Deepwater Disposal 



Traditionally, deepwater disposal of dredged rraterial has been the most 

 frequent disposal method. This was usually economically advantageous over con- 

 fined or unconfined land disposal or confined shallow-water disposal. However, 

 increasing concern about impacts to the water column and bottom sediments from 

 contaminants has resulted in prohibition of the dumping of "polluted" mate- 

 rials into open waters. The definition of what constitutes polluted materials 

 is difficult and controversial. A prime problem is the lack of information 

 about the mere presence versus the actual impact of contaminants on aquatic 

 organisms. 



Several Great Lakes studies indicate that open-water disposal influences 

 the water column for only a few hours because of rapid particle settling and 

 dilution (Fulk et al. 1975, Sly 1977, Sweeney 1978a, K'yeth and Sweeney 1978). 

 With the exception of ammonia, manganese, and zinc, there does not appear to 

 be a significant release of contaminants to the water column during the de- 

 scent of the dredged material to the bottom. Other studies have indicated that 

 dredged material deposited in deepwater may continue to influence overlying 

 waters for as long as 5 yr, apparently through resuspension (Sweeney et al. 

 1975, Sly 1977). 



Overall, Sly (1977) noted that although dredging and ship turbulence 

 caused local turbidities the impacts were small in comparison to those result- 

 ing from wind and wave action. Both Langlois (1941) and Chandler and Weeks 

 (1945) found that turbidity in Lake Erie rose from an average of 40 mg/1 to 

 over 200 mg/1 following disturbance of the bottom by 64 km/hr winds. 



Field studies have indicated that impacts of dredged material disposal to 

 phytoplankton and zooplankton are insignificant (International Working Group 

 1975, Sly 1977). However, stimulation of algal growth has been demonstrated 

 In the laboratory. Large releases of phosphorous and nitrogen have occurred, 

 at least for a few hours, following disposal (International Working Group 

 1975). 



Disposal of dredged m^aterial affects the distribution of fish. Fish may 

 either be attracted to the area of disposal or repelled (International Working 

 Group 1975). Sweeney (1978b) noted a 2- to 30-min absence of fish following 

 disposal. The time of absence varied with species. Turbidity, chemicals of 

 various kinds, chances in substrate, and changes in fish-food orqanisms--al 1 

 affected by disposal--influence fish distribution. Sweeney (1978b) noted 100% 

 mortality of fish eggs within 250 m (270 yd) of a disposal site at Ashtabula 

 in Lake Erie. 



Dredged material has changed the composition of the benthic communities 

 for short periods but long-term, subtle impacts are unknown. Beneficial im- 

 pacts can include improvement of fishery habitat, e.g., disposal mounds may be 

 used for spawning areas and polluted bottom sediments may be covered with 

 cleaner materials (International Working Group 1975). In most instances, the 

 dredged sediments will not be of a suitable grain size or free enough of con- 

 taminants for the above benefits. Most adverse impacts appear to be due to 

 smothering and change in substrate. The extent and duration of impacts depend- 

 ed upon species composition, quantity and type of materials deposited, and the 

 duration of disposal activity (International Working Group 1975). Recovery 



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