A geologic consultant told the Will County Land Code Committee on Oct. 1 that site investigation for a proposed landfill expansion is complete and that preliminary findings support further design work.

The update matters because the committee is considering an expansion of the existing landfill footprint. The committee heard technical results the county must use to evaluate groundwater risks and to prepare a groundwater impact assessment required by state regulators.

Dylan Gallen, a geologic consultant with Geologic Consultants, said the team completed surveys and field tests including piezometer (monitor well) surveying, boring logs and as-built diagrams, groundwater-level measurements and slug testing. He said laboratory permeameter tests on Shelby-tube samples and field slug tests will feed hydraulic-conductivity values into a site-specific groundwater model.

"Vertical expansion is possible," Gallen said. He added the field data show a relatively highly permeable upper bedrock unit — likely from weathering and fractures — and much tighter clay and lower bedrock below that zone. Gallen said the project team will combine the hydraulic-conductivity (k) values from slug tests and lab tests with measured hydraulic gradients (the slope of the potentiometric surface) and porosity to estimate groundwater velocities and contaminant transport for the model.

Committee members asked technical and schedule questions. Gallen said a typical piezometer installation takes roughly four days at a location (drill, sample, install screen, sand pack and seal). He said the team has drafted cross sections, thickness maps and preliminary groundwater-flow maps and expected to present preliminary design drawings at the next meeting.

Gallen described the next steps the county will require before advancing a permit application: finalizing groundwater-flow mapping, completing hydraulic-conductivity analyses, running a site-specific groundwater impact model that incorporates advection, dispersion and diffusion, and preparing an appendix with boring logs and as-built construction records for each piezometer.

Committee members did not take formal action on the project at the meeting; the presentation was an informational update. Gallen and staff said they expect to return with modeling results and preliminary design drawings at a future meeting.

The committee also approved routine meeting minutes and adjourned (see separate item).