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

Abstract

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 km2), 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.

Intermittent streamflow generation in a merokarst headwater catchment

“…A major assumption that we made in the mixing analysis of this study is that groundwater contributions to the stream were only coming from the Upper Eiss Ls and the Morrill Ls. While we provided justification above for excluding the Lower Eiss Ls, N04D contains a total of six different limestone formations that outcrop above the flume, 14 each of which could potentially contribute water to streamflow. However, the Eiss and Morrill members are the stratigraphically lowest of these limestones and have much more contact area with the stream channel than the higher units in the steeper upper reaches of the catchment.…”

“…Indeed, well productivity varies considerably from site to site within the same unit 11 and recent geophysical work has shown a large degree of heterogeneity in the distribution of water within each unit. 14 Previous studies have measured the hydraulic conductivities of the Morrill and Eiss Limestones via rising-head slug tests in the wells and reported values of 10 −5 -10 −3 m d −1 for the Morrill Ls, 10 −8 -10 −5 m d −1 for the Lower Eiss Ls, and 10 −5 -10 −4 m d −1 for the Upper Eiss Ls. 19,20 These values fall in line with those typical of karstic and non-karstic limestones and dolomites, 27 but given the aforementioned heterogeneity they should not be taken as measurements that can be accurately generalized across the entire spatial extent of each limestone.…”

“…11,12 However, they remain largely understudied compared to other carbonate terrains and may be particularly sensitive to environmental change. 13,14 The rapid transport of water generally seen in carbonate terrains amplies the sensitivity of the environment to disturbances, 15 exacerbating the responsiveness of headwater catchments therein.…”

“…Konza Prairie is a part of the broader Flint Hills physiographic province of eastern Kansas and north-central Oklahoma and is classified as a merokarst terrain. 14 Previous studies conducted at Konza Prairie reveal key pieces of information about merokarst streamflow generation and flow intermittency. Costigan et al 9 used long-term precipitation and discharge data (1987–2011) to characterize the overall surface water hydrology of Konza Prairie and observed that headwater discharge patterns were disconnected from precipitation patterns.…”

Carbonates in the Critical Zone

“…Sometimes carbonates are thinly interbedded with impure carbonates, shales, or other non-carbonate rocks, creating a landscape referred to as merokarst (Cvijic, 1925). Merokarst typically displays little surface topographical manuscript submitted to Earth's Future expression of karst but may still behave hydrologically as a karst system (Brookfield et al, 2017;Macpherson and Sullivan, 2019a;Sullivan et al 2020). Thick carbonate layers may be juxtaposed laterally with non-carbonate rocks.…”