Characterizing the heterogeneity of karst critical zone and its hydrological function: An integrated approach

Abstract

Spatial heterogeneity in the subsurface of karst environments is high, as evidenced by the multiphase porosity of carbonate rocks and complex landform features that result in marked variability of hydrological processes in space and time. This includes complex exchange of various flows (e.g., fast conduit flows and slow fracture flows) in different locations. Here, we integrate various “state‐of‐the‐art” methods to understand the structure and function of this poorly constrained critical zone environment. Geophysical, hydrometric, and tracer tools are used to characterize the hydrological functions of the cockpit karst critical zone in the small catchment of Chenqi, Guizhou Province, China. Geophysical surveys, using electrical resistivity tomography (ERT), inferred the spatial heterogeneity of permeability in the epikarst and underlying aquifer. Water tables in depression wells in valley bottom areas, as well as discharge from springs on steeper hillslopes and at the catchment outlet, showed different hydrodynamic responses to storm event rainwater recharge and hillslope flows. Tracer studies using water temperatures and stable water isotopes (δD and δ18O) could be used alongside insights into aquifer permeability from ERT surveys to explain site‐ and depth‐dependent variability in the groundwater response in terms of the degree to which “new” water from storm rainfall recharges and mixes with “old” pre‐event water in karst aquifers. This integrated approach reveals spatial structure in the karst critical zone and provides a conceptual framework of hydrological functions across spatial and temporal scales.

Characterization of karst structures using quasi-3D electrical resistivity tomography

“…In a recent study, Chen et al (2018) utilized hydrometric data alongside measurements of water isotopes in rain water and groundwater to partition event and pre-event water. Their study, was supplemented with a small number of 2D electrical resistivity profiles.…”

Water within, moving through, and shaping the Earth's surface: Introducing a special issue on water in the critical zone

“…Geostatistical interpolation further enabled the characterization and visualization of the 3D CZ structure below the surface. Using electrical resistivity tomography imaging, Chen et al () acquired four profiles across a cockpit karst catchment in southwestern China, revealing complex heterogeneity in the karst subsurface. The authors combined this geophysical characterization with observations of groundwater level, spring discharge, water temperature, and O and H stable isotopic response to precipitation events, to construct a detailed conceptual model of hydrologic connectivity in this complex terrain.…”

“…Subsurface hydrologic storage and connectivity can also be inferred, to some extent, from geochemical and stable isotope studies that trace the origins of stream water based on stream geochemistry, as exemplified by a number of studies in this volume (Carroll et al, ; Chen et al, ; Clow, Mast, & Sickman, ; Foks, Stets, Singha, & Clow, ; Dwivedi et al, 2018; Godsey, Hartmann, & Kirchner, ; Rose, Karwan, & Godsey, ). Several of these studies focus on water sources (e.g., rain or snow) to streamflow and how CZ architecture shapes flowpaths and, consequently, water transit times.…”

“…Indirectly, several studies (e.g., Chen et al, ; Clow et al, ; Dwivedi et al 2018; Floury et al 2018; Foks et al, ; Rose et al, ; Zimmer et al ) partitioned hydrologic flows into paths of different depths and quantified the rate and rhythm of deep groundwater flow, particularly through fractured bedrock, which does the most work in chemical weathering and flushing of weathering products. Additionally, studies of stream solute concentration–discharge (C–Q) relations give insights on flow paths and chemical weathering regimes (dilution, concentration, or chemostatic).…”

“…Electromagnetic images extend our vision into the subsurface, a region that is otherwise opaque to human perceptions. State‐of‐science and inexpensive imaging and inversion techniques, such as reported in Flinchum et al () and Chen et al () in this special issue, hold great promises for quantifying the thickness, porosity profiles, and hydraulic connectivity among different zones in the CZ. Such knowledge should be the target of first order, reconnaissance‐level investigations of CZ structure.…”