6.4.4 Tier III - Groundwater Modeling

6.4.4 Tier III - Groundwater Modeling

Leachate testing provides data regarding the water quality of leachate as it

migrates from the dredged material at the bottom or sides of the CDF. Leachate

pathway evaluations should also consider leachate attenuation, mixing, and

dispersion to determine leachate impacts on a receptor. A variety of groundwater

attenuation and/or mixing or dispersion models are available for this purpose.

These include one-dimensional (1-D) models which simulate vertical migration

and attenuation processes. There are also multidimensional models which may be

used to simulate more complex groundwater flow conditions. Any validated

groundwater model can be used to evaluate CDF leachate attenuation. The

models presented below have been successfully applied to CDF leachate

evaluations.

HELPQ Model for CDF and Vadose Zone. The HELPQ application (Aziz

and Schroeder 1999a, b) of the ADDAMS suite of computer programs (Schroeder

and Palermo 1995) provides a computer program to assist in evaluation of the fate

of leachate as the leachate migrates from the dredged material to the receptors.

HELPQ is the only available leachate attenuation model specifically developed for

evaluation of the CDF leachate pathway.

The HELPQ program accepts data from the leachate tests (such as SBLT or

PCLT) to predict leachate generation and attenuation. Leachate quality and

quantity are predicted as a function of time and location in the vadose zone. The

leachate quality can be compared with applicable water quality standards for

leachate at the appropriate point of compliance. The HELPQ application, along

with documentation, can be downloaded from the USACE DOTS web site at

http://www.wes.army.mil/el/dots.

HELPQ has a quasi-two-dimensional (2-D) hydrologic water budget model

that accounts for the effects of surface storage, runoff, infiltration, percolation,

evapotranspiration, soil moisture storage, lateral drainage to leachate collection

systems, and percolation through liners (Aziz and Schroeder 1999a, b). HELPQ

can model cover soils, dredged material, liner systems, and foundation soils down

to the saturated zone. Alternative scenarios can be selected and evaluated using

the HELPQ model to estimate percolation rates and to compare management

actions. Scenarios which may be evaluated include:

1. Land farming with different lift depths.

2. Different lift depths inside CDFs with no engineering controls other than

routine operation and management for drainage of surface runoff.

3. Extensive CDF management with leachate collection system and a

composite liner (Lee et al. 1992; Brannon, Myers, and Price 1992).

The HELPQ model is developed based on contaminant mass balance and

utilizes the principle of conservation of mass as it applies to the sediment solids,

the percolating fluid (leachate), and the contaminants dissolved in the fluid and

associated with the sediment solids. The hydrologic modeling for contaminant

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Chapter 6

Leachate to Groundwater