Figure 35. One Tree Island Marina project

Figure 35.

One Tree Island Marina project

of the pit was dredged by clamshell into three barges. Next, the clean

conical pit and additional clean material were dredged into an additional

split-hulled barge and disposed at another deep-water site. The pit capacity

was confirmed, and then the three barge loads of contaminated material

were placed in the pit. Finally, more clean material was dredged by clam-

shell directly into the pit to provide the 1.2-m minimum cap over the con-

taminated sediment. During dredging, a 45-m dilution zone extending

radially from the point of dredging was specified, and outside this area,

local water quality standards were maintained. A monitoring program was

conducted to evaluate the effectiveness of the cap.

Simpson Tacoma Kraft

In 1988, the Simpson Tacoma Kraft Company capped approximately

17 acres of in situ contaminated nearshore bottom area with 0.6 to 3.7 m

of sand hydraulically dredged from the Puyallup River (Sumeri 1989). The

contaminated bottom sediments were the result of 37 years of discharging

untreated mill wastewater, log storage and chipping operations, and storm-

water discharges. The site was a designated EPA Superfund site.

The Puyallup River material was predominantly medium sand with some

clay and small fractions of fine and coarse sand and traces of gravel. This

material was determined to be relatively clean by chemical and bioassay

testing and suitable for capping. Twelve- and ten-inch (30.5- and 25.4-cm)

hydraulic dredges were used to dredge approximately 152,910 m3 of cap-

ping material. This material was transported approximately 1 km through

floating and submerged pipeline to a spud barge for distribution over the

contaminated sediment area. A 2.4- by 4.3-m plywood diffuser box with

119

Chapter 10 Case Studies