Characterization of groundwater and surface water mixing in a semiconfined karst aquifer using time‐lapse electrical resistivity tomography

Abstract

Groundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with end-member mixing analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and base flow conditions. To our knowledge, this is the first time EMMA and synthetic geophysical simulations have been combined. Here we interpret an 8 week time-lapse electrical resistivity data set to assess groundwater-surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time-lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m 3 /d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions, it was determined that conduit water composition ranged from 75% groundwater during base flow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time-lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water-rock reactions, in a semiconfined, sink-rise system.

The global distribution of Earth's critical zone and its controlling factors

“…Geophysical methods such as electrical resistivity tomography (ERT) and ground penetrating radar (GPR) show great potential in subsurface characterization [Parsekian et al, 2015]. For example, GPR was used for estimating subsurface porosity and solid dielectric permittivity [Mount and Comas, 2014] and subsurface hydraulic properties [Scholer et al, 2013]; ERT was used to research mixing waters between karst conduits and the matrix [Meyerhoff et al, 2014]. In addition to advances in techniques, much research has attempted to develop some indices that reflect the structure and function of CZs.…”

Time‐lapse electrical resistivity imaging of solute transport in a karst conduit

“…In a karst aquifer, Meyerhoff et al . (, ) recently used time‐lapse ERI to monitor the movement of relatively low‐conductivity surface water through a mapped conduit in response to rainfall. Although they did not verify the interpreted resistivity signal with an in situ monitoring well, their study suggests that time‐lapse ERI is a tool with great potential in karst systems.…”

Insights on geologic and vegetative controls over hydrologic behavior of a large complex basin – Global Sensitivity Analysis of an integrated parallel hydrologic model

“…This type of dual conductivity approach has been used in the past to represent matrix -conduit flow systems at large scales and recent modeling efforts suggest that an equivalent porous media approach is a reasonable approximation of regional groundwater flow processes in eogenetic karst systems such as those found in our study area (e.g. Doummar et al, 2012;Meyerhoff et al, 2014;Panagopoulos, 2012).…”