Transport Considerations

ERDC TN-DOER-C16

July 2000

Pollutant Assessment System (Streile et al. 1996), or a groundwater model, such as the DOD

Groundwater Modeling System (Environmental Modeling Research Laboratory 1999), to allow for

prediction of the contaminant concentrations at the receptors. The saturated zone model will predict

the diffusion of the leachate and the adsorption of contaminants on fine-grained and organic matter.

Important factors in the modeling are the groundwater velocity, the location of the receptors, the

thickness of the aquifer, the heterogeneity of the aquifer, and the quantity of fines and organic matter

in the aquifer. Only those contaminants having a concentration that exceeds its water quality

standard upon entering the saturated zone need to be evaluated with a saturated zone model.

Transport Considerations

Groundwater velocity. Groundwater velocity increases the diffusion and dilution of the leachate

plume, but decreases the time that it takes for the leachate plume to reach a receptor.

Receptor locations. Receptors that are located where the groundwater table is upgradient from the

CDF or significantly off the center line of the leachate plume have a small chance of exposure to

the leachate plume. Additionally, the leachate concentration would be greatly diluted by diffusion

into the groundwater.

Aquifer thickness. Aquifers with greater thicknesses have more water in which the leachate can

diffuse. Greater thickness has the potential to significantly decrease the leachate concentration

exposed to distant receptors.

Aquifer heterogeneity. Aquifer heterogeneity has the potential to increase short-circuiting of the

leachate flow to a receptor. Short-circuiting would decrease diffusion and dilution of the leachate

plume.

Retardation capacity. The capacity of an aquifer to retard the contaminants in the aquifer is a

function of the quantity of fine-grained materials, oxides, sulfides, and organic matter in the aquifer

media. If contaminants are present in significant quantities, the long-term exposure will be greatly

reduced. Most aquifers show little retardation, and the retardation tends to be a short-term

phenomenon.

Evaluation of Transport in Saturated Zone. To evaluate transport effects on leachate

concentration in the saturated zone, site-specific aquifer properties affecting leachate diffusion and

contaminant retardation must be considered. Leachate seeps from the CDF and moves through the

vadose zone to the saturated zone. The leachate is then transported as a plume in the groundwater.

The plume diffuses and entrains groundwater, becoming more dilute as it moves downgradient with

the groundwater. Quantifying the dilution and retardation along the center line of the plume as a

function of distance and time from the entry into the saturated zone for a wide range of aquifer

properties provides a basis for evaluating the leachate at the receptor without running a groundwater

model. Additional refinement of the quantification of the dispersion as a function of distance

perpendicular to the center line can improve the estimate of contaminant concentrations at points

off the center line. This quantification is anticipated in the development of the screening procedure.

A groundwater model could be run for the site-specific conditions if increased accuracy in the

predictions were needed to pass the screening.