Port Newark/Elizabeth project (Cont.)

The final overall dimensions of the contaminated sediment mound, as

defined by the 0.3-m contour, were approximately 630 m in the north/south

direction and 645 m in the east/west direction. If the 0.15-m contour is de-

fined as the edge of the main mound, then the mound dimensions increase

to approximately 745 m in each direction as shown in Figure 32. SPI sur-

veys of the contaminated sediment apron showed the apron extended out

approximately 400 m in each direction beyond the outer edge of the dis-

posal lanes, creating a roughly circular area to be capped with an average

diameter of 1,370 m (4,500 ft) (Figure 32).

Based on nine SPC transects with three to six stations per transect that

contacted the apron, the average thickness was about 3 to 5 cm. On some

transects, the thickness decreased regularly out from the mound, while on

others the variation was more random. The native bottom was visually dis-

tinct, allowing a visual resolution of a minimum thickness of contami-

nated sediments of 1 to 2 cm. Thus, the edge of the apron was defined as

areas with less than 1- to 2-cm thickness of dredged material.

Prior to the start of the capping operation, New York District and EPA

Region II staff decided to cap the contaminated mound including the

apron with 1 m of sand. This required what was initially estimated as

1,500,000 m3 of sand to cap the area shown in Figure 32. On 11 July 1994,

hopper dredges began placing cap material, 0.4 mm sand from Ambrose

Channel, over the contaminated sediments. At least two intermediate sur-

veys and additional capping were required before capping was completed

in February 1994, when an estimated total of 1,870,000 m3 of sand had been

placed covering the entire contaminated footprint with close to a meter or

more of sand. The additional 370,000 m3 (480,000 yd3) over the original

estimate (a 25-percent increase) was due to the requirement to provide a

1-m cap everywhere as opposed to an average of 1 m. Capping the con-

taminated main mound as defined by the 0.15-m contour with 1 m of sand

would have required an estimated volume of approximately 450,000 m3.

If instead of the 1-m cap placed over the apron, a 0.30-m cap had been

placed over the apron, it would have required an estimated 308,000 m3,

for a total cap volume of 758,000 m3. Increasing that total by 25 percent

to provide a minimum 1-m cap over the main mound and a 30-cm cap over

the apron would have brought the total to 940,000 m3, or approximately

half the amount actually placed.

Due to concerns about the possible adverse effects of contaminated

sediment resuspension during the cap placement, EPA Region II required

that the initial 15 cm of cap placed impact the bottom with as little down-

ward velocity as possible (i.e., sprinkled at the individual particle settling

velocity). This required modification of previous capping procedures rou-

tinely used where barge or hopper dredges perform conventional bottom

dumping operations. Randall, Clausner, and Johnson (1994) discuss modifi-

cations made to the STFATE model (and now incorporated into the MDFATE

model), based on experiments using planar laser-induced fluorescence

(Roberts, Ferrier, and Johnson 1994), used to model cap placement.

The capping procedure consisted of using the spit-hull hopper dredges

Dodge Island and Manhattan Island and the hopper barge Long Island dis-

charging over predetermined lanes to cover the contaminated mound. The

split-hull dredges "sprinkled" their average 2,000-m3 loads over a period

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Chapter 10 Case Studies