3.5 m at the center to layers less than about 20 centimeters thick 

 beyond a radius of about 150 to 200 meters (SAIC, 1980) . Cap 

 thickness also varied in different places on the mound. This 

 suggests that the original cap layer could have been relatively 

 thin at some of the outlying stations which showed contaminant 

 concentrations above levels detected at the Center station in the 

 present study. Bioturbation in these areas could have resulted in 

 mixing of underlying contaminated sediments with the cap material, 

 thereby elevating contaminant concentrations in surface sediments 

 above those measured at stations such as the , where cap material 

 alone was sampled. The fact that STNH-N had the deepest RPDs and 

 a relatively high percentage of Stage III organisms compared to the 

 other CLIS mounds gives strength to this interpretation. 



Comparisons of data from samples collected at stations 

 in the vicinity of the STNH-N disposal mound in 1982 and 1984 did 

 show some apparent trends with time (Table 3-26) . The most 

 striking trend was the consistency in statistical results for Top 

 and Bottom samples at individual stations: when 1982 or 1984 

 results were significantly different from the 1986 results, this 

 was generally true for both the Top and Bottom samples. However, 

 no systematic patterns beyond the individual station were apparent. 

 It is therefore difficult to attribute the increases at station 

 200E from 1982 and 1984 to 1986 to transport of dredged material. 

 Localized washing of the mound could account for some changes from 

 year to year at individual stations. However, there were very few 

 significant differences in concentrations at the mound apex 

 (station Center) , where such washing presumably occurs with greater 

 frequency and/or intensity. 



4.5 Benthic Community Analysis 



Another objective of the 1986 field operations was to 

 analyze the benthic community structure at selected disposal mounds 

 and the new CLIS reference station. The faunal assemblage 

 recovered at the CLIS reference station was similar to that found 

 at the same location in the EPA/COE Field Verification Program. 

 That long-term study showed that many of the dominant species go 

 through irregular cycles of abundance. Since Mulinia is one of the 

 most variable species, it is likely that the mature specimens found 

 in the July samples may be absent in future samples. Dominance was 

 shared by a large number of polychaete and mollusc species, while 

 crustaceans were relatively uncommon, presumably because of the 

 negative effects of high water content, fine-grained sediment. 



Benthic community analyses were performed at the FVP 

 mound because it had long-term data for comparison. Despite the 

 presence of sand and shells in the FVP Center samples, most of the 

 species recovered were ones adapted for soft bottoms. The number 

 of species was similar to that found at the reference station (37 

 at FVP Center vs. 35 at the reference station) , but numbers of 



40 



