Evaluation of CDF and Vadose Zone Effects

ERDC TN-DOER-C16

July 2000

is high in acid volatile sulfides. Oxidation and pH reduction increase the concentration of a number

of metals of concern in the leachate. Additionally, oxidation increases the quantity (leachable

fraction) of the metals in the dredged material that can leach.

Disposal sequence. Sandwiching the placement of the most contaminated dredged material

between layers of less contaminated material or placing clean fine-grained material at the bottom

of the CDF provides a sorptive capacity within the CDF to retard the contaminant migration and to

improve the leachate quality. This type of layering and encapsulating greatly improves the initial

leachate quality, and the improvement may last for decades. In addition, the layering serves

effectively to homogenize or equalize the dredged material quality in terms of the long-term leachate

quality, significantly reducing the maximum contaminant concentrations in the leachate.

Control features. Liners and drains are the primary control features for leachate. Liners can greatly

restrict leachate flow rates from CDFs. Liners also divert leachate to drains that collect the leachate

and route it to a treatment facility. The control measures prevent nearly all of the leachate from

reaching any of the receptors.

Evaluation of CDF and Vadose Zone Effects. To evaluate CDF and vadose zone effects on

leachate concentration, site-specific considerations of 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 factors. To predict time-varying leachate flow, all

of 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 processes and

factors include precipitation, temperature, solar radiation, wind speed, and humidity. Important

hydrologic processes include infiltration, snowmelt, 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 1999) can be used to simulate these processes for selected disposal scenarios.

Quantifying the CDF and vadose zone effects on leachate concentration from its source to the

saturated zone for a wide range of the dominant CDF factors including interactions among the factors

provides a basis for evaluating the leachate at the point of entry into the saturated zone without

running the HELPQ model. This quantification is anticipated in the development of the screening

procedure. The HELPQ model could be run for the site-specific conditions if increased accuracy

in the predictions were needed to pass the screening.

SATURATED ZONE AND RECEPTOR FACTORS: Output from the HELPQ model giving

the leachate flow rate and contaminant concentration from the bottom of the CDF or vadose zone

as a function of time can be linked to a multimedia model, such as the Multimedia Environmental