To develop a sound and informed understanding of both the waste type and

waste management scenario, and of the geology, hydrogeology,

andgeochemistry of the vadose zone, the uppermost aquifer, and its confining

layer(s); and

To predict the movement of contaminants into and through the uppermost

aquifer.

The conceptual model is the product of the review and interpretation of the data

presentation/reduction outputs (e.g., maps, cross sections) described in the previous sections.

An accurate conceptual model of the site should be the final output of the site

characterization program. The conceptual model should incorporate all essential features of

the hydrogeologic system and waste type under study. The final conceptual model should be

a site specific description of the hydrogeology of the vadose zone, the uppermost aquifer, and

its confining units; it should consider the characteristics of the wastes managed at the facility;

and it should contain all of the information necessary to design an adequate monitoring

system. The degree of detail and accuracy that is necessary to develop a conceptual model

varies according to hydrogeologic setting and waste type. For example, a homogeneous

unconfined aquifer may demand only simple cross sections and water table maps to illustrate

the conceptual model. In contrast, more complicated settings with multiple aquifers, multiple

confining layers, and complex waste types will demand more complex hydrogeologic models

such as flow nets, potentiometric surface or water table maps for each aquifer, geochemical

diagrams, and a series of structure contour and isopach maps. In formulating the conceptual

model, the hydrogeologist should only consider those geologic features that affect

ground water flow, quality, and contaminant transport.

A preliminary conceptual model is formulated early in the site investigation process

using data obtained during the preliminary investigation to establish the hydrogeologic and

waste management setting. The model is gradually refined by building an understanding of

the site specific information obtained during the boring program and other field investigations.

Interpretation of data through cross sections and maps improves the model until a final

integrated picture of the site's hydrogeologic and waste management setting is established.

The development of the conceptual model is an ongoing process that should continue

throughout the entire site characterization program. Interim conceptual models developed at

the various stages of the site characterization are invaluable for planning subsequent field

investigation activities so that they are properly directed towards supplying missing

information.

After the detection monitoring system has been installed, the conceptual model of a

site should be further refined as additional information regarding the site is obtained. For

instance, various natural and artificial factors (e.g., the installation of a water supply well in

the vicinity of the site, salt water intrusion, the construction of a dam) may affect

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