Figure 6-2. Illustration of the vadose zone, saturated groundwater flow zone, and leachate pathway to groundwater receptors

of leachate exposed to a receptor (such as a well ) is further impacted by disperion

or mixing as the leachate is transported from the CDF locale to the receptor

through the coarse-grained layers of an aquifer. In effect, the contaminant

concentration in the leachate is diluted by the groundwater flow. Attenuation by

adsorption to organic matter and interactions with fine-grained materials will also

occur in the aquifer, but the effect is generally small as a result of low

concentration of organic and clayey materials in the main regions of saturated

groundwater flow.

Figure 6-2. Illustration of the vadose zone, saturated groundwater flow zone, and leachate pathway to

groundwater receptors

Leachate generation and transport depend on site-specific hydrology,

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

initial water content, and nature of contaminants. Therefore, evaluation of

potential leachate impacts will be greatly affected by the nature of the site and the

engineering controls in place. Varying the engineering controls during the

evaluation also allows selection of the optimum controls.

Two aspects of leachate generation from CDFs are of particular concern:

1. Leachate contaminant concentrations. If maximum leachate

contaminant concentrations do not exceed applicable groundwater

standards, this may be sufficient to indicate no need for leachate

management actions. However, maximum leachate concentrations

exceeding such standards, without consideration of leachate flow and

dispersion, do not provide sufficient basis for a decision to implement

leachate control measures.

2. Leachate flow. The flow of leachate from the CDF and its interaction

with groundwater flow is the mechanism for migration to a receptor. The

most significant effect of a CDF leachate management action is in the

leachate mass flow. For example, mass flow through a 1-m lift of the

same dredged material will be higher from a 2-ha site than from a 1-ha

site with the same precipitation and climate. Leachate concentrations at

6-3

Chapter 6

Leachate to Groundwater