ANTHROPOGENIC IMPACTS ON THE DEEP SEA
441
• bacteria dominated by methanogenic species, which
continue to ferment the sludge, coliform bacteria
and human pathogens;
• industrial contaminants, including heavy metals and
hydrocarbons.
Dredge spoils may largely contain the same classes
of anthropogenic contaminants as sludges: hydrocarbons, heavy metals and synthetic organic substances.
However their origin is different. They are dredged
from river, channel and harbour systems to keep
the waterways open or deepen them for ship traffic.
Uncontaminated dredge spoil may be used for building
material, land reclamation or road construction, since
particles of sand and gravel size may be abundant
(GESAMP, 1990; OSPARCOM (Oslo/Paris Commission), 1993). Whereas contaminants may be higher in
dredge spoil than in sewage sludge, the opposite is
found for the content of organic matter.
Contaminated dredge spoils, specifically the fineparticulate-mud component, leaves the same problems
for permanent storage as sewage sludge. Repositories
on land are rare, dumping in coastal areas may contaminate shallow-water ecosystems, and in most countries
this type of disposal is banned through national or
international regulations. Although contaminants and
pathogens may be returned to inshore waters and
threaten the local human population (Collins et al.,
1980), dumping of dredge spoils in shallow waters still
occurs and may continue to be permitted (McIntyre,
1995). Certainly, the most important reaction of society
would be the avoidance of sewage production and
river pollution. Progress of waste-water treatment at
industrial plants has led to reductions in the emission
of contaminants. But deep-sea ecologists should be
prepared to conduct environmental impact assessments
comparing terrestrial and deep-sea disposal. Such
evaluation may suggest that deep-sea disposal would be
less of a threat both to human societies and to already
impacted terrestrial communities (see also Valent and
Young, eds, 1997; Young and Richardson, 1998).
The more knowledge deep-sea ecologists can accumulate, the more valuable will be their arguments in
the decision process. Essential prerequisites for impact
evaluations are acquaintance with disposal techniques,
the composition of wastes, and the reactions of
ecosystems and natural communities to the disposal.
For all considerations it is of paramount importance
to remember that the dumping of sewage sludge and
dredge spoil are large-scale intrusions into the deep sea.
The amount to be disposed of into an abyssal dump site
may need to reach some 10
6 m
3 per year to ensure that
the process is economically feasible. Since the general
problems of disposal techniques and the environmental
research requirements are similar, sewage sludge and
dredge spoil are not further distinguished in this article
and they are both referred to as mud or waste.
Basically, two types of techniques are considered,
differing in their mode of waste delivery to the
seafloor: uncontained discharge or contained dumping.
Uncontained waste might be released at the ocean
surface, as it was at DWD 106. Because of its low
water content and high compactness it may sink as one
large parcel, with more or less turbulent mixing and
plume creation, and particulate transport by currents.
Since great depth is preferable, the water column may
be impacted over depths from 4000 m to 5000 m. For
this reason, a riser technique would be preferable –
carrying the muds in a pipe system down to about
100 m above the seafloor, where the material would be
released for settlement. Such a riser system may be
shipborne and deployed at the disposal site, or it may
be an anchored floating system, at which the transport
ships would dock to pump the mud down into the abyss
(Fig. 13.5). On release from the pipe the slurry would
spread out over the seafloor, and some light materials
would drift for some distance with the current. The riser
technique would avoid any contamination of the water
column except in its deepest part.
Even better isolation from the environment would
result from the disposal of muds contained in strong
flexible plastic bags (De Bruin and Loos, 1995) with
a capacity of 350 m
3 . Properly sealed, these parcels
would be released from the transport ship to sink to
the seafloor as single units (Fig. 13.6), or they would
be dropped close to the seafloor from some sort of
autonomous underwater vehicle (AUV) (Fig. 13.7).
Several of them could be carried to the dump site
loaded on a transporter. Liberated from the mother
ship, the AUVs would approach the seafloor, dump the
bags, probably above a field marked by transponders,
and return to the surface for recovery by the carrier
(Valent and Young, 1995). If the bags were stable, the
waste material would remain contained, if the synthetic
geotextile is degradable, the waste material would be
released after a longer or shorter lag time.
The results obtained by monitoring pollution indication at and around DWD 106 cannot be applied fully
to an abyssal dump site, since the former is at about
half the depth and governed by a more dynamic regime.
