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4.0 CAPPED MOUND CORING 

 INVESTIGATION 



Despite the lack of evidence of cap 

 failure, questions concerning the chemical 

 integrity of mounds have persisted. 

 Previous investigations of capped mounds 

 have suggested that there is a distinct 

 physical and chemical boundary between 

 mound and cap. Coring investigations of 

 an experimentally capped mound at the 

 New York Bight Mud Dumpsite revealed 

 that the sand-mud interface was distinct 

 visually and could be recovered with 

 vibracoring operations (Bokuniewicz 

 1989). Grain size analysis of cores 

 showed that the transition from sand to 

 mud occurred over a distance of less than 

 a few centimeters. Preliminary chemistry 

 results of vertical core studies of sand- 

 capped mounds in the Duwamish waterway 

 supported the conclusion that the mound 

 and cap material formed a sharp, relatively 

 unmixed interface (Truitt 1986). 



A coring investigation was initiated in 

 1990 to revisit three of the capped mounds 

 located at COS (STNH-N, STNH-S, and 

 CS-2). The initial assumptions of this 

 investigation were that the caps should 

 have relatively low levels of contaminants 

 and should be visually and chemically 

 distinct from the underlying contaminated 

 material. The guiding hypotheses were: (1) 

 if the interface was distinct visually, then 

 the mound material has been physically 

 isolated, and (2) if the interface was 

 distinct chemically, then the mound 

 material has been chemically isolated by 

 the capping operation. If chemical 

 gradients existed in the cap, then 



contaminants may have migrated from 

 mound to cap. 



It is important to note that there was 

 no independent criterion for distinguishing 

 the cap/mound interface. Distinct 

 interfaces were discernible, but there is no 

 conclusive method of determining the 

 original interface between the mound and 

 the cap. For management purposes, this 

 distinction is not crucial as long as the 

 contaminants remain isolated from the 

 biotic communities. However, information 

 concerning the fine-scale distribution of 

 sediments and contaminants within 

 historical capped mounds can be used to 

 evaluate the assumptions behind the design 

 of capped mounds and to guide future 

 investigations and capping operations. 



Capped mounds were cored in roughly 

 cross-shaped sampling arrays (CS-2, 

 Figure 2-10; STNH-N, Figure 3-3; STNH- 

 S, Figure 4-1) located away from the peak 

 heights in an attempt to sample three 

 distinct (cap, mound, and base) layers 

 from each capped mound. Cores were 

 named according to their location: e.g., a 

 core taken from the center of the capped 

 mound was identified as CTR; a core 

 taken from 80 m north of the center station 

 was identified as 80N. Sediment samples 

 were analyzed for Cd, Cu, and Zn, as well 

 as for total recoverable petroleum 

 hydrocarbons (TRPHs) and grain size. 

 One core from each capped mound was 

 analyzed for pesticides and PCBs, and 

 three cores from each capped mound were 

 analyzed for PAHs. 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



