Modeling a Large‐Scale Historic Aquifer Test: Insight into the Hydrogeology of a Regional Fault Zone

Abstract

Faults can act as flow barriers or conduits to groundwater flow by introducing heterogeneity in permeability. We examine the hydrogeology of the Sandwich Fault Zone, a 137 km long zone of high-angle faults in northern Illinois, using a large-scale historic aquifer test. The fault zone is poorly understood at depth due to the majority of the faults being buried by glacial deposits and its near-vertical orientation which limits geologic sampling across faults. The aquifer test-perhaps one of the largest in terms of overall withdrawal in North American history-was conducted in 1942 at a facility adjacent to the fault zone. More than 34,000 m 3 /day was pumped for 37 days from nine multiaquifer wells open to the stratified Cambrian-Ordovician sandstone aquifer system. We modeled the aquifer test using a transient MODFLOW-USG model and simulated pumping wells with the CLN package. We tested numerous fault core/damage zone conceptualizations and calibrated to drawdown values recorded at production and observation wells. Our analysis indicates that the fault zone is a low-permeability feature that inhibits lateral movement of groundwater and that there is at least an order of magnitude decrease in horizontal hydraulic conductivity in the fault core compared to the undeformed sandstone. Large head declines have occurred north of the fault zone (over 300 m since predevelopment conditions) and modifying fault zone parameters significantly affects calibration to regional drawdown on a decadal scale. The flow-barrier behavior of the fault zone has important implications for future groundwater availability in this highly stressed region. Article impact statement: Modeling a large-scale historic aquifer test determined the hydraulic conductivity of a deep vertical fault zone in a multiaquifer system.

Using Production Well Behavior to Evaluate Risk in the Depleted Cambrian‐Ordovician Sandstone Aquifer System, Midwestern USA

“…The High and Severe‐risk zones along the SFZ are persistent despite Joliet switching water sources because high‐capacity wells operated by industries and communities south of Joliet are expected to still maintain and even increase their pumping rates near the SFZ. Due to continued pumping at wells near the SFZ and the flow‐barrier behavior of the SFZ (Hadley et al., 2020) recovery in 2030 is fairly limited (Figure 6b).…”

“…Loss of these multi‐aquifer connections means that there is likely a significant decrease in the amount of artificial leakage to the Ironton‐Galesville sandstone (see Mannix et al., 2019). Both the loss of artificial leakage and the presence of the SFZ, which limits recharge to the cone of depression (Hadley et al., 2020), could be contributing to accelerated head declines in southern Joliet (Figure 6b).…”

“…The near vertical orientation and lack of surface exposure make mapping of the fault zone difficult, and thus is represented with simple conceptualizations in groundwater flow modeling of the CO-aquifer Roadcap et al, 2013). Hadley et al (2020) simulated a large-scale aquifer test that occurred adjacent to the SFZ and found that bulk horizontal hydraulic conductivity (K h ) of the fault core is at least an order of magnitude less than the surrounding host rock at a regional scale. Kelly et al (2018) also found that structural features such as the LaSalle anticlinal belt influence groundwater chemistries and recharge to the deep sandstone units.…”

“…Hadley et al. (2020) simulated a large‐scale aquifer test that occurred adjacent to the SFZ and found that bulk horizontal hydraulic conductivity ( K h ) of the fault core is at least an order of magnitude less than the surrounding host rock at a regional scale. Kelly et al.…”

The Effects of Fault‐Zone Cementation on Groundwater Flow at the Field Scale

“…Faults can be important hydrogeologic features, capable of either inhibiting or facilitating fluid flow between different geologic formations (Caine et al 1996;Bense and Person 2006). As a result, faults can affect groundwater supply, contaminant transport, petroleum migration and trapping, and carbon sequestration (Shipton et al 2004;Sorkhabi and Tsuji 2005;Cartwright et al 2007;Mayer et al 2007;Hadley et al 2019). Most models evaluating the potential for faults to act as barriers to fluid flow (e.g., Lindsay et al 1993;Fristad et al 1997;Fulljames et al 1997;Welbon et al 1997;Childs et al 2002;Yielding 2002;Bretan et al 2003) do not account for fault-zone cementation, despite the widespread recognition that it is common and can dramatically decrease fault zone porosity and permeability (Bense et al 2003(Bense et al , 2013.…”