6.4.4 Tier III - Groundwater Modeling (Cont.)

routing in the soil profile is composed of balancing the water budget at the ground

surface and then routing the infiltrated water and the available contaminants

throughout the soil profile. The Hydrologic Evaluation of Landfill Performance

model (HELP) is used for surface water hydrology, infiltration, and drainage in

the soil.

Since the HELP model was developed for evaluating landfill performance, it

offers additional features that are useful in CDF design and performance

evaluation. These features include the use of sand or gravel layers for lateral

drainage or leachate collection and clay and synthetic materials as liners. To

allow for flexibility in the design of confined disposal facilities, lateral drainage of

leachate and barrier liners can also be used in HELPQ for preliminary design and

CDF performance evaluation.

Contaminant routing in the soil profile relies heavily on the results of the

subsurface water routing performed by the HELP model. Routing of

contaminants begins after vertical drainage, lateral drainage, and soil moisture

contents are computed. Except for lateral drainage layers, contaminants enter a

layer from above and leave from below. In lateral drainage layers, contaminants

may also leave the layer laterally to a drain, and hence out of the CDF, thus

reducing the amount of contaminant entering the barrier soil liner and eventually

contaminating the groundwater. Since the HELP model allows for

evapotranspiration, contaminant mass may increase in the soil segments affected

by this process; volatilization of contaminants is not modeled in HELPQ. When

lateral drainage layers are used, lateral drainage occurs at the top of liner systems

or barrier soils. Therefore, lateral drainage in the contaminant routing model is

taken into consideration in the mass balance for contaminants at the bottom of

lateral drainage layers. The net result is a decrease in the amount of contaminants

that may percolate into the underlying barrier soil.

The HELPQ program requires partitioning coefficient data for the

contaminants to be considered, initial concentrations of the contaminants in each

soil layer, and the salinity (conductivity) in each layer if the dredged material is of

estuarine origin. Equilibrium-partitioning data for pollutants that are typically

present in dredged material are classified as one of the following types: a constant

partitioning coefficient, a point Kd a data-averaged Kd, a best fit Kd, or a salinity-

dependent Kd. The partitioning data could be conservative values from the

literature, past dredging projects, or testing. In addition, HELPQ requires the

same data needed to run the HELP model such as weather data (precipitation,

temperature, evapotranspiration) and soil and design data (soil properties, layer

types, etc). The HELP model input requirements are explained in Schroeder et al.

(1994a and 1994b).

The use of the water budget method for routing contaminants in CDFs

provides an economic method for preliminary design and for evaluating the

performance of various CDF design alternatives. The HELPQ model produces

results that can be used by management and planning personnel for assessing the

potential contamination of surrounding waters due to the construction of a CDF.

Moreover, the use of lateral drainage layers and clay liners to control and restrict

6-14

Chapter 6

Leachate to Groundwater