Toward a new conceptual model for groundwater flow in merokarst systems: Insights from multiple geophysical approaches

Merokarst aquifers - relatively thin (<1-2 m) karstified carbonate units interbedded between mudstone, shale, or sandstone - constitute a significant proportion of carbonate terrain and underlie a large portion of the west- and south-central USA, yet few advances have been made in our understanding of porosity development and flow-path generation in these complex systems in decades. Toward this end, we used a multi-geophysical approach at the well-studied Konza Prairie Biological Station (KPBS), a part of the larger Flint Hills (25,734 km(2)), underlain by thin limestone units (1-2 m thick) interbedded with mudstone/shale units (2-4 m thick), to elucidate hydrologic connectivity and potential controls on known groundwater flow directions. We combined electrical resistivity tomography (ERT), surface and borehole nuclear magnetic resonance (NMR), and ground penetrating radar (GPR) measurements across a low order catchment where over 25 boreholes and groundwater wells sampling perched aquifers could be used to constrain interpretation of lithology, potential flow paths, and permeability. Data revealed that groundwater export may be an unappreciated component of lateral-flow-dominated models used to represent merokarst in that: (a) potentiometric surfaces from two limestone units showed groundwater flows toward a hydrologic depression, opposite the direction of stream flow, in the upstream portion of the catchment, (b) long term measures of groundwater levels revealed a greater variance and overall water storage in this same upstream area compared to wells near the outlet, and (c) ERT and NMR results indicate pronounced lateral heterogeneity within a given unit, suggestive of a greater degree of vertical hydrological connectivity than usually considered for horizontally-layered merokarst. Our data suggest vertical connectivity can shunt water to depth in these "sandwiched" merokarst aquifers, yielding atypical groundwater flow directions and unrealized deep export of weathering solutes and carbon.