Large and Small Scale Geologic Mapping: The Foundation for Determining Sensitivity of Aquifers at Various Scales

Richard C. Berg, David R. Soller, Curtis C. Abert

It is essential that the texture and three-dimensional relationships of geologic materials be considered when assessing the sensitivity of aquifers to potential contamination because geology controls the movement of groundwater and contaminants as well as influencing groundwater quality. Several large and small scale aquifer sensitivity models have been developed for Illinois and the surrounding states that assess aquifer sensitivity and which are useful for regional and local groundwater protection planning.

Models for the regional assessment of aquifer sensitivity often rely on data generalizations, while models for municipalities or counties are based on specific data points. An aquifer sensitivity model, honoring about 3,000 water-well data points in Will County, Illinois (1:100,000 scale), was developed based on depth to aquifer, aquifer thickness, drift thickness, and bedrock lithology. This mapping has been used for county landfill siting assistance. A statewide aquifer sensitivity model (1:500,000 scale), generalizing data from about 400,000 data sources, was devised for Illinois based on depth to aquifer and water-transmitting capabilities of aquicludes. This mapping has been used for statewide screening for a low-level radioactive disposal site as well as for screening regions of the State for potential contamination problems. Finally, a multi-state aquifer sensitivity model (1:1,000,000), generated from statewide geologic maps, has recently been completed for portions of five Midwestern states and was based on texture of surface sediments, actual and probable occurrence, of aquifers, hydraulic conductivity of the uppermost bedrock, and drift thickness. The multi-state mapping will assist federal agencies in assessing contamination potential across state lines.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995