36 



If the sediment originally passed 

 the permit requirements, why might 

 the disposed sediment fail a second 

 round of bioassay tests? Samples 

 taken for the initial permit may have 

 missed contaminant "hot spots", or 

 contaminants in the original test may 

 have been in a form unavailable to the 

 test organisms. As materials are 

 excavated and disposed, redox 

 conditions can change; this in turn 

 may affect the partitioning coefficient 

 of a pollutant compound between 

 particles, interstitial water, and organic 

 carbon. This phenomenon possibly 

 can have an effect on the 

 bioavailability of contaminants. One 

 needs to examine in detail the 

 difference in mortalities among the 

 disposal, reference, and control 

 sediments and use best professional 

 judgment to explore the most 

 parsimonious explanation for a toxic 

 response at this point. 



Underlying Assumptions : Results 

 of the bioassay test are assumed to 

 explain the failure of species (e.g. 

 spionid polychaetes) other than the 

 test organism(s) to colonize a disposal 

 mound. Other assumptions about the 

 adequacy of sediment bioassay testing 

 were given in Section 3 and the 

 discussion of Figure 1 (see Box 1.7). 



Sources of Uncertainty: The 

 assumption that the bioassay test is an 

 adequate "surrogate" test for other 

 organisms is untested. For example, 

 amphipod crustaceans (used as a mid- 

 range sensitivity species in the 

 laboratory bioassay) are probably more 

 sensitive to a wide range of sediment 

 contaminants than spionid or 

 capitellid polychaetes which are 



known to colonize highly 

 contaminated sediments. This 

 inference is based on the observation 

 that spionids and capitellids are found 

 in contaminated sediments that are 

 not colonized by amphipods. If a 

 tested sediment kills a large 

 proportion of the tested amphipods 

 (Box 2.8), these results may be 

 sufficient to explain the failure of 

 polychaetes to colonize a disposal 

 mound but the species-to-species 

 extrapolation is one based largely on 

 faith. The best use of the amphipod 

 test would be to explain the failure of 

 amphipods (a Stage II taxon) to 

 colonize a mound in the n+1 years 

 (Box 2.4). Other sources of 

 uncertainty related to bioassay testing 

 are discussed in detail earlier (see 

 discussion in Box 1.7). 



Box 2.9 "Assume Due to Physical or 



Biological Processes; Reassess 

 in 6-12 Months " 



If no toxic response is observed in 

 the bioassay test species, the logic path 

 leads to an apparent paradox. The 

 cause for a failure of recruitment is 

 not identified from the measured or 

 observed physical-chemical features of 

 the deposit. 



Underlying Assumption : The 

 conclusion is that the cause for the 

 observed colonization anomaly is 

 related to a physical or biological 

 factor that either has been overlooked 

 or not been measured adequately in 

 the monitoring program. For example, 

 intrinsic properties of the recruitment 

 process itself such as decreased 

 reproductive success of parent stocks 

 or external factors such as bottom 



An Integrated, Tiered Approach to Monitoring and Management of Dredged Material Disposal Sites 



