6.1.1 Leachate and Contaminant Transport Considerations

groundwater, and infiltration of precipitation. Immediately after dredging and

disposal, dredged material is saturated (all voids are filled with water). As

evaporation, consolidation, and seepage remove water from the voids, the amount

of water stored and available for gravity drainage decreases. Thus, leachate

generation and transport in a CDF depend on site-specific hydrology and

geohydrology, engineering controls at the disposal site, dredged material hydraulic

conductivity, initial water content, and nature of any contaminants in the dredged

material. The potential leaching pathway and processes are shown in Figure 6-1.

Figure 6-1. Illustration of potential CDF leachate pathways

If there is leachate from upland CDFs, it typically seeps through the vadose

zone (soil above the water table) and/or the saturated groundwater zone where it

can affect groundwater quality. Leachate from upland CDFs can also seep

through the dikes to the surface of adjacent lands but this seepage typically

evaporates or infiltrates and does not generally pose an environmental concern. If

the site is situated so that groundwater will flow through the dredged material

within the CDF (typically, a nearshore CDF), percolating groundwater may be the

primary source of water through the material. If the CDF is a nearshore or island

facility, surface water may be in contact with the dredged material as a result of

fluctuating water levels and transport contaminants from the CDF in a process

termed "tidal or wave flushing" (Schroeder 2000).

6.1.1 Leachate and Contaminant Transport Considerations

Contaminant migration via leachate seepage is a porous medium contaminant

transport problem (Figure 6-2). Solid particles will not migrate with the leachate,

but the contaminants in the aqueous phase are convected with pore water in the

dredged material as leachate. As leachate is transported through the porous media

of the vadose zone, the contaminant concentrations are reduced as the leachate

passes through cleaner layers of dredged material, foundation soils, and fine-

grained soils. This process is called attenuation. The contaminant concentration

6-2

Chapter 6

Leachate to Groundwater