二维砂箱模型中DNAPL运移过程的电阻率成像法研究

本研究通过设有黏性土透镜体的二维砂槽中开展重非水相液体(DNAPL)入渗运移试验,采用电阻率成像法(ERT)探讨DNAPL在非均质多孔介质中的运移分布规律.试验过程中利用电阻率成像系统进行动态监测,采集电压电流数据后得到相应的电阻值并将其转换为视电阻率值,运用Surfer软件处理数据进而获取DNAPL运移过程的相对电阻率变化分布图像.结果表明,相对电阻率值随着污染物浓度的增加而不断增大,不同时刻相对电阻率变化分布图像与DNAPL的运移过程相对应,可以清晰圈定出DNAPL的运移路径及分布范围;DNAPL在入渗过程中受到重力作用与毛细力的控制,以垂向运移为主,在遇到黏土透镜体时出现明显横向运移,绕过透镜体继续向下入渗最后聚积在砂槽底部.本研究为进一步丰富对DNAPL污染物在实际环境条件下运移机理和分布特征的认识,制定可行的DNAPL污染物治理方案等方面具有重要的科学价值.     

融合电阻率成像法及图像法的DNAPL污染过程研究

本文应用图像法和电阻率成像法(ERT)监测二维模型中DNAPL在饱和非均质多孔介质中的运移分布特征,研究DNAPL在实际环境条件下的运移机理与分布特征.图像法是利用数码相机对污染物注入过程进行实时拍摄,以提供一个直观的角度以观察二维模型里多孔介质中DNAPL运移状态,与此同时利用ERT对模型中的电阻率变化进行监测,并导出数据到Surfer软件中绘制成电阻率分布图像.将两种方法的结果图进行对比分析,采用图像法验证ERT的数据结果,结果表明,DNAPL的运移、扩散方向会倾向于介质松散的位置,在黏土裂隙分布区域产生运移速度更快的优势运移通道;在DNAPL的重分布过程中,DNAPL的运移主要受重力作用的影响,污染区域下移,污染区域的DNAPL含量减小,电阻率相对值减小.ERT的监测结果基本可以反映DNAPL在多孔介质中运移和分布,但是局部污染区域监测受到限制,说明ERT分辨率需要进一步提高.本文融合两种方法进行污染物运移监测,实验结果对于理解DNAPL的运移机理与分布特性,以及确定污染范围和污染程度方面具有重要意义.     

饱水和排水过程中岩石电阻率各向异性特征的电阻率成像法研究

从电阻率的角度研究岩石裂隙介质的各向异性特征是一种方便而有效的方法,但多限于空间单个点上的测量数据分析.通过在砂岩岩样上的饱水与排水实验以及同步进行的高密度电阻率成像监测,探讨了应用高密度电阻率成像法获得图像研究岩石各向异性特征的可能性,分析了饱水与排水过程中岩石电阻率在不同方向上的响应特性.结果表明,电阻率成像法在分析岩石裂隙介质的各向异性方面具有多方向成像和动态监测的优点,可以通过对不同方向上获得的电阻率图像的分析,提取出岩石沉积结构的空间分布模式,清晰地反映出岩石在饱水和排水过程中电阻率变化空间分布模式的各向异性特征.     

LNAPLs迁移的数值模拟和土壤介电性质的变化分析

包气带是地面以下潜水面以上的地带,由非饱和多孔介质构成.LNAPLs泄漏到土壤中后会对包气带和地下水造成严重污染.为了准确探测出LNAPLs的迁移范围,有必要对其迁移规律及迁移对土壤介电常数的影响进行研究.本文从扩散方程出发,采用数值模拟研究了LNAPLs泄露速率对其在包气带中迁移的影响,分析了LNAPLs迁移过程中土壤相对介电常数的变化特征.研究结果表明,LNAPLs在土体中的迁移范围受到渗漏速率的影响:对于均匀渗漏,渗漏速率越大,迁移范围越大;对于非均匀渗漏,单位时问内渗漏量一定时,LNAPLs在土体中的扩散范围短期内受到渗漏速率非均匀性的影响,但长期内并不受此影响.LNAPLs的泄露会使区域中土壤相对介电常数发生变化:在毛细水带以上的非饱和区域,相对介电常数变化较小,但由于LNAPLs的扩散,驱使土壤中残余水向两侧迁移,进而形成明显的"山形";在毛细水带,相对介电常数随LNAPLs量的变化而明显变化,并有明显的横向变化趋势.     

