Quantitative characterization of solute transport processes in the laboratory using electrical resistivity tomography

The cultivation of date palms (Phoenix dactylifera) is widespread in hyper-arid regions and relies on high-frequency irrigation to achieve satisfactory yields. Adequate irrigation management is of great importance, and requires understanding of the dynamics of sap flow and water storage within the date palm stem. Traditionally, sap flow estimates are obtained using heat dissipation probes. This method provides point estimates that may not represent the spatial distribution of sap flow within the date palm stem. The aim of this study is to investigate whether electrical resistivity tomography (ERT) measurements on date palm stems can be used to obtain information on the spatial distribution of sap flow in order to obtain improved estimates of transpiration. In a first step, laboratory experiments were used to improve understanding of the electrical and hydraulic properties of date palm stems. A laboratory set-up was developed that induced flow in a date palm stem segment using vacuum pressure while making time-lapse ERT measurements. It was found that such ERT monitoring allows to visualize changes in radial flow variability due to different flow conditions. In addition, the electrical conductivity of the outflow was considerably higher than that of the introduced solution, which suggest the presence of stored salt in the stem segment. The relationship between bulk electrical conductivity and water content of date palm stem segments was investigated on smaller samples using multi-step-outflow experiments combined with bulk electrical conductivity measurements. The results showed that the water redistribution in the sample was slow after the initial desaturation, which suggests that the water is tightly bound as in a clay soil. The observed relationship between bulk electrical conductivity and saturation could be described with models established for porous media. In a second step, field experiments were performed that combined ERT and sap flow measurements on both juvenile date palm trees growing in lysimeters and mature date palm trees. For this, a custom-made measurement system was used to acquire high-speed ERT measurements with a temporal resolution of several minutes. The high-resolution monitoring of both the juvenile and mature date palms showed a high spatial variability in electrical conductivity within both the juvenile and mature date palm stems. This has obvious implications for the installation of sap flow sensors, where low-conductivity areas likely indicating regions without flow should be avoided. ERT monitoring also revealed diurnal changes in the spatial distribution of the electrical conductivity that are associated with the tree response to irrigation. An induced drought period for the juvenile date palm in the lysimeter also resulted in a noticeable decrease in the mean electrical conductivity on the second day after irrigation was stopped, suggesting that ERT may also provide an early indicator of water stress.

The integrated use of electrical resistivity tomography (ERT) and ground penetrating radar (GPR) measurements, and in particular the joint analysis of 2D and 3D data, can represent a valid solution for target identification at complex archaeological sites. A good example, in this respect, is given by the case study of a Phoenician–Punic necropolis in the archaeological site of Nora, in southern Sardinia (Italy), where GPR and ERT measurements were collected before site excavation. In this specific case, the mix of soil and air in the buried chambers, as well as the orientation and the complex spatial distribution of these structures into the sandstone bedrock, generated a number of anomalies difficult to interpret only using 2D results. Only the integration of all GPR and ERT data in a 3D view, and the comparison with archaeological evidence after the excavation, allowed a solid interpretation of geophysical anomalies visible in the 2D sections. Overall, this case study demonstrates the efficiency of the combined use of GPR and ERT acquisitions and shows how, in general, only the joint analysis of 2D data and in a 3D view can help the interpretation of the real distribution of the buried archaeological remains at similar archaeological complex sites.

Large-scale and small-scale high-resolution electrical resistivity tomography (ERT) measurements were carried out to investigate the sedimentological architecture of the Shen Mitri hill and its close surroundings near the Butrint archaeological site in Albania. The ERT measurements were conducted in combination with archaeological excavations of an Upper Paleolithic/Mesolithic open-air site with the aim to better understand the early modern human migration history. The resulting inversion models are meaningful and span a wide range of subsurface resistivities. The Shen Mitri hill is mainly composed of low-resistive Pliocene clayey sediments less than 5 $$\Omega$$ m and is covered by resistive sandstone towards the hill-top reaching values more than 100 $$\Omega$$ m. The Vrina plain surrounding the hill consists of sediments of Quaternary origin with rather low resistivities. High-resistive features in the Vrina plain correspond to Pliocene units of sandstone and outcrops of evaporates with several hundred $$\Omega$$ m of resistivity. The results revealed that the neighboring hill Xarre is likely to be of older age than the hill Shen Mitri as it is composed of sediments from an older Pliocene unit with higher resistivities. The results improve the knowledge of the actual geological understanding of the hills and give detailed insight to their architecture, such as thickness and distribution of the occurring formations. Thus, not the main focus of the article, small-scale ERT results reveal zones with a high-resistive surface layer which is identified as a archaeological layer-containing stone artifacts and ceramic sherds. However, undisturbed archaeological layers were not identified yet and ERT measurements can help to clarify the taphonomic history of the Shen Mitri site. Prior to the inversion, an elaborate estimation of ERT data errors including geometrical errors and the data variation coefficient is presented. The estimated error model is meaningful and included in the two-dimensional error-weighted inversion scheme. Subsequently, the effect of surface topography on ERT data is estimated by a modeling study and, as a consequence, included in all the inversion models.

