Operational Controls

ERDC TN-DOER-C18

August 2000

Containment refers to the ability of the site with associated design features to hold the contaminants

within the site as opposed to treatment approaches intended to destroy or degrade contaminants or

chemically (not physically) immobilize the contaminants within the sediment. Control measures

may include operational modification, selective placement of dredged material, engineered site

controls or containment features, and other site management actions (USACE/USEPA 1992).

Operational Controls. Site operations can be used as a control measure for CDFs to reduce the

loss of contaminants through the surface water, volatilization, and leachate pathways. Operational

controls may include management of ponded water on the site during and after disposal operations.

Mobilization of some contaminants from dredged material depends on a variety of factors, including

the oxidation state of the chemical species. Most metals are much less mobile when maintained in

an anaerobic and reduced condition. On the other hand, oxidizing conditions tend to favor aerobic

biodegradation of organic contaminants. Moist, exposed sediments generally present the greatest

potential for volatilization of organic contaminants. Ponded conditions that normally exist in

nearshore and in-water CDFs can limit volatilization. Water depth also may affect plant and animal

uptake. Maintaining ponded water on the site produces a hydraulic gradient that increases the

potential for movement of leachate through the site. Plant and animal propagation affects contami-

nant uptake. Management of the site for contaminant controls both during filling and after dredged

material placement requires a comprehensive understanding of the migration pathways and the

effects various operations have on the overall mass balance and rate of contaminant releases

(USACE/USEPA 1992). Oftentimes, trade-offs are required to balance effectiveness and cost and

to reduce environmental risk to acceptable levels. Operational techniques for CDFs that have been

used for contaminant control include:

Placement sequencing or sandwiching: filling the CDF with alternating layers of clean and

contaminated material to provide for attenuation (sorption, ion exchange, filtration, biode-

gradation, etc.) or containment of contaminants.

Self-sealing/self-lining: taking advantage of the fine-grained nature of dredged material,

which yields low permeability when subjected to consolidation in a CDF.

De facto covers: placing dredged material with suitable chemical and physical properties as

the final layer in a CDF.

Drainage layers: placement of sand layers to enhance dewatering and consolidation.

Control of ponded water to reduce hydrostatic head or maintain a negative hydraulic gradient

(conditions causing seepage flow into the CDF as opposed to flow from the CDF).

Selective Placement Configurations and Sequencing. Selective placement is the place-

ment of contaminated sediments within the CDF where contaminants remain relatively immobile

or the placement of clean dredged material to intercept or attenuate contaminant migration from

contaminated dredged material. Selective placement configurations with respect to water levels are

possible for nearshore and in-water CDFs. Selective placement below the groundwater or surface

water elevation keeps that portion of the CDF fill anaerobic, which reduces the potential for release

of some classes of contaminants of concern (especially metals) to the dissolved phase. Selective

placement can also take the form of configuring the CDF fill with a greater depth and smaller surface

area. This technique reduces the "footprint" of the site subject to erosion, plant and animal uptake,