Groundwater Flow Models of Illinois: Data, Processes, Model Performance, and Key Results

The Illinois State Water Survey (ISWS) has a long history of developing groundwater flow models to simulate water supply and groundwater contamination issues in the state of Illinois. However, past local- and regional-scale models developed by the ISWS have traditionally been project based; thus models are archived when the project is completed and may not be updated for many years. This report presents the first version of the Evolving Network of Illinois Groundwater Monitoring and Modeling Analyses (ENIGMMA), which is the framework of data, procedures, protocols, and scripts that facilitate the development of a single, continuously updated groundwater flow model and other outputs (hydrographs, maps, animations of groundwater potentiometric surfaces). This report focuses on five aspects of ENIGMMA: 1. The archived models and high-resolution datasets that serve as inputs to ENIGMMA 2. The procedures for developing model-ready datasets from these inputs 3. The Illinois Groundwater Flow Model (IGWFM), which serves as the single model that will be continuously updated by ENIGMMA 4. The ISWS Calibration Toolbox, used to facilitate a transient calibration of the IGWFM 5. Animations of groundwater potentiometric surfaces using head-specified models This report is a living document that will be updated periodically. Future updates to this report will focus on additional aspects of ENIGMMA, including the automated development of model-ready inputs and display of model outputs. Updates to this report will also chronicle any additional geologic data added to ENIGMMA, and subsequently, to the Illinois Groundwater Flow Model. Updates will also highlight both local- and regional-scale advancements made with the model, including any key results from these models. The current version of the IGWFM combines and expands on two existing groundwater flow models: 1) the Northeastern Illinois Cambrian-Ordovician Sandstone Aquifer model and 2) the East-Central Illinois Mahomet Aquifer model. In addition, the model incorporates new geologic information developed by the Illinois State Geological Survey in the Middle Illinois Water Supply Planning region. The current model domain covers large portions of Illinois, Wisconsin, Indiana, and Michigan. This large spatial extent is necessary to capture the far-reaching regional head declines in the deep Cambrian-Ordovician sandstone aquifer system, which can extend beyond state boundaries. Depicting some shallow, unconsolidated aquifers also requires a simultaneous simulation of the deep sandstone to account for flow exchange between units. This is because the low-permeable stratigraphic units (aquitards) overlying the sandstone aquifers are absent over large areas of northern Illinois or are locally punctured by wells with long, open intervals. To capture these complex flow pathways, the three-dimensional IGWFM explicitly simulates all geologic materials from the land surface to the impermeable Pre-Cambrian crystalline bedrock. The IGWFM does not currently include a groundwater flow simulation of the southern portion of the state where the deep basin sandstones are highly saline and not used for water supply. Incorporating the shallow aquifers in the southern portion of the state into the IGWFM is a long-term goal. The primary datasets currently incorporated into IGWFM include surface water elevations, annual groundwater withdrawals, well information such as open intervals, geologic 2 surfaces, measured water levels, and aquifer properties inferred from previous modeling studies. These datasets are input at their best available spatial and temporal resolutions, allowing for the development of refined local-scale models. Such local-scale models are essential for simulating groundwater-surface water interactions, well interference, and contaminant transport. Major local-scale models already exist for the Mahomet Aquifer, Kane County, and McHenry County. The IGWFM can address a number of water supply planning questions, particularly the impacts of historic, modern, and future high-capacity groundwater withdrawals on heads and groundwater discharging to surface waters. In addition, where detailed geologic information of the shallow aquifers is available, the IGWFM can also simulate the subsurface migration of point (e.g., volatile organic compounds) and nonpoint (e.g., chloride and nitrate) contaminants.