management unit, one in the direction of ground water flow and one orthogonal

to ground water flow;

Identify zones of suspected high hydraulic conductivity, or structures likely to

influence contaminant migration through the unsaturated and saturated zones;

Compare findings with other studies and information collected during the

preliminary investigation to verify the collected information; and

Determine whether laboratory and field data corroborate and are sufficient to

define petrology, effective porosity, hydraulic conductivity, lateral and vertical

stratigraphic relationships, and ground water flow directions and rates.

After the hydrogeologic data are interpreted, the findings should be reviewed to:

Identify information gaps;

Determine whether collection of additional data or reassessment of existing

data is required to fill in the gaps; and

Identify how information gaps are likely to affect the ability to design a RCRA

monitoring system.

Generally, lithologic data should correlate with hydraulic properties (e.g., clean, well

sorted, unconsolidated sands should exhibit high hydraulic conductivity). If the investigator is

unable to: 1) correlate stratigraphic units between borings; 2) identify zones of potentially

high hydraulic conductivity, their thickness and lateral extent; or 3) identify confining

formations/layers, their thickness and lateral extent, then additional boreholes should be

drilled and additional samples should be collected to adequately describe the hydrogeology of

the site.

Owners and operators should evaluate the potential for confining units to degrade in

the presence of site specific waste types. In pristine areas, the possible future chemical

degradation of a confining layer should be of concern during any assessment monitoring or

corrective action at the facility. Marls, limestones, and dolomites, for instance, are chemically

attacked by low pH wastes because of their carbonate content. Studies have shown that

certain concentrated organic liquids can cause desiccation of clay minerals, which can lead to

cracking and to a significant increase in permeability (Daniel et al., 1988). Smectitic, and to

a lesser extent illitic, clays are ineffective barriers to the migration of many highly 

concentrated organic chemicals. In contaminated areas, a clay rich, but chemically  degraded

confining layer may lead to unanticipated contaminant migration. An example of how a

contaminant may affect the integrity of a confining layer is shown in Figure 6.

November 1992

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