CHAPTER FOUR

CHARACTERIZING SITE HYDROGEOLOGY

The adequacy of a ground water monitoring program largely depends upon the

quantity and quality of the hydrogeologic data used in designing the program. Clearly, if the

design of the monitoring well system is based on incomplete or inaccurate data, the system

will not fulfill its intended purpose. Because of the complexity of site characterization and

ground water monitoring system design, owner/operators should discuss the intended approach

with the appropriate State or EPA Regional office prior to finalizing site characterization

plans.

When characterizing the hydrogeology of a site prior to designing a monitoring well

network, owner/operators should be concerned with questions relating to data quantity and

quality:

(1)

Has enough information been collected to identify and adequately characterize

the uppermost aquifer and potential contaminant migration pathways? Does the

information allow for the placement of monitoring wells that are capable of

immediately detecting releases from the regulated unit(s) to the uppermost

aquifer?

(2)

Have appropriate techniques been used to collect and interpret the information

that will be used to support the placement of monitoring wells, and is the

quality and the interpretation of the information satisfactory when measured

against the program's DQOs?

The answers to these questions will establish whether or not the site characterization is

adequate. The Agency recognizes that the quantity of site characterization information and

the appropriateness of investigation techniques vary according to site specific conditions.

Sites in complex geologic settings require more hydrogeologic data for ground water

monitoring system design than do sites in less complex settings. Likewise, investigatory

techniques that may be appropriate in one geologic setting or for one waste type, may be

inappropriate in another setting or for a different waste type.

This section identifies techniques that can be used to characterize a site prior to

installing a monitoring well network, and describes the factors that should be considered when

evaluating whether a particular method is appropriate in a specific case.

Table 1 lists a number of investigatory techniques commonly used to conduct

hydrogeologic investigations. Also listed are preferred methods for presenting the data

generated from a hydrogeologic investigation. Many States and Regions also may request or

November 1992

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