132 



heavier than seawater. Thus, geotextile bags filled only with sewage sludge would sink very 

 slowly through the water column and drift laterally for long distances. Methane gas generated in 

 the sludge would decrease the bulk density even further, and the bags could float! Therefore, 

 sewage sludge would require blending with a fine-grained weighting material to facilitate 

 transporting the sludge to a specified abyssal seafloor isolation site and then ensuring that the 

 sludge remains in place. Fly ash and dredged material, while not completely free of handling and 

 placement problems, did not pose significant problems in the technical assessment. 

 Concept 1 above was eliminated at an early stage fi-om the technical assessment because it 

 became clear that the rate of placing waste material on the abyssal seafloor would be one-tenth 

 the rate estimated for the remaining concepts; therefore, the bucket concept was eliminated as not 

 being cost competitive. Both risk and capital-operating cost analyses revealed Concept 3, that of 

 free-falling the waste-filled bags from the ocean surface to the abyssal seafloor, to be the best 

 option (see Figure 2) (Hightower et al. 1 995a, b, c). 



Environmental Assessment - 



Site Selection - We sought first to identify areas of the abyssal seafloor within 1800 km (1000 

 nautical miles) of the US mainland where environmental isolation would be maximized, where 

 the environmental impact of placing the wastes on the seafloor would be minimized, and where 

 economic zones of other countries would be excluded. A site assessment model was developed 

 to quantitatively compare the suitability for waste isolation within 10-degree (latitude-longitude) 

 squares of the abyssal seafloor. Included in the analysis were environmental and anthropogenic 

 factors. Areas in the Hatteras Abyssal Plain (Atlantic) and the abyssal hills province west of 

 southern California (Pacific) were shown to be the most suitable for waste isolation because of 

 low currents, low eddy kinetic energy, favorable sediment type, favorable weather, and low 

 anthropogenic activity. Atlantic sites scored somewhat better because of lower seafloor slopes 

 and less roughness. Even the best areas in the Gulf of Mexico were shown to be poorly suited 

 due to the high near-seafloor currents and high eddy kinetic energy (see Figure 3). 



Hydrodynamic Processes - Simulations were conducted using the NRL six-layer, basin scale, 

 ocean circulation model. Model results show that, if any dissolved contaminants were to be 

 released into the water column during waste placement, these contaminants would not be 

 advected shallower than 1000 m water depth for a 10-year simulation period. Well within this 

 period of time, we would expect that the contaminants will have been adsorbed on/scavenged by 

 particulates in the water column and would have settled to the seafloor. Model results, validated 

 with existing data, indicate that near-seafloor currents at the most suitable abyssal seafloor sites 

 will not be strong enough to erode/suspend uncontained dredged material or fly ash (Valent and 

 Young 1995). 



Biological Processes - The overall response of abyssal animals to the placement of one or more 

 million cubic meters of organic-rich material, containing varying degrees of adsorbed 

 contaminants, on the abyssal seafloor is not known with any certainty. It is clear that all resident 

 animals buried under the bags and the sediment apron resulting from the impact plumes 



