THREE-DIMENSIONAL GROUNDWATER MODELING OF A COMPLEX GLACIATED AQUIFER SYSTEM AT THE ROCHELLE, IL MUNICIPAL LANDFILL

Serving as a decision/analysis tool for the existing Rochelle, IL Municipal Landfill for expansion to adjacent properties, a three-dimensional numerical groundwater flow model was constructed and calibrated. The regional geology consists of very slightly dipping sedimentary rock layers overlain by gently rolling hills composed of unconsolidated glacial deposits. The bedrock aquifer system consists of a thick multi-layered complex of high conductivity Cambrian and Ordovician aged sandstones, limestones, and dolomite units interbedded with lower conductivity shale and dolomite zones. Low conductivity Wisconsinan and Illinoian aged glacial tills mantle the Cambro-Ordovician aquifer system and acts as a semi-confining unit. Localized, discontinuous sand and gravel deposits are also present in the glacial tills.

The unconsolidated glacial drift was deposited by multiple Pleistocene glaciation events. The Tiskilwa Formation is prevalent at the site, being a uniform basal till most likely deposited by lodgement, but also includes supraglacial till and interbedded zones of lacustrine, fluvial, and ice-contact deposits. Thin lenses of sorted gravel, sand, silt or clay interbedded with till of variable grain size is typical at the top of the formation, while some lower portions contain beds of sorted fine-grained deposits.

Extensive geologic and hydrogeologic investigations were performed at the site over many years. Borings and/or monitoring wells were installed in every hydrostratigraphic zone across the site. Hydrostratigraphic data collected included slug tests, packer tests, and laboratory conductivity tests. To fully appreciate the complexity of the geologic and hydrogeologic regimes, 16 detailed cross-sections were prepared and analyzed, utilizing all field and laboratory data. Based on the cross-sections and field and laboratory data, a MODFLOW groundwater flow model was constructed and calibrated to steady-state conditions. Model-calibrated values of horizontal and vertical hydraulic conductivity matched the field and laboratory estimates very well. Overall, model-calibrated results accurately represented the multi-hydrostratigraphic regime currently present at the site, allowing for effective decision/analysis of future conditions at the site.