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.
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