Characterization of water content dynamics and tracer breakthrough by 3-D electrical resistivity tomography (ERT) under transient unsaturated conditions

Abstract

Characterization of preferential flow and transport is still a major challenge but may be improved employing noninvasive, tomographic methods. In this study, 3-D time lapse electrical resistivity tomography (ERT) was employed during infiltration on an undisturbed, unsaturated soil core in a laboratory lysimeter. A tracer breakthrough was conducted during transient conditions by applying a series of shortterm infiltrations, simulating natural precipitation events. The electrical response was quantitatively validated using data from a multicompartment suction sampler. Water content probes were also installed for ground-truthing of ERT responses. Water content variations associated with an infiltration front dominated the electrical response observed during individual short-term infiltration events, permitting analysis of water content dynamics from ERT data. We found that, instead of the application of an uncertain petrophysical function, shape measures of the electrical conductivity response might be used for constraining hydrological models. Considering tracer breakthroughs, the ERT observed voxel responses from time lapse tomograms at constant water contents in between infiltration events were used to quantitatively characterize the breakthrough curve. Shape parameters of the breakthrough derived from ERT, such as average velocity, were highly correlated with the shape parameters derived from local tracer breakthrough curves observed in the compartments of the suction plate. The study demonstrates that ERT can provide reliable quantitative information on both, tracer breakthroughs and water content variations under the challenging conditions of variable background electrical conductivity of the pore solution and non steady-state infiltration.

Characterization of reactive transport by 3-D electrical resistivity tomography (ERT) under unsaturated conditions

“…The experiment was carried out on a 0.48 m deep soil core contained in a laboratory lysimeter (0.5 m length, 0.5 m width, 0.6 m height). The setup of this lysimeter was described previously [ Wehrer and Slater , ] and is only summarized here. An undisturbed soil core was retrieved from an agricultural field near Riverside, Long Island, NY, USA, with the procedure described in Wehrer and Slater [].…”

“…Considerable development in the field of electrical resistivity tomography for the characterization of solute breakthroughs has occurred over the last 20 years [ Binley et al ., ; Day‐Lewis and Singha , ; French et al ., ; Garre et al ., ; Irving and Singha , ; Koestel et al ., ; Olsen et al ., ; Singha et al ., ; Slater et al ., ; Wehrer et al ., ; Wehrer and Slater , ]. The potential of ERT to visualize heterogeneous conservative transport was shown by cross‐borehole tomography [ Daily et al ., ], 2‐D imaging along a trench [ French et al ., ] and 3‐D imaging in lysimeters [ Binley et al ., ; Garre et al ., ; Koestel et al ., ; Olsen et al ., ; Wehrer et al ., ; Wehrer and Slater , ]. Temporal and spatial moments derived from time‐lapse images were validated for breakthroughs of conservative tracers [ French et al ., ; Koestel et al ., ; Wehrer and Slater , ] and show that quantitative information on transport processes can be extracted using this technique.…”

“…The potential of ERT to visualize heterogeneous conservative transport was shown by cross‐borehole tomography [ Daily et al ., ], 2‐D imaging along a trench [ French et al ., ] and 3‐D imaging in lysimeters [ Binley et al ., ; Garre et al ., ; Koestel et al ., ; Olsen et al ., ; Wehrer et al ., ; Wehrer and Slater , ]. Temporal and spatial moments derived from time‐lapse images were validated for breakthroughs of conservative tracers [ French et al ., ; Koestel et al ., ; Wehrer and Slater , ] and show that quantitative information on transport processes can be extracted using this technique. Observation and validation of reactive tracer breakthroughs using ERT in the unsaturated zone are rare [ Wehrer et al ., ].…”

“…Observation and validation of reactive tracer breakthroughs using ERT in the unsaturated zone are rare [ Wehrer et al ., ]. Observations of heterogeneous flow and transport done by ERT can be validated using dye tracer breakthroughs [ Binley et al ., ; Koestel et al ., ] or using a multicompartment sampler (MCS) [ Wehrer and Slater , ]. In contrast to dye tracing techniques, the validation by MCS does not require destruction of the soil core.…”

Estimation of Specific Yield and its Variability by Electrical Resistivity Tomography

“…Geoelectrical methods have been widely used to characterize flow dynamics in the unsaturated zone (Oberdörster et al, ; Rucker, ; Wehrer & Slater, , and several more) and to assess hydraulic parameters such as permeability or transmissivity (Revil & Cathless, ; Niwas et al, ; among many others) as well as to monitor mass solute, mean solute advection and spreading (Singha & Gorelick, ). Despite the extensive literature on the relationship between resistivity and hydraulic properties, studies regarding Sy are less abundant.…”