Hydrogeophysical investigations of merokarst aquifers in the Flint Hills, Kansas

Merokarst represents a unique hydrogeologic setting and occupies a large portion of the west- and south- central USA that consists of thin karstified carbonate aquifers interbedded with shale, mudstone, or sandstone. Understanding the preferential flow-paths that connect surface and subsurface systems in these highly heterogenous systems is challenging and geophysical imaging has emerged as a way to to image the inaccessible karst subsurface. In this study, our goal is to use a multi-geophysical approach to study the spatial heterogeneity of merokarst and its hydrologic connectivity in the well-studied Konza Prairie Biological Station, a Long-Term Ecological Research site, in northeastern Kansas. Specifically, we combined electrical resistivity tomography (ERT), surface and borehole nuclear magnetic resonance (NMR), and ground penetrating radar (GPR) measurements across a headwater catchment. Hydrologic records in past 28 years from over 25 boreholes and groundwater wells are used to constrain and calibrate geophysical interpretation of the geology, potential flow paths, and porosity development. Our results show that the merokarst system exhibits both lateral and vertical structural heterogeneities that likely control groundwater flow. The ERT and NMR results indicate vertical heterogeneity and a greater degree of vertically connected mobile water through limestone layers in some parts of the watershed. Such vertical connectivity plays a more important role than currently appreciated in simplified horizontal flow models. The water saturation degree and suggested vertical and horizontal water flow patterns in also contribute to understand the weathering and porosity evolution in the merokarst system. Our study shows that a combination of different geophysical tools is necessary to derive robust interpretations of the hydrological patterns in complex merokarst systems and the joint application of geophysical tools can be used as proxy hydrological observables for advancing our understanding of merokarst aquifers.