Kane County hydrology update: shallow groundwater supply limited; chloride projected to persist under business-as-usual

Dan Hadley, a hydrologist with the Illinois State Water Survey, updated the Kane County Committee of the Whole on an ARPA-funded shallow groundwater study that models water availability, stream baseflow impacts and chloride accumulation across the county. The presentation, led by Jody Wallnick, director of Environmental and Water Resources, summarized model calibration to municipal water use, watershed-scale sustainable supply estimates and projected chloride concentrations to 2050.

The study team estimated a countywide sustainable shallow-groundwater supply of about 15,000,000 gallons per day using an 85 percent baseflow retention target — i.e., allowing up to 15 percent reductions in stream baseflow — a threshold the presenters said is informed by prior ecological studies. Hadley said the modeling shows several watersheds have been withdrawing more than that sustainable supply for decades. “If you look at each watershed and add those up, the sustainable supply to the shallow groundwater system is about 15,000,000 gallons per day,” Hadley said.

Why it matters: thousands of homeowners and multiple municipalities draw from sand-and-gravel and shallow bedrock aquifers that also provide baseflow to creeks during dry periods. Reduced baseflow can stress aquatic habitat and affect streamflow availability for downstream users.

Most impacted subwatersheds. The Mill Creek subwatershed, which overlies the St. Charles bedrock-valley aquifer, has been shown in the model to exceed its sustainable supply since about 1990; Hadley said the model’s sustainable supply for Mill Creek is less than 1,000,000 gallons per day. Other modeled declines include a roughly 23 percent baseflow reduction in the Blackberry Creek watershed, a 19 percent reduction in the South Branch Kishwaukee, and an 11 percent decline in the Mid Fox River watershed. Hadley noted that actual streamflow in some places has been buffered by treated wastewater returns the model does not fully represent.

Chloride (road salt) projections. The study simulated chloride accumulation under a business-as-usual road-salting scenario and under an elimination scenario beginning in 2025. Hadley showed measured chloride rises at monitored wells and said modeled concentrations generally match observations, though some wells (for example Aurora 119) are highly variable and require parameter refinement. He reminded the committee that the EPA’s secondary maximum contaminant level for chloride is 250 milligrams per liter and said some wells in the county already exceed that level. “Aurora is aware of this. Yes. Aurora is, is keenly aware of the chloride issues,” Hadley said.

Targeting mitigation. Hadley described maps that identify persistent chloride “hot spots” where reductions in salt application should be prioritized because chloride lingers in the aquifer and clears slowly. He said the team is preparing a middle scenario representing “sensible salting practices” that falls between business-as-usual and complete elimination of road salt.

Monitoring network and next steps. County staff and partners maintain a real-time shallow groundwater monitoring network of about 11 wells, with two additional monitoring wells planned. Hadley and Wallnick said they will finalize model calibration and deliver a final report and supporting maps to the county and municipal stakeholders; Hadley said, “we will be back with the final report.” County staff also discussed overlaying model results with municipal districts and road GIS so local officials can identify neighborhoods and streets associated with high chloride levels.

Discussion versus decisions. The meeting record shows technical presentation and questions but no formal motions or votes. Staff were directed in discussion to refine model parameters, prepare a “sensible salting” scenario, expand monitoring coverage and share detailed maps and data with municipalities ahead of winter salting season.

Ending: The presenters said the final calibrated report and GIS overlays will be provided to the board and municipal partners to guide targeted road-salt reduction and local water-supply planning.