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
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
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
November 1992
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