, there was an extremely luxuriant plankton
bloom which occurred just below the thermocline. When the plankton algae died off
and sank into deeper water, they had, for degradation, an enormous oxygen demand
(see Chap. 2.1). The deep water therefore was quickly depleted of oxygen, the
anoxic situation was established. Certainly natural processes and the addition of
surplus organic substance to the oxygen-consuming degradation process acted in the
same direction. However, with the present knowledge at hand, it is impossible to
decide what was the man-made effect, and whether, under similar meteorolOgical
conditions, the anoxic situation would not also have been established without
pollution.
64
Conventionally, dredge spoil from harbors and from shipping channels is dumped
into the sea. The same is done with a variety of earth material, pardy used as land fill
when reclaiming shallow coastal areas for urban or industrial purposes. Waste water
treatment plants release rather clean treated effluents, but have to dispose of the
sludge. Waste water treatment plants close to the sea conventionally dump the sludge
into the sea. In all these cases, the material dumped at sea cannot be classified as
highly toxic, because toxic substances are only in trace concentrations. But because
large quantities of material are concerned, the effect of such disposal upon the coastal
ecosystem has to be carefully evaluated.
Great quantities of sewage sludge from sewage treatment plants in New York and
New Jersey have been dumped onto the Continental Shelf off these two states,
anually up to 4 million m 3 containing 200,000 t of organic solids. This import of
organic matter into the sea is about ten times the amount of organic substance produced by phytoplankton photosynthesis on the shelf off New York. An area of
36 kmz is affected, where, if distribution is even, 4 mm of sludge settle annually.
Anaerobic conditions in the sediment result; poisonous hydrogen sulfide (HzS) form,
and the fauna dies out or is restricted to the few species that are able to resist (compare Fig. 4). In some years anoxic conditions spread from the waste disposal area
southward. Oxygen concentration in the seawater of the disposal area is in general
2-3 mg/llower than in the surroundings; in summer, however, sometimes it is only
about 2 mg/I in a large area of New York Bight, so that a further reduction in oxygen
concentration means anoxic conditions (National Academy of Science 1975a).
In July 1976 dead fish, crustaceans, and molluscs were reported from the sea area
south of the waste disposal locality in New York Bight. Subsequent hydrographical
investigations (Fig. 39) revealed an area of 165 km 2 with less than 2 mg/I of oxygen in
the seawater close to the sea bottom, and some patches with anoxic conditions and
even with hydrogen sulfide in the lower half of the 35 cm deep water. A total of
12,000 km 2 was concerned. It may well be that the even worse conditions in the
dumping area triggered this large-scale catastrophe, where 143,000 t of Spisula clam
died in an area of 6,700 km 2 • However, this assumption is not certain.
The year 1976 was an extreme situation. Spring and summer were very warm, without storm action, therefore a thermocline could develop better than in other years,
and light warm water at the sea surface was separated by a stable transition zone
from the heavy, cold water at the sea bottom. The bottom water, therefore, was cut
off from atmospheric oxygen. Second, there was an extremely luxuriant plankton
bloom which occurred just below the thermocline. When the plankton algae died off
and sank into deeper water, they had, for degradation, an enormous oxygen demand
(see Chap. 2.1). The deep water therefore was quickly depleted of oxygen, the
anoxic situation was established. Certainly natural processes and the addition of
surplus organic substance to the oxygen-consuming degradation process acted in the
same direction. However, with the present knowledge at hand, it is impossible to
decide what was the man-made effect, and whether, under similar meteorolOgical
conditions, the anoxic situation would not also have been established without
pollution.
64