The same hypothetical worst-case scenario can be used to estimate the 

 potential changes to the physical nature of the sediments. The deposition of 

 this drilling mud would contribute at most 10 percent to the mass of natural 

 sediment in the upper 1 cm over the 1 square km area. Barium sulfate, the 

 only component of drilling mud with significantly higher density than natural 

 sediment, represents a variable fraction (one half to two thirds) of the total 

 drilling mud by weight. 



Generally, deposition of particulates from drilling muds beyond the 

 wellsite should be at a relatively low rate: 0.1 cm/yr, compared with 0.06 

 cm/yr for natural deposition (also a low rate, based on measurements near the 

 canyon head).* Such deposits are unlikely to threaten megafauna, including 

 commercial fish and invertebrate species, except possibly in their early life 

 stages. At the expected low rate of deposition, material derived from 

 drilling muds should be quite thoroughly mixed into natural sediments by 

 physical and biological processes. Moreover, depositions from the drilling 

 mud represent only a fraction of total depositions from all sources in a 

 highly active environment. Thus the physical effects on the substrate beyond 

 500 m should be quite small. 



Because, at least in some canyons, some turbulence and resuspension 

 occurs normally, some or most canyon fauna are not particularly sensitive to 

 sediment in the water. Therefore, the principal effect of the drilling muds 

 on the biota, beyond the near-field blanketing of the bottom, is likely to be 

 chemical. Although chemical constituents of drilling muds may not cause 

 direct mortality, there may nevertheless be an effect upon sensitive 

 organisms. Chemical cues may affect larval settlement or behavior and hence 

 recruitment. 



'See also reviewer comment in Appendix A. 



99 - 