Feasibility of Electrical Resistance Tomography for measurements of liquid holdup distribution in a trickle bed reactor

Quantitative inverse modelling of a cylindrical object in the laboratory using ERT: An error analysis

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection

ABSTRACTThe impact of untreated sewage irrigation and waste disposal practices on groundwater is investigated by 3D joint inversion of radio magnetotelluric and electrical resistivity tomography data. In this case study, electrical resistivity tomography and radio magnetotelluric field measurements were carried out on several profiles near a waste disposal site which was irrigated with untreated sewage water for agriculture purpose. In addition, radio magnetotelluric and electrical resistivity tomography measurements were carried out, far away from the waste site, to derive the uncontaminated geology. The data were analysed earlier using 2D inversion techniques. However, for the 2D inversion of the electrical resistivity tomography and radio magnetotelluric data, assumptions about the strike direction are required. As no clear strike direction is evident for the contamination, we considered the problem as 3D and interpreted the present data set using the 3D inversion algorithm ‘AP3DMT‐DC’. The inverted 3D resistivity model shows an unconfined aquifer of low resistivity which is overlain by an unsaturated slightly resistive near surface formation. With increasing distance from the waste sites, an increase in the resistivity of the shallow unconfined aquifer is observed. Furthest away from the waste site undisturbed geology is expected. We derived consistent and meaningful 3D resistivity models. The uncontaminated reference site indicates an increased resistivity for the aquifer layer. A synthetic 3D study was carried out to demonstrate and validate algorithm performance as well as convergence capabilities. The study demonstrates that the two methods, electrical resistivity tomography and radio magnetotelluric, complement each other. Besides, a better resolved inverted model is obtained through a 3D joint inversion, in comparison to individual 2D and 3D inversions.

A 3D ERT study of solute transport in a large experimental tank

Electrical resistivity tomography (ERT) has proved to be a valuable tool for imaging solute transport processes in the subsurface. However, a quantitative interpretation of corresponding ERT results is constrained by a number of factors. One such factor is the nonuniqueness of the ERT inverse problem if no additional constraints are imposed. In the vadose zone, further problems arise from the general ambiguity of the imaged bulk electrical conductivity in terms of water content and solute concentration. In this study we address these issues in detail for a solute tracer experiment conducted in an undisturbed unsaturated soil monolith where the tracer transport was monitored by means of 3‐D smoothness‐constrained ERT and time domain reflectometry (TDR) measurements. The experimental design allowed the determination of solute tracer concentrations directly from imaged bulk electrical conductivity. Independent TDR data and effluent tracer concentrations provided a “ground truth” for the ERT‐derived apparent convection‐dispersion equation transport parameters. The apparent transport velocity calculated from the ERT results was consistent with that based on TDR data and the sampled effluent, independent of the degree of smoothness imposed in the ERT inversion. On the other hand, the apparent dispersivity calculated from the ERT results was larger than that estimated from TDR data but smaller than that estimated from the sampled effluent, with the magnitude of deviations dependent on the degree of smoothing. Importantly, no mass balance problems were observed in the ERT results. We believe that this is largely a consequence of the uniform application of the tracer as a front and of the configuration of the electrode array with respect to the main transport direction. In conclusion, the study demonstrates that ERT can yield unprecedented quantitative information about local‐ and column‐scale solute transport characteristics in natural soils.

