Geologic and environmental assessment data available in state and

Federal project reports for local dams, highways, subway systems, and

other major construction projects;

Logs from local private or public water supply wells; and

Logs from building construction and quarry activities.

Appendix 4 provides a comprehensive list of sources of information that may be

consulted during the preliminary investigation stage of the hydrogeological investigation.

Information collection activities should be supplemented by a site reconnaissance to

substantiate concepts developed from the preliminary investigation and to help identify

problems that require resolutions during subsequent site investigation activities.

A properly conducted preliminary investigation is necessary for planning the direction

and scope of subsequent field investigations. For example, information on stratigraphy,

depositional environment, and tectonic history can be used to estimate the distribution and

types of geologic materials likely to be encountered at the site. Topographic maps can assist

in defining the locations of recharge or discharge areas, such as lakes, swamps, springs, and

streams, and the locations of faults or fractures as indicated by surface drainage patterns.

Geologic maps depict the locations of geologic contacts and provide the lithology of geologic

units, as well as depicting the locations of faults, fractures, and folds. Information on

regional ground water flow rates and directions, depth to ground water, potentiometric surface

elevations, water quality and chemistry, local ground water pumping, evapotranspiration rates,

transmissivities, storativities, and surface water hydrology allows for an effective first

approximation of the site specific hydrogeologic setting.

The owner/operator should develop a preliminary conceptual model of the site based

on the information collected during the preliminary investigation. The conceptual model

should incorporate all essential features of the system under study, and should be tailored to

the amount, quality, and type of information available at each stage of the investigation. This

model is an essential element for planning the subsequent field investigation (e.g., the initial

placement of boreholes) and should be revised and updated as additional information becomes

available and as new interpretations are made. A final conceptual model, incorporating the

information collected during the site characterization activities described in the following

section, is essential for designing an adequate detection monitoring system.

4.2

Characterizing the Geology of the Site

After completion of the preliminary investigation, subsurface samples (e.g., soil

samples, unconsolidated material samples, rock borings) should be collected and lithologically

or pedologically classified so that the lithology, stratigraphy, and structural characteristics of

the subsurface are identified. As stated previously, indirect methods of geologic investigation

November 1992

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