Recharge and Groundwater Flow Within an Intracratonic Basin, Midwestern United States by Samuel V. Panno 1 , Zohreh Askari 2 , Walton R. Kelly 3 , Thomas M. Parris 4 , and Keith C. Hackley 5 Abstract The conservative nature of chloride (Cl − ) in groundwater and the abundance of geochemical data from various sources (both published and unpublished) provided a means of developing, for the first time, a representation of the hydrogeology of the Illinois Basin on a basin-wide scale. The creation of Cl − isocons superimposed on plan view maps of selected formations and on cross sections across the Illinois Basin yielded a conceptual model on a basin-wide scale of recharge into, groundwater flow within and through the Illinois Basin. The maps and cross sections reveal the infiltration and movement of freshwater into the basin and dilution of brines within various geologic strata occurring at basin margins and along geologic structures. Cross-formational movement of brines is also seen in the northern part of the basin. The maps and cross sections also show barriers to groundwater movement created by aquitards resulting in areas of apparent isolation/stagnation of concentrated brines within the basin. The distribution of Cl − within the Illinois Basin suggests that the current chemical composition of groundwater and distribution of brines within the basin is dependent on five parameters: (1) presence of bedrock exposures along basin margins; (2) permeability of geologic strata and their distribution relative to one another; (3) presence or absence of major geologic structures; (4) intersection of major waterways with geologic structures, basin margins, and permeable bedrock exposures; and (5) isolation of brines within the basin due to aquitards, inhomogeneous permeability, and, in the case of the deepest part of the basin, brine density effects. Introduction The geochemical origin and the movement of brines within intracratonic sedimentary basins have been the subject of numerous studies for much of the latter half of the 20th century (e.g., Bethke et al. 1988; Hanor 1994). An understanding of these influences on the groundwater chemistry within the basin is important for water resource evaluation and natural resource exploitation. Moreover, reservoirs in intracratonic basins have been used for storage of natural gas, and the disposal of a variety of industrial, oil field, and other types of wastes (e.g., Bergstrom 1968). The magnitude of this disposal has 1 Corresponding author: Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, IL 61820; (217) 244-2456; E-mail: s-panno@illinois.edu 2 Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, IL 61820. 3 Illinois Water Survey, 2204 Griffith Drive, Champaign, IL 61820. 4 Kentucky Geological Survey, 504 Rose St., Lexington, KY 40508. 5 Isotech Laboratories, Inc., 1308 Parkland Ct., 61821. Article impact statement : Development of a conceptual model of recharge to and groundwater flow within and through the Illinois Basin using chloride concentrations on plan-view maps and cross section. Received October 2016, accepted May 2017.  2017, National Ground Water Association. doi: 10.1111/gwat.12545 increased considerably with injection of flow-back waters from unconventional oil and gas reservoirs and could increase with geologic carbon sequestration (Locke et al. 2013). The Illinois Basin in the U.S. Midwest (Figure 1) is an anomaly among intracratonic basins because it contains no known halite-bearing evaporite sequences that could con- tribute to brine formation. The chemical composition and origin of brines in the Illinois Basin have been shown to be unique for Cambrian, Ordovician, Silurian – Devonian, and Mississippian–Pennsylvanian strata (Stueber et al. 1987; Walter et al. 1990; Stueber and Walter 1991, 1994; Panno et al. 2013). Groundwater movement within the Illinois Basin was studied by Cartwright (1970) using thermal anomalies in the southern half of Illinois and by Siegel (1989) using stable isotopes in northern Illinois. Both con- cluded that groundwater flows toward the center of the basin becoming more sluggish with depth, and discharges upward across confining units. Cartwright (1970) further suggested that upward movement of groundwater should occur along fractures associated with geologic structures. Total dissolved solids (TDS) and chloride (Cl − ) concentrations of brines within intracratonic basins typ- ically increase with depth (Domenico 1972). Within the Illinois Basin, Cl − concentrations range from about 1 mg/L in shallow aquifers to over 200,000 mg/L for the deepest Paleozoic strata (Panno et al. 2013). Variable permeability and hydraulic gradients within Paleozoic NGWA.org Groundwater 1