Thesis
Evaluating the hydrostratigraphy of the Dakota Formation in a highly monitored borehole in Northwest Iowa
University of Iowa
Master of Science (MS), University of Iowa
Autumn 2025
DOI: 10.25820/etd.008236
Abstract
Hydrostratigraphic units (HSUs) form the foundational framework for modeling and managing groundwater systems. HSUs represent units with distinct hydraulic conductivity (K) and contrasts in K between them. In layered and fractured sedimentary systems where strong anisotropy is common, recognizing contrasts in bulk vertical hydraulic conductivity (Kᵥ) with depth is essential for accurately defining HSUs. However, their delineation traditionally relies heavily on lithostratigraphic boundaries, which may not accurately represent HSU boundaries. This study evaluates whether lithostratigraphic units are a reliable proxy for intervals with distinct Kᵥ within the Cretaceous Dakota aquifer system at one location in northwest Iowa. A research borehole in Cherokee County Iowa was continuously cored to ~ 99 m below ground surface (bgs); logged with normal and full spectrum spectral gamma and acoustic and optical televiewer tools; and instrumented with a high resolution (i.e., 5 monitoring intervals/10 m) Westbay multilevel system (MLS). The continuous core was logged for sedimentological parameters and then scanned using a Multi-Sensor Core Logger – Standard (MSCL-S) producing detailed depth profiles of elemental chemistry and seismic wave velocities. Hydraulic heads (i.e., heads) were collected from the MLS 3 times over a 5-month period and used to assess changes in head and the vertical component of hydraulic gradient (i.e., vertical gradient) with depth. Changes in vertical gradient did not consistently align with lithostratigraphic boundaries. Instead, changes in Kᵥ appeared to be more strongly associated with subtle transitions in clay mineralogy indicated by changes in elemental composition in the core, fracture abundance, and potential sequence boundaries. Subtle yet consistent differences in hydraulic behavior were observed between the illite-rich upper mudstone and the mixed-layer clay-rich lower mudstone of the Woodbury Member. Fracture data from the core and borehole televiewer logs suggested that mechanical stratigraphy influenced vertical connectivity, though mechanical property contrasts, derived from seismic wave velocities, alone were insufficient predictors of hydraulic behavior. These findings offer further evidence that high-resolution head profiles, combined with multi-proxy datasets, are valuable tools for delineating HSUs in a depositional setting where the head profile approach has not previously been applied. Furthermore, the potential HSUs identified in this study are different from those typically used in regional groundwater modelling studies of the Dakota aquifer in NW Iowa suggesting a need for further hydrostratigraphic evaluation of the Dakota Aquifer in this region.
Details
- Title: Subtitle
- Evaluating the hydrostratigraphy of the Dakota Formation in a highly monitored borehole in Northwest Iowa
- Creators
- Shubhamita Das
- Contributors
- Jessica Meyer (Advisor)Bradley Cramer (Committee Member)Keith Schilling (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Autumn 2025
- DOI
- 10.25820/etd.008236
- Publisher
- University of Iowa
- Number of pages
- viii, 41 pages
- Copyright
- Copyright 2025 Shubhamita Das
- Grant note
The research was made possible through generous funding from the National Science Foundation. Data collection and core components of this project were supported by NSF EPSCoR grant (#2119551; PI: Cramer). I was also supported as a Research Assistant through NSF IUSE grant (#2119888; PI: Cramer).
- Language
- English
- Date submitted
- 08/04/2025
- Description illustrations
- Illustrations, graphs, charts, tables
- Description bibliographic
- Includes bibliographical references (pages 37-41).
- Public Abstract (ETD)
Aquifers and aquitard units, more generally referred to as hydrostratigraphic units (HSUs) are used to represent differences in the groundwater flow and storage properties of geologic materials in the subsurface. Identification of these units is critical to understanding and managing groundwater resources. These units are often classified based on the boundaries between units of rock. But these rock boundaries do not always correspond to variation in groundwater storage or flow properties. In this study, we tested whether boundaries between different rock units are a good way to identify changes in the property that controls groundwater flow, hydraulic conductivity, in the Dakota Aquifer of northwest Iowa. We collected detailed data from a 99 m deep well in Cherokee County. This was a specialized well called a Westbay multilevel system that allowed us to divide a single borehole up into 24 distinct monitoring intervals. This is much more cost, time, and space efficient than drilling and installing 24 separate wells to different depths all within a very small area. The Westbay multilevel system was used to collect hydraulic heads (i.e., a measure of the energy of the groundwater from each of the 24 monitoring intervals. We combined these head data with information on rock types, minerals (especially clays), rock strength, and natural fractures to advance our understanding of HSUs in the Dakota Formation. We found that HSUs do not align with boundaries between rock units. Instead, subtle variations in clay mineralogy, fracture orientation and abundance, and depositional conditions more accurately reflect HSUs. These findings provide additional evidence that examining rock type alone is not sufficient to understand HSUs. A more accurate understanding requires high-resolution head profiles along with detailed information on clay mineralogy, fracture networks, and depositional environments. Together, these data can significantly enhance groundwater resource assessments and improve the management of challenges such as contamination and water scarcity.
- Academic Unit
- School of Earth, Environment, and Sustainability
- Record Identifier
- 9985135148302771
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