Water Quality of Springs in the Driftless Region of Illinois
Studying a unique area within the Rock River Region
Figure 1. Fractured dolomite exposed at a quarry in Jo Daviess County. (Photo credit Sam Panno)
Northwest Illinois, particularly in Jo Daviess County, is a unique area in the Rock River region. This corner of Illinois is the Illinois Driftless Region . Glaciers from the last ice age (10,000 years ago) did not reach this region and alter the landscape. Because of this history , the hydrogeologic story is different here than the rest of the Rock River Region.
Water resources in the Driftless Region are especially susceptible to contamination. Soils are thin and the dolomite aquifers can exhibit karst features, such as sinkholes, springs, and conduit flow in the bedrock. Another karst feature, crop lines, are visible with aerial photography of alfalfa fields and show karst fractures throughout the landscape. Groundwater in the Driftless Area is very hard, with high concentrations of calcium, magnesium, and bicarbonate, which reflects the dolomite aquifer material.
Karst aquifers are susceptible to contamination because water tends to move rapidly from the surface into and through the subsurface, with little chance for biogeochemical reactions that might retard contaminant transport.
Figure 2. Cartoon showing how water flows in a karst aquifer. Water moves rapidly from the land surface to the aquifer and surface streams, and is often contaminated.
Natural springs are common in the region , discharging from crevices and bedding planes. They are typically found where the underlying Galena Dolomite or overlying Silurian dolomite aquifer intersects the low permeability Maquoketa Shale. When these units intersect, the shale becomes a flow barrier and water is forced to discharge at the land surface.
Figure 3. Geologic cross section of Jo Daviess County. Springs tend to form where contacts between the Maquoketa Shale and the underlying Galena Dolomite and the overlying Silurian dolomite occur. Dashed blue lines show groundwater levels, arrows show direction of groundwater flow.
Scientists from the ISWS and Illinois State Geological Survey have sampled wells and springs in Jo Daviess County in order to assess the county’s shallow groundwater quality and see if human activity has impacted the chemical composition of the aquifer. Twelve shallow domestic wells and 34 springs were sampled between 2014 and 2019.
Figure 4. Location of springs sampled in Jo Daviess County by ISGS and ISWS scientists.
The homeowner’s wells that were sampled generally did not show contamination, with some exceptions, but many of the springs, which amalgamate the shallowest groundwater flow in a region, were contaminated. The primary contaminants were nitrate and bacteria, including E. coli. The main source of contamination was suspected to be septic system discharge. The results of this investigation revealed that the Galena and Silurian dolomite aquifers are susceptible to surface-borne contaminants.
Figure 5. Nitrate concentrations in springs sampled in Jo Daviess County. The drinking water standard is 10 mg/L.
Figure 6. E. coli concentrations in springs sampled in Jo Daviess County.
If you’re interested in learning more about your private well and how to protect your well water quality, we encourage you to take the free, on-line Private Well Class developed at the Illinois State Water Survey.
To attain the full report of this research study, please see this link or contact the authors.
