Application of electrical imaging to leachate plume evolution studies under in-situ and model conditions

Electrical imaging (resistivity tomography) is increasingly used as a geophysical exploration technique in contaminated land research. The present work demonstrates the efficiency of electrical imaging in monitoring pollution plume evolution under both in-situ and model conditions. The in-situ campaign was done at an unlined landfill site situated in the city of Patras, Hellas. A partially saturated zone of alluvial fan deposits underlies the site, which retards leachate percolation to the underlying water table. Electrical imaging provided details of the internal structure of the waste tip, and confirmed the presence of a leachate plume beneath the base of the landfill. This field data component provided the constraints for the design of a generic model of contaminant infiltration into partially saturated sand. The aim was to study leachate plume evolution in a laboratory environment. For this purpose, a miniaturised resistivity tomography technique, developed at Cardiff University, was used to image the resistivity distribution before during and after contaminant infiltration. Comparison of resulting two-dimensional tomography with observed plume geometry at the end of the test showed the miniaturised electrical imaging technique to be highly effective. Experiment showed that contaminant evolution taking place in the model was gravity-driven, with capillary water in the vadose zone being displaced by the denser contaminant solution.     

Diesel transport monitoring in simulated unconfined aquifers using miniature resistivity arrays

The principal aim of this study is to assess the scope of monitoring diesel plume migration in a scaled aquifer model with a miniaturised electrical resistivity array. Respectively 1000 and 500 ml of diesel were injected in both the unsaturated and water-saturated zones of a sand body overlying a clay aquitard, and diesel migration was monitored with a miniature electrode array and an off-the-shelf resistivity meter. Inverted time-lapse electrical resistivity tomography (ERT) data reflect downward and lateral spreading of the diesel plume away from the injection point in the unsaturated zone. Diesel was also imaged to spread upwards and laterally away from the injection point in the saturated zone, as controlled by capillary rise. In both cases later-time ERT images reflected preferential pooling of diesel on the water table, as well as vertical smearing of pooled diesel in response to simulated water-table fluctuations. Repeat fluid electrical conductivity (EC) and dissolved oxygen (DO) measurements validate the observed changes in bulk resistivity caused by both diesel injections. Artefacts introduced by 2D inversion of 3D contaminant transport were abound. Time-lapse ERT imaging of diesel transport is therefore inferred to be feasible and well-suited to complementing conventional techniques of intrusive site investigation, although time-lapse 3D or 4D ERT imaging is strongly advocated.     

Clay fine fissuring monitoring using miniature geo-electrical resistivity arrays

This article describes a miniaturised electrical imaging (resistivity tomography) technique to map the cracking pattern of a clay model. The clay used was taken from a scaled flood embankment built to study the fine fissuring due to desiccation and breaching process in flooding conditions. The potential of using a miniature array of electrodes to follow the evolution of the vertical cracks and number them during the drying process was explored. The imaging technique generated two-dimensional contoured plots of the resistivity distribution within the model before and at different stages of the desiccation process. The change in resistivity associated with the widening of the cracks were monitored as a function of time. Experiments were also carried out using a selected conductive gel to slow down the transport process into the cracks to improve the scanning capabilities of the equipment. The main vertical clay fissuring network was obtained after inversion of the experimental resistivity measurements and validated by direct observations.     

Olive-oil mill wastewater transport under unsaturated and saturated laboratory conditions using the geoelectrical resistivity tomography method and the FEFLOW model

An integrated approach for monitoring the vertical transport of a solute into the subsurface by using a geophysical method and a simulation model is proposed and evaluated. A medium-scale (1 m³) laboratory tank experiment was constructed to represent a real subsurface system, where an olive-oil mill wastewater (OOMW) spill might occur. High-resolution cross-hole electrical resistivity tomography (ERT) was performed to monitor the OOMW transport. Time-lapse ERT images defined the spatial geometry of the interface between the contaminated and uncontaminated soil into the unsaturated and saturated zones. Knowing the subsurface characteristics, the finite element flow and transport model FEFLOW was used for simulating the contaminant movement, utilizing the ERT results as a surrogate for concentration measurements for the calibration process. A statistical analysis of the ERT measurements and the corresponding transport model results for various time steps showed a good agreement between them. In addition, a sensitivity analysis of the most important parameters of the simulation model (unsaturated flow, saturated flow and transport) was performed. This laboratory-scale study emphasizes that the combined use of geophysical and transport-modeling approaches can be useful for small-scale field applications where contaminant concentration measurements are scarce, provided that its transferability from laboratory to field conditions is investigated thoroughly.     

