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impact. It is, of course, often advisable to schedule dredging and disposal operations 
to avoid disrupting spawning activities and fish migrations. However, studies 
showed that most adult organisms can tolerate turbidity levels and durations far in 
excess of what dredging and disposal operations produce. These studies, conducted 
in the laboratory and verified in the field, involved a variety of marine, estuarine, 
and freshwater organisms. 
With regard to benthic or bottom-dwelling organisms, their resiliency, once 
beyond the larval stage, was demonstrated. Disposal sites can be and are rapidly 
recolonized by the establishment of new populations, by migration of organisms 
from adjacent unaffected areas, and by survival of the organisms buried. Coloniza- 
tion by opportunistic species can occur within weeks and by the original species 
within months. When the type of dredged material disposed at a site is of the same 
grain-size distribution as the natural botton (e.g., sand deposited on sand or silt on 
slit), survival of existing organisms is maximized. Conversely, a mismatch of sedi- 
ment type can be quite detrimental. The condition that could be most unjurious to 
benthic organisms is when the disposal operations, primarily hydraulic pipleline op- 
erations, produce a fluid mud or “fluff” layer that is a difficult and alien environ- 
ment for many organisms. 
It was shown that certain aquatic organisms will uptake chemical contaminants 
from dredged material. However, the patterns of uptake were found to be unpredict- 
ably erratic and there were no clear trends. 
Different types of organisms will uptake different quantities of contaminants such 
as heavy metals depending on an apparent variety of environmental and biological 
factors. The complexity of this process and the low level of predictive capability 
have been controlling factors in the decisions that bioassays must be an integral 
part of the evalutive criteria used in implementing the Section 404 and 103 pro- 
grams. It is fully realized that bioasay tests are expensive and time consuming, but 
the state-of-the-art allows no effective alternative for determining how organisms 
will be affected by contaminated dredged material. 
Determining the effects of open-water disposal has been somewhat like trying to 
strengthen a chain. Once the weakest link is found and strengthened, attention is 
necessarily then directed to the next weakest link. Major DMRP field studies of 
open-water disposal sites strengthened several links. They verified several major 
laboratory findings and showed that short-term impacts are quite brief and are not 
of major environmental significance. These indeed can occur, but are certainly going 
to be the exception rather than the rule. In addition, studies have called attention 
to situations where open-water disposal has even had benefical environmental ef- 
fects and have identified operational procedures that can be used to reduce impacts 
without new technology or major cost increases. 
The next weakest link in the strengthened chain involves long-term biological im- 
pacts. Certain selected field test sites will be monitored for 3 years beyond the end 
of the DMRP to provide some much-needed information on this subject; however, 
many answers still will not be forthcoming. Among these will be ones relating to 
chronic or sublethal effects of very long-term exposure of benthic organisms to con- 
taminated material and effects on reproduction. 
Thus far, mention has been made primarily of assessing the effects of open-water 
disposal and very little about controlling or mitigating effects when they occur. This 
aspect was not overlooked, and even when an effect was found to be an unlikely 
event, it was presumed there could be instances where control or regulation would 
be advisable for one reason or another. A good example is turbidity. Even though 
adverse biological effects are highly unlikely, there may be reasons why excess tur- 
bidity should be minimized. One study called attention to how simple equipment 
maintenance and efficient operation can reduce turbidity and another extensively 
evaluated and developed guidelines of using silt curtains or ‘diapers,’ pointing out 
when they can be effective and when they will only mask the problem and not alle- 
viate it. For example, silt curtains are ineffective where currents exceed 1 knot and 
will be both ineffective and uncontrollable under moderate wave conditions. 
The DMRP included considerations of dredging equipment development in very 
few cases as this was largely beyond its scope. However, because of the peculiar 
nature of the problem of turbidity, a concept was developed for the submerged dis- 
charge of material from a hydraulic pipeline dredge through a specially designed 
underwater diffuser. Model tests of the diffuser showed it has excellent potential for 
reducing turbidity as well as for reducing the extent of the potentially harmful fluid 
mud layer that so often develops. 
On a related subject, various studies considered the feasibility of treating contami- 
nated dredged material to reduce the impact of disposal operations. Because of the 
large volumes and variable nature of the material involved and the very low con- 