However, several detrimental effects can be predicted,
441
• bacteria dominated by methanogenic species, which
continue to ferment the sludge, coliform bacteria
and human pathogens;
• industrial contaminants, including heavy metals and
hydrocarbons.
Dredge spoils may largely contain the same classes
of anthropogenic contaminants as sludges: hydrocarbons, heavy metals and synthetic organic substances.
However their origin is different. They are dredged
from river, channel and harbour systems to keep
the waterways open or deepen them for ship traffic.
Uncontaminated dredge spoil may be used for building
material, land reclamation or road construction, since
particles of sand and gravel size may be abundant
(GESAMP, 1990; OSPARCOM (Oslo/Paris Commission), 1993). Whereas contaminants may be higher in
dredge spoil than in sewage sludge, the opposite is
found for the content of organic matter.
Contaminated dredge spoils, specifically the fineparticulate-mud component, leaves the same problems
for permanent storage as sewage sludge. Repositories
on land are rare, dumping in coastal areas may contaminate shallow-water ecosystems, and in most countries
this type of disposal is banned through national or
international regulations. Although contaminants and
pathogens may be returned to inshore waters and
threaten the local human population (Collins et al.,
1980), dumping of dredge spoils in shallow waters still
occurs and may continue to be permitted (McIntyre,
1995). Certainly, the most important reaction of society
would be the avoidance of sewage production and
river pollution. Progress of waste-water treatment at
industrial plants has led to reductions in the emission
of contaminants. But deep-sea ecologists should be
prepared to conduct environmental impact assessments
comparing terrestrial and deep-sea disposal. Such
evaluation may suggest that deep-sea disposal would be
less of a threat both to human societies and to already
impacted terrestrial communities (see also Valent and
Young, eds, 1997; Young and Richardson, 1998).
The more knowledge deep-sea ecologists can accumulate, the more valuable will be their arguments in
the decision process. Essential prerequisites for impact
evaluations are acquaintance with disposal techniques,
the composition of wastes, and the reactions of
ecosystems and natural communities to the disposal.
For all considerations it is of paramount importance
to remember that the dumping of sewage sludge and
dredge spoil are large-scale intrusions into the deep sea.
The amount to be disposed of into an abyssal dump site
may need to reach some 10
6 m
3 per year to ensure that
the process is economically feasible. Since the general
problems of disposal techniques and the environmental
research requirements are similar, sewage sludge and
dredge spoil are not further distinguished in this article
and they are both referred to as mud or waste.
Basically, two types of techniques are considered,
differing in their mode of waste delivery to the
seafloor: uncontained discharge or contained dumping.
Uncontained waste might be released at the ocean
surface, as it was at DWD 106. Because of its low
water content and high compactness it may sink as one
large parcel, with more or less turbulent mixing and
plume creation, and particulate transport by currents.
Since great depth is preferable, the water column may
be impacted over depths from 4000 m to 5000 m. For
this reason, a riser technique would be preferable –
carrying the muds in a pipe system down to about
100 m above the seafloor, where the material would be
released for settlement. Such a riser system may be
shipborne and deployed at the disposal site, or it may
be an anchored floating system, at which the transport
ships would dock to pump the mud down into the abyss
(Fig. 13.5). On release from the pipe the slurry would
spread out over the seafloor, and some light materials
would drift for some distance with the current. The riser
technique would avoid any contamination of the water
column except in its deepest part.
Even better isolation from the environment would
result from the disposal of muds contained in strong
flexible plastic bags (De Bruin and Loos, 1995) with
a capacity of 350 m
3 . Properly sealed, these parcels
would be released from the transport ship to sink to
the seafloor as single units (Fig. 13.6), or they would
be dropped close to the seafloor from some sort of
autonomous underwater vehicle (AUV) (Fig. 13.7).
Several of them could be carried to the dump site
loaded on a transporter. Liberated from the mother
ship, the AUVs would approach the seafloor, dump the
bags, probably above a field marked by transponders,
and return to the surface for recovery by the carrier
(Valent and Young, 1995). If the bags were stable, the
waste material would remain contained, if the synthetic
geotextile is degradable, the waste material would be
released after a longer or shorter lag time.
The results obtained by monitoring pollution indication at and around DWD 106 cannot be applied fully
to an abyssal dump site, since the former is at about
half the depth and governed by a more dynamic regime.
However, several detrimental effects can be predicted,