盐溶液入渗过程的电阻率成像法时空监测

采用电阻率成像法探讨了室内实验尺度上岩体中盐溶液的入渗过程。在岩柱试样上进行蒸馏水和盐溶液的入渗实验，在入渗过程中通过多通道高密度电阻率勘测系统对岩柱的电性特征在三维空间上进行动态监测。然后对入渗过程岩柱进行电阻率成像，从而获得不同时刻岩柱中的电阻率分布。各勘测截面上电阻率的分布和变化基本反映了岩柱内盐溶液的时空变化特征，表明电阻率成像法动态监测室内实验尺度岩石中盐溶液的入渗是可行的，岩柱5个截面的电阻率图像表明溶液的入渗是不均匀的。     

基于高密度电阻率成像技术的土体干缩开裂过程监测研究

为了探究土体干缩开裂问题,文章采用ERT技术,对黏性土开展了一维干缩开裂动态监测试验。配制初始状态饱和 的泥浆试样,在自然条件下干燥,采用ERT技术获得试样干燥过程中的电阻值变化。结合试样的电阻值图像和裂隙图像, 对土体干缩开裂规律进行了分析。研究结果表明：在干燥蒸发初期,土体电阻值随时间增加缓慢减小,其原因在于土体干 燥收缩导致土颗粒间接触面积增大,颗粒水化膜变薄,进而使得土颗粒表面双电层导电性增强。随着干燥继续进行,气体 进入土体内部,土体由初始饱和状态转变为非饱和状态,电阻值转为缓慢增加。当土体产生裂隙时,裂隙周围土体电阻值 急剧增大,而未发育裂隙的土体电阻依然保持缓慢增加的趋势。通过对比试样电阻值变化曲线和裂隙图像,发现两者所呈 现的裂隙发育位置和状态存在良好的一致性。因此,ERT技术能对干燥过程中土体裂隙发育进行有效的动态监测,准确掌 握裂隙发育的时空动态信息,并且能提前预测裂隙发育的可能位置,为研究极端干旱气候作用下土体的工程性质响应提供 了理想手段。     

Imaging of wastewater percolation in heterogeneous soil using electrical resistivity tomography (ERT): a case study at east of Tenth of Ramadan City,Egypt

The objective of this study is to evaluate the effectiveness of DC resistivity surveys for imaging the wastewater percolation around the stabilization ponds in the Tenth of Ramadan City, the desert fringes of East Nile Delta, Egypt. Detailed resistivity surveys, including DC soundings and electrical resistivity tomography (ERT), were carried out along several profiles. Furthermore, synthetic modeling of ERT was designed to optimize the survey configurations and interpretation of the results. A 2D modeling of smoothness-constrained least-squares inversion scheme was applied to delineate the possible wastewater infiltration zones from oxidation ponds. Because the geoelectrical interpretation has a degree of non-uniqueness, the resistivity inversion was constrained using borehole lithological information and soil sample laboratory measurements. The DC inversion results indicate decreasing resistivity down to a depth of 15 m around waste disposal sites. The inferred soil zone close to the oxidation ponds was a mixture of sand, silt and clay. Moreover, the clay minerals were characterized by moderate swelling that could have reduced the vertical infiltration speed, causing wastewater seepage, especially around unlined disposal sites and open surface drains. Accordingly, the medium-to-low resistivity values can be attributed to wastewater leakage in clayey sand soil. Because the area slopes generally toward the northeast, the surface seepage was dominant in the shallow impermeable sandy clay subsoil. Therefore, measuring soil parameters is a complementary method to optimize resistivity interpretation, with potential for mitigating environmental hazards from wastewater leakage around disposal ponds.     

