Geologic Hazards Investigations

Proper evaluation of geologic structure, soil and groundwater conditions and other tectonic factors are critical to site stability. These factors are also important for devising remedial action plans in areas where contaminant migration may become hazardous to the environment and possibly human health. Using boring/well logs* to generate geologic cross-sections* and results of analyses of groundwater samples to create isoconcentration maps* are just two of the many methods of evaluating subsurface conditions of a site.

Terms used:

*Boring/Well logs:
A sketch and written description of what was encountered in a soil boring. A well log would also contain a description of the well construction details.

A small scale depiction (sketch) of a boring identifying the locations sampled, the types of soil encountered and where changes in soil type occurred. Also included are descriptions of the various soil types and the location (depth) the groundwater was encountered. A well log will also include details of the construction of the well. These logs are often used to develop geologic cross-sections of a site and area. Cross-sections are developed to aid in understanding subsurface conditions across the site and area.

*Geologic cross-sections:
A graphic representation of the subsurface area

A geologic cross-section is prepared using boring/well logs of a site and/or area. The boring logs are scaled for presentation on a figure or plate in relative vertical and horizontal positions and boundaries between soil types are extrapolated outward from each boring log to the next. Where and if these boundaries meet and the nature of their meeting is not generally known, but is an interpretation of the available data. Also, there may be more than one interpretation of the data. The resulting cross-section is intended to be a small scale graphic representation of the subsurface stratigraphy of the site and/or area.

*Isoconcentration map:
Contours of the amount of a chemical substance present in the subsurface are depicted on a map

Laboratory analyses of groundwater form monitoring wells indicate the amount of a particular contaminant that is present in the water. A map including the approximate boundary of the various amounts of the contaminant can be prepared based on the locations of the wells and the concentrations detected in the samples. An isoconcentration map is often used in understanding the distribution of various constituents of a particular contaminant.