Developing Potentiometric Surfaces and Flow Fields with a Head‐Specified MODFLOW Model

Abstract

Investigating changes in an aquifer system often involves comparison of observed heads from different synoptic measurements, generally with potentiometric surfaces developed by hand or a statistical approach. Alternatively, head-specified MODFLOW models, in which constant head cells simulate observed heads, generate gridded potentiometric surfaces that explicitly account for Darcy's Law and mass balance. We developed a transient head-specified MODFLOW model for the stratified Cambrian-Ordovician sandstone aquifer system of northeastern Illinois to analyze flow within its 275 m deep cone of depression. Potentiometric surfaces were developed using static heads from production wells regardless of open interval; hence assuming no vertical head difference. This assumption was tested against steady-state, head-specified models of each sandstone strata for 1980 and 2014. The results indicate that the original conceptual model was appropriate in 1980 but not 2014, where a vertical head difference had developed at the center of the cone of depression. For earlier years, when the head difference was minimal, the transient head-specified model compared well with a traditional, flow-specified model. In later years, the transient head-specified model overestimated removal of water from storage. MODFLOW facilitates the development of a time-series of potentiometric surfaces and can easily be modified to test the impacts of different conceptual models, such as assumptions on vertical head differences. For this study of a deep confined aquifer, MODFLOW also offers advantages in generating potentiometric surfaces and flow fields over statistical interpolation techniques, although future research is needed to assess its performance in other settings.

Conceptualizing leakage and storage contributions from long open interval wells in regional deep basin flow models

“…Removal from storage in both the Galena–Platteville Dolomite and sandstone aquifers (St. Peter—Ironton–Galesville) exhibit distinct inflections when transitioning to unconfined conditions. In the original model, zones of desaturation in the St. Peter Sandstone began earlier and were more widespread than observed in the head‐specified model (Abrams et al, ), resulting in exaggerated removal from storage in this unit and corresponding calibration error through this period (Figure ). In contrast, for the final model, removal of water from storage from the overlying Galena–Platteville Dolomite was more important, comprising approximately half of the total.…”

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

“…Given the large width of the fault zone (up to 3.2 km) and the unknown geometry of individual faults that comprise the zone, we conceptualized the fault zone within the St. Peter and Ironton‐Galesville sandstones as an inner 1‐cell wide fault core surrounded by a 1‐cell wide damage zone (Figure ). This is based on low specific capacities of sandstone wells near the fault (Abrams et al ) and the physical properties of faults observed elsewhere (Caine et al ; Shipton and Cowie ; Kim et al ; Caine and Minor ; Ball et al ). We tested various conceptualizations of the fault zone by varying K h of the damage zone and fault core but only present results for three scenarios: (1) an unrestricted flow scenario in which the fault zone is identical to the host rock, (2) a restricted flow scenario in which the K h of the damage zone is reduced by half and the K h of the fault core is reduced by an order of magnitude, and (3) a highly‐restricted flow scenario in which the K h of the damage zone and fault core are reduced by two orders of magnitude (Table ).…”

“…Roadcap et al () modeled the Sandwich fault zone as a low horizontal hydraulic conductivity ( K h ) zone of 3 × 10 −5 m/day compared to a K h of 1.5 m/day for the undeformed sandstone units. The fault zone has also been modeled as a no‐flow barrier in contouring models of the sandstone system (Abrams et al ). Most recently, the fault zone has been modeled as a broad region using a K h of 0.76 m/day (Mannix et al ; Abrams et al ).…”

“…The fault zone has also been modeled as a no‐flow barrier in contouring models of the sandstone system (Abrams et al ). Most recently, the fault zone has been modeled as a broad region using a K h of 0.76 m/day (Mannix et al ; Abrams et al ). In all cases, there was little constraint on K h , which was determined through the regional calibration process.…”

“…The St. Peter and Ironton‐Galesville sandstones are heavily utilized in Kane, Will, and Kendall Counties which has led to partial and complete desaturation of the St. Peter sandstone in some areas, prompting drillers to install wells only open to the underlying Ironton‐Galesville sandstone (Abrams et al ; Mannix et al ). This has led to increased head separation between the sandstone aquifers in the center of the cone (Abrams et al ). Communities and industries are expected to increase withdrawals and to drill additional sandstone wells, many of which will be located near or within the fault zone as development continues in western Will and eastern Kendall Counties.…”