一种用于突发性场地污染模拟的解析模型

为了及时有效地应对各种突发性环境污染事故,有必要开发一种简单实用、适于各类型污染物的场地污染数学模型。通过污染事故发生后污染物在包气带、饱和带迁移转化的概化,建立了污染物运移的自由入渗模型以及降雨入渗模型并给出各自相应的解析解。无降雨时,考虑污染物在重力作用下随包气带向下渗透的作用,建立一维垂直入渗模型。有降雨时,考虑污染场地（包气带）中污染物迁移和转化的对流作用、扩散作用及挥发、生物降解、吸附、根系吸收等作用,建立包气带剖面二维溶质运移模型和饱和带平面二维溶质运移数学模型。建模过程中,假定降雨量的平均分布及土壤质地、水力参数以及有机物成分、种类均相同,同时假定污染物与多孔介质间的作用为线性吸附,植物根系对污染物的吸附遵循一级动力学。基于模型的解析解,实现案例的模拟计算。模拟结果表明：该模型具有适用范围广、模拟高效快捷等优点,能够较准确预测污染发生后污染物在土壤中的动向、到达饱和带的时间以及饱和带中污染物的迁移情况。     

黄土斜坡内部水分二维成像研究——以黑方台罗家坡滑坡为例

黄土高原地区受构造活动及河流侵蚀形成的大量黄土斜坡, 在降雨及农业灌溉等条件影响下, 每年会有大量黄土滑坡发生, 因此, 准确揭示黄土斜坡内部水分空间分布规律及运移特征对于理解黄土滑坡机理和预警预报极其重要。本文利用电阻率层析成像技术(ERT)及模型模拟的方法对斜坡内部电阻率与含水量关系进行量化, 并结合surfer软件进行斜坡内部水分二维成像。黄土电阻率与地层物理化学参数相关性分析表明:黄土斜坡中地层含水率是地层电阻率发生变化的主导因素;基于Archie公式的模型拟合结果能更准确地量化电阻率与含水量特征关系;地层水分二维成像结果可以直观地显示斜坡内部水分时空演化。本研究定量化且更直观地对黄土斜坡内部的水分分布特征进行描述, 可为黄土滑坡预报预警提供最直接有效的数据资料。     

二维孔隙介质中重非水相液体饱和度测定与分析

在二维均质饱和孔隙介质中利用光透法与高密度电阻率法定量监测重非水相液体(DNAPL)的运移过程与饱和度。 将光透法与高密度电阻率法相结合,进行高密度电阻率法对 DNAPL 在饱和多孔介质中渗流过程的定量监测。在二维砂箱 中进行 DNAPL 的入渗试验,应用光透法与高密度电阻率法进行动态监测,分别利用 CCD 相机、LCR 数字电桥采集数据。 光透法的数据处理采用“水-DNAPL”两相中 DNAPL 饱和度的计算公式。而高密度电阻率法的数据处理是将获得的电阻值 数据转化为电阻率值,然后利用 Archie 公式获得重非水相液体的饱和度空间分布。Archie 公式中参数β值本文采用与光透法 相结合的新方法来获取,克服了传统方法获取β值时存在的困难。根据不同时刻的饱和度空间分布可以估算注入砂箱内的 DNAPL 总量,并将其与实测的入渗量进行对比。结果表明,与光透法结合后的高密度电阻率成像法能定量监测饱和孔隙介 质中 DNAPL 的入渗过程,且估算的 DNAPL 入渗量与实测值比较吻合,提出的基于光透法计算β值的新方法是有效且可行的。     

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.     

