Hydrogeologic framework of LaSalle County, Illinois

Water-supply needs in LaSalle County in northern Illinois are met by surface water and groundwater. Water-supply needs are expected to increase to serve future residential and mining uses. Available information on water use, geology, surface-water and groundwater hydrology, and water quality provides a hydrogeologic framework for LaSalle County that can be used to help plan the future use of the water resources.

The Illinois, Fox, and Vermilion Rivers are the primary surface-water bodies in LaSalle County. These and other surface-water bodies are used for wastewater disposal in the county. The Vermilion River is used as a drinking-water supply in the southern part of the county. Water from the Illinois and Fox Rivers also is used for the generation of electric power.

Glacial drift aquifers capable of yielding sufficient water for public supply are expected to be present in the Illinois River Valley in the western part of the county, the Troy Bedrock Valley in the northwestern part of the county, and in the Ticona Bedrock Valley in the south-central part of the county. Glacial drift aquifers capable of yielding sufficient water for residential supply are present in most of the county, although well yield often needs to be improved by using large-diameter wells. Arsenic concentrations above health-based standards have been detected in some wells in this aquifer. These aquifers are a viable source for additional water supply in some areas, but would require further characterization prior to full development.

Shallow bedrock deposits comprising the sandstone units of the Ancell Group, the Prairie du Chien Group, dolomite of the Galena and Platteville Groups, and Silurian-aged dolomite are utilized for water supply where these units are at or near the bedrock surface or where overlain by Pennsylvanian-aged deposits. The availability of water from the shallow bedrock deposits depends primarily on the geologic unit analyzed. All these deposits can yield sufficient water for residential supply in at least some parts of the county, and sandstone deposits in the Ancell and Prairie du Chien Groups can yield sufficient water for residential or public supply in much of the county.

The Cambrian-Ordovician aquifer system comprises the most widespread, productive aquifers in northern Illinois and is used for water supply by a number of municipalities in the county. Water levels in the aquifer system have declined by as much as 300 feet in parts of LaSalle County. The aquifer contains naturally occurring concentrations of radium that are higher than established health guidelines in much of the county.