134 



over 12,000 years ago have not reached geochemical equilibrium (Valent and Young 1995). 



Regarding the contaminants contained in the wastes, US Army Corps of Engineers experience 

 with dredged material placed at shallow water depths indicate that the contaminants would 

 remain adsorbed on the clay mineral particles with some of the organic contaminants adsorbing 

 on organic particulates in the dredged material. Geochemical changes in the waste deposit could 

 cause iron and manganese compounds to go into solution, but the iron and manganese would 

 then precipitate as oxides when reaching the deposit interface with the oxygenated overlying 

 water. The oxides would then serve as scavengers of other heavy metal contaminants that may 

 leach to the deposit-seawater interface (Spencer 1991, p 78). Sewage sludge poses possibly a 

 somewhat greater problem due to a potential deficit of appropriate clay minerals to provide 

 adsorptive surfaces for scavenging heavy metal ions. This shortcoming could be corrected by 

 blending clay mineral material into the sewage sludge during handling, which would increase 

 overall volumes to be isolated and cost per unit volume. Fly ash poses the largest problem to the 

 waste isolation option due to its easily leachable, high content of lead, cadmium, dioxins and 

 furans. To isolate fly ash on the abyssal seafloor, the fly ash would probably have to be blended 

 into a fme-grained, organic-rich dredged material to provide sufficient adsorptive surfaces for the 

 contaminants. Developing an adequate approach to isolation of contaminants within a sewage 

 sludge or fly ash deposit would require some laboratory experimentation which was beyond the 

 scope of the DoD tasking for this study. 



Conclusions - 



(1) Placement of subject wastes on the abyssal seafloor with no accompanying loss to the water 

 coluitm appears feasible using technology that could be developed. 



(2) Model predictions suggest that for reasonable waste isolation scenarios the placed wastes 

 would (a) likely be contained locally within a defmed site, (b) bury local faima which will be 

 replaced by a new abyssal community of animals, and (c) impact local geochemical processes for 

 thousands to tens-of-thousands of years. 



(3) Regarding potential abyssal sites for the isolation of wastes (a) the Atlantic offers the most 

 favorable sites, (b) the Pacific sites are favorable but less so than those in the Atlantic, and (c) the 

 Gulf of Mexico offers poor choices for isolation sites. 



(4) Overall impacts of placed wastes are predicted to be localized in extent. However, very little 

 is actually known about the environmental impacts of such emplacement. 



Reconmiendations - Before deep ocean isolation of wastes could safely occur, significant 

 additional research would be needed. 



(1) The models upon which this study was based, and disposal activity would be based, need to 

 be refined. Some additional models need to be developed. This is imderway in the ARPA study, 

 discussed below. 



(2) In-situ research would be needed to learn more about the abyssal environment, including its 

 processes and inhabitants, to accurately assess potential enviroimiental impacts. 



(3) Research must be undertaken to better predict the performance of waste-filled geotextile bags 



