6.1.2 Water Quality Standards for Leachate

the site boundaries (interface between dredged material and the bottom of

the CDF) will generally be similar regardless of the leachate management

actions used.

Leachate flow in conjunction with leachate contaminant concentration

determines the mass of contaminant that can potentially leave the site boundaries.

Contaminant mass leaving the site boundaries is particularly important when

comparing various leachate management actions such as depth of fill, drainage of

surface water, collection, and treatment.

To determine leachate mass flow, site-specific factors affecting leachate

generation must be considered. After dredging and disposal, dredged material is

initially saturated (all voids are filled with water). As evaporation and seepage

remove water from the voids, the amount of water stored and available for gravity

drainage decreases. After some time, usually several years for conventional CDF

designs, a quasi-equilibrium is reached in which water that seeps or evaporates is

replenished by infiltration through the surface. The amount of water stored when

a quasi-equilibrium is reached and the amount released before a quasi-equilibrium

is reached depend primarily on local hydrology, dredged material properties, and

facility design features. To predict time-varying leachate flow, all these factors

must be considered.

Preproject estimation of leachate flow, therefore, requires coupled simulation

of local weather patterns and hydrologic processes governing leachate generation.

Important climatic variables include precipitation, temperature, wind, and

humidity. Important hydrologic processes include infiltration, runoff, and

evaporation. Important subsurface processes include evaporation from dredged

material voids and flow in unsaturated and saturated zones. The Hydrologic

Evaluation of Leachate Production and Quality (HELPQ) model (Aziz and

Schroeder 1999a and 1999b) can be used to simulate these processes for selected

disposal scenarios.

6.1.2 Water Quality Standards for Leachate

It is the position of the USACE that drinking water standards should be

considered applicable in evaluation of potential leachate discharges only for CDFs

constructed over freshwater aquifers with potential for use for drinking water.

Drinking water standards should not be applied for evaluation of leachate from

nearshore or island CDFs or upland CDFs constructed near or adjacent to

shorelines with underlying brackish or saline aquifers. In such cases, comparison

of potential leachate with applicable surface water standards would be more

appropriate.

Section 230.10(c), CWA Guidelines, prohibits the discharge of dredged

material that might cause significant adverse "effects on municipal water

supplies," and is a guiding principle when determining whether to perform

leachate evaluations. Unless there are overriding navigation factors outlined in

Section 404(b)(2), CWA, discharges of dredged material into CDFs should be

avoided if leachate evaluations reveal the potential for impacts to municipal water

6-4

Chapter 6

Leachate to Groundwater