岩柱中水体入渗过程的高密度电阻率成像法研究

本文应用高密度电阻率成像法探讨了室内实验尺度岩柱中的这一人渗过程。首先建立了电法勘探的电位场数学模型，用有限元法进行了电位场正问题的求解。然后应用基于序列二次规划(SQP)方法的电阻率反演算法，由测量数据反演得到了岩石电阻率在二维空间上的分布。由岩柱中水体人渗过程的电法测量数据反演得到的电阻率图像反映了岩柱中水分的空间变化过程。岩柱中电阻率的空间分布及其随时间的变化表明水体的人渗是不均匀的，在岩柱周边渗流较快，而在中部存在滞后现象。得到的相对电阻率图像也反映了岩柱周边存在的微裂隙。     

Modelling coupled heat and contaminant transport in groundwater

This paper presents the results of a research programme conducted on the geotechnical centrifuge at The University of Western Australia to investigate coupled heat and contaminant transport in the soil surrounding a buried waste source. The phenomena which govern heat and contaminant transport through porous media are discussed, the principles of geotechnical centrifuge modelling are outlined, and relevant scaling laws that govern the relationship between a centrifuge model and the prototype, with respect to the problem of coupled waste transport, are presented. A model test, simulating two-dimensional migration from a buried heat and contaminant source, is described, and the results from four model tests are presented. The experimental data show that hydraulic instability is responsible for the transport of contaminant in the soil around the source and that the mode of instability is determined by the magnitude of the effective Rayleigh number.     

Tracing subsurface oil pollution leakage using 2D electrical resistivity tomography

This study provides an evaluation of the subsurface oil contaminant flow in an oil production field at the Wadi Feiran fan, along the Gulf of Suez, Egypt. The 2D electrical resistivity tomography technique has been applied along four profiles surrounding the spill location of oil dump site. Theoretical soil resistivity response versus fluid resistivity for different cation exchange capacity values has been studied. Then, a physical model for the current situation of flowing the oil over the groundwater (sea invaded water) of saturated fluvial sediments has been introduced. It was supposed that the crude oil has higher resistivity values and turned to be conductive far away from the oil spill source under the effect of microbial biodegradation. Such conditions of contaminated distribution and its resistivity values have been considered during the inversion and interpretation of the acquired data sets. The processed 2D resistivity profiles reveal that oil plumes are accumulated in the seawater invasion direction, whereas it has low signatures along the profiles parallel to the shore line.     

Electrical resistivity mapping of oil spills in a coastal environment of Lagos,Nigeria

Three years after the oil spillage and pipeline explosion that claimed about 100 human lives at Ijegun Community of Lagos–Nigeria, a combination of carefully designed 2D Electrical Resistivity Profilling and Vertical Electrical Sounding methods was deployed to map and characterise the subsurface around the contaminated site. Data acquired were processed, forward modelled and tomographically inverted to obtain the multi-dimensional resistivity distribution of subsurface. The results of the study revealed high resistivity structures that indocate the presence of contaminant (oil plumes) of different sizes and shapes around the oil leakage site. These high resistivity structures are absent in the tomograms and resistivity-depth slices computed for Iyana—a linear settlement not affected by oil spillage. The five geo-electric layers and the resistivities delineated in the area are the top soil layer, 220–670 Ωm; clayey sand layer, 300–1072 Ωm; top sand layer, 120–328 Ωm; mudstone/shale layer, 25–116 Ωm and the bottom sand layer, 15–69 Ωm. The base of the first four geo-electric layers corresponds to 3.9, 8.4, 27.2 and 34.6 m respectively. The two groundwater aquifers delineated correspond to the third and fifth geo-electric layers. The top aquifer has been infiltrated by oil plumes. The depth penetrated by the oil plume decreases from 32 m to about 24 m across the survey profiles from the two ends. It was concluded that the contaminant plumes from the oil spillage are yet to be completely degraded as at the time of the study. It is recommended that the contaminated site be remediated to remove or reduce the contaminant oil in the subsurface.     

The application of GPR and electrical resistivity tomography as useful tools in detection of sinkholes in the Cheria Basin (northeast of Algeria)

Sinkhole collapse is one of the main limitations in the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Ground-penetrating radar (GPR) and electrical resistivity imaging or tomography (RESTOM) are well suited to mapping sinkholes because of the ability of these two techniques for detecting voids and discriminating subtle resistivity variations. Nine GPR profiles and two-dimensional electrical resistivity tomography have been applied, with relative success, to locate paleo-collapses and cavities, and to detect and characterize karst at two sinkhole sites near Cheria City where limestone is covered by about 10 m of clayey soils. The survey results suggest that GPR and RESTOM are ideal geophysical tools to aid in the detection and monitoring of sinkholes and other subsurface cavities.     