Improved understanding of water flow and solute transport through the unsaturated zone is important for the sustainable management of soils. As soils are complex and heterogeneous systems, quantification of the transport processes is difficult. More knowledge on the relationship between solute transport process, soil structure, hydrologic initial and boundary conditions, and observation scale is needed here.Modeling unsaturated flow and transport with mathematical or numerical methods is an important tool for predicting the infiltration and redistribution of soil water and the transport of solutes in the unsaturated zone. Flow and transport models are commonly used to support the decision making process in agricultural management, environmental impact assessment, toxic waste control, remediation design, and subsurface cleanup monitoring. The movement of contaminants through porou media describs by the combination of advection, diffusion-dispersion and chemical retardation. The most common model that describes solute transport by convection and dispersion is the convection-dispersion equation (CDE). This equation describes the change in concentration at any point along the flow path as a function of time. This paper is mainly dedicated to a discussion of basic processes for modelling of water flow and contaminant transport in saturated and unsaturated soils. After a brief description of the classical approach for simulating water flow and solute transport in porous media, issues related to water and solute trasport equation in soil.

Geophysical and petrological studies of the former lead smelting waste dump in Sławków, Poland

A methodology is developed for discrimination among models of transient solute transport in porous media. The method utilizes nonlinear regression on observations of solute concentration. Discrimination requires comparisons of model predictions to observations, systematic error in residuals, stability in parameter estimates from regression on different observation sets, and other measures of model fit among hypothesized models of transport. The set of observations of solute concentration to which models are fitted strongly influences the assessment of these discrimination criteria. The most desirable observation set for discrimination amplifies the weaknesses of those models that appear to describe existing conditions but are in fact unsuitable for prediction. The inadequacies of various observation sets are illustrated in four examples of discrimination between one‐dimensional models of solute transport. Our purpose in these examples is to understand the physical, deterministic basis of sampling design for model discrimination. In addition to physical attributes such as transport processes, boundary conditions, and flow geometry, the assumed distribution of random error in the regression model is also treated as a model attribute to be tested by the designed experiment. A common problem in field studies occurs when the set of available observations does not include sufficient information with which to discriminate among hypothesized models, hence supporting the need to design a second round of sampling specifically for discrimination. A proposed objective function in the sampling design problem favors design points at locations and times when two hypothesized transport models display the greatest differences in predicted concentration. Two hypothetical examples demonstrate the effectiveness of the objective function and the application of the discrimination criteria.

To improve the efficiency of irrigation water application it is necessary to understand 3 important aspects. The depth of wetting reached by each irrigation episode, the overlap between irrigation emitters and the depth where the largest root volume is found. The joint use of electrical resistivity tomography (ERT) with mechanistic hydrological model allows establishing an appropriate irrigation schedule for the particular condition of each irrigation sector, considering aspects for intra-farm soil variability and variations in the root volume of the orchard. For a correct characterization of the existing soil variability in a field, we propose the use of high-resolution ERT (several measurements per hectare) and clustering using k-means for the definition of sites of interest where it is necessary to obtain a petrophysical relationship. (Waxman & Smits, 1968) that allows obtaining the moisture content of the soil from the electrical resistivity of the soil. For the calibration of the mechanistic hydrological model in these same sites, the use of disk infiltrometers measurements is proposed. Through time-lapse ERT measurements in periods between irrigation, it is possible to observe areas of greater water absorption and define areas where there is a greater root volume. Through time-lapse ERT measurements in irrigation episodes, it is possible to determine the depth of wetting reached and use this information to calibrate model parameters of the mechanistic hydrological model. Finally, the use of computer simulations in the defined clusters makes it possible to establish irrigation times and frequencies that ensure a correct overlap between emitters and a wetting depth that reaches the areas of greatest water absorption.

Foam is to be used as a blocking agent for confining a pollutant source zone and avoid spreading in an aquifer. To this end, it is necessary to determine where injected foam flows and stays inside a porous medium. This study examines the use of electrical resistivity tomography for this purpose. Foam is injected in a large-scale 3D heterogeneous porous medium (0.84 × 0.84 × 0.84 m). During the injection, electrical resistivity tomography measurements are performed. We show that combining a large number of measurements with inversion techniques allows for the monitoring of a foam front in 3D during the injection process.

Modeling of solute transport is a key issue in the area of soil physics and hydrogeology. The most common approach (the convection-dispersion equation) considers an average convection flow rate and Fickian-like dispersion. Here, we propose a solute transport model in porous media of continuously expanding scale, according to the combinatorics principle. The model supposed actual porous media as a combinative body of many basic segments. First, we studied the solute transport process in each basic segment body, and then deduced the distribution of pore velocity in each basic segment body by difference approximation, finally assembled the solute transport process of each basic segment body into one of the combinative body. The simulation result coincided with the solute transport process observed in test. The model provides useful insight into the solute transport process of the non-Fickian dispersion in continuously expanding scale.