THE STRATIGRAPHIC APPROACH TO HYDROGEOLOGIC AND GEOTECHNICAL INVESTIGATIONS

Fermilab, a Department of Energy (DOE) high energy physics research laboratory, is located in the glaciated Midwest outside of Chicago, Illinois. Tritium (3H) has been shown to be the radionuclide of most concern for migration through the subsurface. Accurate information of the depositional sequence and hydraulic properties of these sediments is important in identifying regulated groundwater, determining transport directions and velocities, and installing monitoring systems.

Choosing a suitable groundwater monitoring zone in a sequence of glacial deposits is often complicated by the difficulty in determining which sand-and-gravel bodies are discontinuous and which are sufficiently continuous to constitute a preferential contaminant pathway. Upland glacial settings in the Midwestern United States are typically underlain by a sequence of fine-grained glacial sediments deposited by successive glacial advances. Stratified sand and gravel may occur at various stratigraphic positions within this fine-grained succession. Laterally extensive sand-and-gravel units are most likely to occur between the deposits of different glacial advances. The key to determining a suitable groundwater monitoring zone thus depends on the ability to determine the boundary between different glacial advances in these successions of primarily fine-grained deposits. Unfortunately, use of the Unified Soil Classification System (USCS) as the means for classifying and differentiating fine-grained units in glacial settings is often inadequate because different fine-grained stratigraphic units typically classify as the same in the USCS. Other properties must be recognized and described to provide suitable information for recognizing and correlating fine-grained glacial stratigraphic units, including sedimentary structure and secondary weathering zone properties. In certain situations, it may be important to select a drilling method that provides adequate sample recovery and high quality samples.