Hydrogeological modeling constraints provided by geophysical and geochemical mapping of a chlorinated ethenes plume in northern France

A methodological approach is described which combines geophysical and geochemical data to delineate the extent of a chlorinated ethenes plume in northern France; the methodology was used to calibrate a hydrogeological model of the contaminants’ migration and degradation. The existence of strong reducing conditions in some parts of the aquifer is first determined by measuring in situ the redox potential and dissolved oxygen, dissolved ferrous iron and chloride concentrations. Electrical resistivity imaging and electromagnetic mapping, using the Slingram method, are then used to determine the shape of the pollutant plume. A decreasing empirical exponential relation between measured chloride concentrations in the water and aquifer electrical resistivity is observed; the resistivity formation factor calculated at a few points also shows a major contribution of chloride concentration in the resistivity of the saturated porous medium. MODFLOW software and MT3D99 first-order parent–daughter chain reaction and the RT3D aerobic–anaerobic model for tetrachloroethene (PCE)/trichloroethene (TCE) dechlorination are finally used for a first attempt at modeling the degradation of the chlorinated ethenes. After calibration, the distribution of the chlorinated ethenes and their degradation products simulated with the model approximately reflects the mean measured values in the observation wells, confirming the data-derived image of the plume.     

Delineation of contamination plume around oxidation sewage-ponds in Southwestern Nigeria

Integrated surface electrical resistivity and electromagnetic (EM) surveys were conducted in a hard-rock terrain of Southwestern Nigeria in the vicinity of active oxidation sewage treatment ponds. The aim was to detect soil contamination due to the spread of sewage effluent, locate possible leachate plumes and conductive lithologic layers, and access the risk of groundwater pollution in the vicinity of the sewage-ponds. Dipole–dipole resistivity profiling and very low frequency (VLF) data were acquired at 10-m intervals along five 200-m long east-west geophysical traverses. Resistivity sections obtained revealed four subsurface geologic layers comprised of lateritic clay, clayey sand/sand, weathered/fractured bedrock, and competent bedrock. A distinct low resistivity zone corresponding to the contamination plume (labeled B) was delineated from all the resistivity sections. This low zone extends into the weathered bedrock and possibly suggests contamination of this layer. The filtered real component of the processed VLF data detected three distinct anomaly zones that are representative of fractured zones filled with conductive fluids and/or lithologic boundaries that possibly serve as conduits for the movement of contaminated effluents. The results obtained from the two methods suggest possible contamination of the subsurface soil layers and groundwater in the vicinity of the sewage-ponds. The existence of this contaminated plume poses a serious threat to the ecosystem and health of the people living in the vicinity of the sewage-ponds.     

Electrical resistivity anomalies over hydrocarbon bearing structures

The present study is aimed at working out the possible resistivity anomalies associated with hydrocarbon bearing structures. The anomaly due to a typical two-dimensional anticlinal structure filled with hydrocarbon, and overlying a basement of infinite resistivity was computed using the conformal mapping technique. A more realistic and elaborate model, which could not be simplified by conformal mapping, was studied using the finite element method. This model consisted of a two-dimensional anticlinal structure filled with oil or gas-embedded midway in a homogeneous layer which itself overlies a half-space of infinite resistivity, the electrical resistivity of the hydrocarbon bearing structure being simulated as infinite.     

非饱和黏性土的电阻率特性及其试验研究

在讨论黏性土导电特性的基础上,假定非饱和黏性土中存在3种电流流通路径,针对已有的非饱和土电阻率结构模型存在的缺陷,推导了适用于非饱和黏性土的电阻率结构模型,探讨了土电阻率的主要影响因素,并将其归纳为3类。最后,通过对合肥膨胀土相关试验研究,分析了含水率、孔隙水电阻率、温度以及孔隙率对膨胀土的电阻率的影响特征,探讨了非饱和膨胀土的电阻率基本特性。