10.3.1 Barrier systems

and the dredged material characteristics. Use of management actions such as

liners, slurry walls, groundwater pumping, and subsurface drainage can be

considered for CDFs. Graded stone dikes with low-permeability cores or steel

sheet-pile cutoffs have been used or proposed at CDFs to control leachate

migration. The low permeability of fine-grained sediments following compaction

can reduce the need for liners in many cases, but it can also limit the effectiveness

and implementability of groundwater pumping and subsurface drainage.

10.3.1 Barrier systems

Barriers are layers of low-permeability materials designed to prevent vertical

or lateral migration of water and minimize groundwater contamination. Soil

barriers can use natural geologic formations of low-permeability material if

available at a site or constructed layers. Barrier systems might utilize soils,

synthetic membranes, grout mattresses, and slurry walls.

10.3.2 Surface covers

A surface cover is a barrier layer placed on top of a filled CDF. The term

surface cover is used here to describe both a cap and cover layer for CDFs to

distinguish this option from a subaqeous cap as used for contaminant control in

the aquatic environment. A cover can be highly effective in reducing leachate

generation by avoiding precipitation infiltration, isolation from bioturbation and

uptake by plants and animals, limiting direct human contact, minimizing

volatilization of contaminants from the surface, and eliminating detachment and

transport of contaminants by precipitation and runoff. A layer of clean material

can achieve the last three benefits mentioned. However, prevention of infiltration

requires a barrier of very low permeability, such as a flexible membrane or a

compacted clay layer, both of which are not easily or reliably implemented for

CDFs.

10.3.3 Liners

Liners are commonly considered as a leachate or seepage control measure and

can be placed on the sides and bottom of a CDF. However, liners have not been

used extensively for contaminated dredged material sites because of the inherent

low permeability of fine-grained dredged material, the retention of contaminants

on solids, and the difficulty and expense of construction of a reliable liner system

for wet dredged material.

Liners may be designed using utilize soils, synthetic membranes, or grout

mattresses. Fine-grained sediments may have permeabilities comparable to clay

barriers following compaction. Leachate collection systems and groundwater

pumping systems may also be considered in conjunction with liners to control

leachate.

10-4

Chapter 10

CDF Contaminant Management Actions