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placement of the cap material over the mound accomplished with the aid
of a taut-wired buoy and accurate navigational controls proved to be suc-
cessful. The stability of these mounds has been tested by 11 years of
monitoring and the passage of Hurricane David in 1979, although the hur-
ricane's passage was coincident with the predicted exponential compac-
tion phase of the mound, and Hurricane Gloria (Fredette et al. 1989). It is
desirable for the mound/cap formation to occur well before any storm win-
dows in order that natural settlement and compaction has time to occur.
All three mounds showed normal biological recolonization rates in sub-
sequent monitoring. Sediment chemistry data show the surface sediment
remained at or below background concentrations of the contaminants
measured. Coring data show a clear visual and chemical boundary in
many of the cores.
The historical record of the successful capping of the STNH mounds
and CS-2 provided comparative insight as to why other capping projects
were not as successful. For example, accurate placement of dredged sedi-
ments is less reliable without the use of both a buoy and an accurate navi-
gation system, and their lack of use was attributed to the offset of the cap
and mound at CS-1. The Mill-Quinnipiac River mound (MQR) demon-
strated the importance of controlling operational factors and maintaining
vigilant monitoring. Biological monitoring at the MQR showed subnor-
mal recolonization rates relative to the other CLIS mounds. The disposal
operations that included the Mill-Quinnipiac River and Black Rock and
New Haven harbors were not conducted as distinct mound and cap deposi-
tional phases. The overlapping cap/mound deposition may have affected
the recolonization rate at MQR. Similarly, the Norwalk mound was not
formed in distinct cap and mound operations. The contaminant concentra-
tions for both the mound and cap at Norwalk were well below those of
Black Rock and MQR, and there was no evidence of adverse effects due to
disposal operations at Norwalk in subsequent monitoring. Sediment chem-
istry results from MQR show that the surface chemistry of the mound was
not similar to Black Rock sediments; instead, concentrations were at the
high end of the range of most constituents analyzed in New Haven sedi-
ments. However, these monitoring results have allowed NED to detect
and take corrective management actions.
During a 1993 NED capping project, maintenance sediments from New
Haven Harbor and private terminals were placed in the CLIS. A total of
approximately 500,000 yd3 of contaminated material was dredged from
New Haven Harbor and private terminals followed by capping with about
660,000 yd3 of cap materials. Placement of the contaminated sediments
was controlled with a taut-wire buoy, while a total of 18 separate place-
ment points (using LORAN-C) were specified for the cap placement.
Throughout the cap placement process, continuous monitoring allowed for
adjustment of disposal points to optimize cap coverage and avoid point
dumping.
The unique aspect of this project was that the mounds created from five
previously placed projects were used to make a bowl in which to place the
500,000 yd3 of New Haven sediments (Fredette 1994). At the center of
the bowl, the depth was 62 ft, while the surrounding depths were generally
0.6 to 10 ft shallower. Surveys showed that the planned depression was
110
Chapter 10 Case Studies

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