Step 4 - Evaluate Compatibility of Site, Materials, and Equipment

ERDC TN-DOER-R6

December 2004

Step 4 - Evaluate Compatibility of Site, Materials, and Equipment. At this point in the design,

the contaminated material has been characterized; a site has been identified and characterized; liner and

cover materials have been selected and characterized; equipment and placement techniques have been

selected for both materials; and positioning needs have been addressed. These essential components of the

design (Blocks 2, 3, 4, 5, 6, and 7 in Figure 3) must now be examined as a whole, with compatibility in

mind, to evaluate the efficacy of liner and cover placement for the sediments, site conditions, equipment

availability and capabilities under consideration, and cost. The primary concern with compatibility relates

to contaminated material and the ability of the subbase to support the liner, dredged material, and surface

cover, considering the material characteristics and dredging and placement techniques.

Guidance on the compatibility of various dredging and placement techniques for differing material types

has been developed based on field experience and knowledge of the resulting physical stability of the

materials. If the various site, sediment, and selected equipment components are compatible, additional

and more detailed design requirements can be addressed. If there is a lack of compatibility at this point, a

different CDF site, a different liner or cover material, or different dredging and placement equipment and

techniques must be considered.

Specific attention should also be given to chemical compatibility of the liner materials with the leachate.

Chemical degradation of liner systems can result from interactions of the contaminants and/or the water in

the leachate with the liner system, potentially leading to defects in the liner and increased leakage rates

for leachate transport. For example, a leachate possessing a highly acidic pH can potentially dissolve

metal species normally associated with the clay structure. This leaching of the clay matrix can cause

increased formation of void spaces in the soil matrix, leading to increased hydraulic conductivity. Further,

a study by Anderson (1982) demonstrated that clay can be more permeable to concentrated organic

liquids than to pure water. Special consideration must also be given to employment of bentonite-based

clay materials as liners in the presence of saltwater environments or where the leachate may contain large

concentrations of salt, since the presence of salt has been shown to increase the hydraulic conductivity of

bentonite clays (Petrov et al. 1997, Theriault and Mitchell 1997, Martin et al. 2000).

Chemical degradation effects should be evaluated with falling head permeameters containing at least two

to three samples of soils compacted in the lower portion of the acceptable zone for dry unit weight and

moisture content (Qian 1995). The permeant should reflect the chemical characteristics of the leachate

expected from water infiltrating through contaminated sediments. Compatibility tests for synthetic liner

materials are provided in EPA Method 9090A (USEPA 1992b), while ASTM D6141-97 (ASTM 1997)

provides guidance for evaluation of clay portions of geosynthetic clay liners.

Step 5 - Determine Liner System (Leachate Control and Cover) Design. The liner must be

designed to adequately isolate the contaminated material from the environment and achieve the intended

liner functions. Determination of the required liner thickness is dependent on the physical and chemical

properties of the contaminated sediments and liner materials, and it includes the liner thickness required

for contaminant isolation (physical isolation plus any thickness needed for control of contaminant flux),

plus that required for consolidation. The potential for consolidation and the resultant expulsion of pore

water from the contaminated sediment must also be evaluated. Since the potential for liner consolidation

also depends on the total liner thickness, some iterative calculations may be required. The HELPQ

module (Hydrologic Evaluation of Leachate Production and Quality) (Aziz and Schroeder 1999)) of the

Automated Dredging and Disposal Alternatives Management System (ADDAMS (Schroeder and

Palermo 1995)) can be employed to assist the designer in determining the appropriate thickness of liner

components for contaminant adsorption.

One of the most important design parameters influencing liner material selection is hydraulic

conductivity. Soil and dredged material liners should provide a field hydraulic conductivity of 1x10-8 to