Three-dimensional Hydrogeologic Mapping Begins in McHenry County

Natural resource management, specifically groundwater resource management, is a key factor to sustainable development in McHenry County, Illinois. Using a base of previous mapping efforts, historical records, and new fieldwork, the Illinois State Geological Survey (ISGS) has begun a county-funded three-dimensional (3-D) geologic mapping project. The project is aimed at better understanding the distribution, variability, and character of sand and gravel aquifer systems throughout the county. This 3-D map will be used to assess the long-term availability and sensitivity to contamination of groundwater resources throughout the county. The map will also be serve as a basis for regional groundwater flow modeling studies by the Illinois State Water Survey, for more detailed groundwater flow modeling efforts by consultants for municipal or county agencies, and for guiding sand and gravel quarrying operations throughout the county.

The 2008 fieldwork in McHenry County was supported by funding from several different projects, including the McHenry County 3-D Mapping Project, Northeast Illinois Water Supply Planning Project, and the Central Great Lakes Mapping Coalition. Drilling was conducted at nine locations. At each site, continuous core was collected to bedrock, and a groundwater observation well was installed. A variety of geophysical data was also gathered throughout the county through 2-D seismic techniques, 2-D electrical resistivity methods, and borehole methods. Approximately 13 miles of 2-D geophysical profile data were collected, and downhole geophysical data were collected at seven locations. The 2009 fieldwork plans include more drilling and geophysics with associated laboratory analyses. These field data, together with available historical data, are being incorporated into a 3-D geologic map. Supplemental 3-D visualization tools are being developed by ISGS staff members to greatly simplify geologic interpretations and product development.

The progression of this project depends on the collaboration of many ISGS staff and university students with other state, county, and local agencies in the study area. Particularly important is the continued collaboration with the McHenry County Conservation District, which is hosting a majority of fieldwork locations. Furthermore, drilling data shared from a monitoring well network project, initiated by the county and conducted by the US Army Corps of Engineers, will greatly aid this project.