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Title: Evaluating the hydrostratigraphy of the Dakota Formation in a Highly Monitored Borehole in Northwest Iowa.

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 vertical hydraulic conductivity (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 4-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 corresponded to subtle transitions in clay mineralogy indicated by changes in elemental composition in the core, fracture frequency, and 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 hydrostratigraphic units 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 modeling studies of the Dakota aquifer in NW Iowa suggesting a need for further hydrostratigraphic evaluation of the Dakota Aquifer in this region.