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Page Title: 6.1.2 Water Quality Standards for Leachate
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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

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