PFAS污染砂土的电阻率特性试验

PFAS新型污染物是当前环境水文地球物理学的研究热点,但其电阻率特性尚未清楚．为了探究PFAS溶液电特性及砂样含水溶液饱和度与溶液浓度对PFAS污染砂土电阻率的影响．本研究以PFOA为PFAS污染物的代表,设置不同浓度PFOA溶液并改变砂样盒的含水溶液饱和度,利用高密度电法仪监测砂样盒进行电阻率试验,结果表明：（1）砂样被PFOA溶液饱和的过程中,砂样电阻率随着砂样含水溶液饱和度的增大逐渐减小,砂样完全饱和时砂样电阻率值降到最低．对两者之间的关系均可用Archie公式较好拟合,拟合结果的相关系数R2均接近1.(2）在砂样被PFOA溶液完全饱和的状态下,砂样电阻率值会受PFOA溶液浓度的影响．砂样电阻率随溶质浓度的增大呈现幂函数关系的下降,幂函数拟合的相关系数R2值为0.8977,拟合效果较好．（3）在砂样饱和与非饱和过程试验中,砂样电阻率值会随着介质含水溶液饱和度的改变呈相应的动态变化．对于砂样电阻率与介质含水溶液饱和度的关系曲线可以用Archie公式很好的拟合,相关系数R2为0.9980．本研究获取了砂样被PFOA溶液饱和过程中的电阻率数据,并得到了砂样电阻率与多孔介质含水溶液饱和度...     

滑坡体降雨入渗的高密度电法模型试验研究

降雨作为影响滑坡体稳定性的主要因素之一，其在坡体中的入渗过程难以监测．高密度电法作为一种无损监测手段，其具有操作便捷、对监测对象扰动小的特点，可实现滑坡体水分运移的高效实时监测．基于高密度电法监测技术，通过饱和度测定试验建立岩土体电阻率与饱和度之间的数学关系，开展间隔降雨条件下滑坡体的高密度电法监测模型试验．电阻率反演结果揭示了滑坡体在间隔降雨条件下水分的运移规律和坡体渗透破坏过程，验证了采用高密度电法监测滑坡体降雨入渗过程中水分迁移的可行性．     

大斜度井和水平井井地三维电阻率数值模拟和联合反演

井地电阻率成像法利用井套管作电流源向井下供入大功率直流电流,在地表测量由地下介质的电性变化形成的电位分布,通过反演可得到地下介质的电阻率分布.针对大斜度井和水平井开展井地三维电阻率数值模拟和反演研究,对油田注水及压裂效果监测具有重要意义.基于井地电阻率成像法原理,采用有限差分法和不完全切勒斯基共轭梯度法进行了三维正演模拟,研究了大斜度井和水平井的井地电位响应特征.提出了采用层状约束阻尼最小二乘法由浅到深地进行大斜度井和水平井的多层联合反演,并对实际水平井井地电位各个层段数据进行了三维反演.模拟结果表明,倾斜线源和水平线源会对地面电位响应产生明显影响,在反演中需要考虑线源形态.实际水平井井地电位反演成像表明,考虑倾斜线源或者水平线源的联合反演得到了准确的水平井三维注水层成像图,得出注水层的真电阻率分布,能够判断注水运移方向.     

露天煤矿地下水电磁探测与类型判别

为准确确定露天煤矿地下水分布与类型,根据不同赋存条件电磁差异,基于磁场强度与电阻率参数进行异常划分与类型判别。以新疆某露天煤矿为例,采用高精度磁法通过磁异常强度确定烧变岩边界,并采用高密度电法和瞬变电磁法通过电阻率圈定低阻异常区,结合异常响应差异分别推断地下水分布范围与赋存类型。地面钻探验证探测成果的准确性。结果表明:露天煤矿烧变岩水与砂岩裂隙水电磁响应差异明显,基于磁场强度与电阻率的双参数综合电磁法能准确圈定其分布范围,并判别其赋存类型。     

高密度电阻率法成像

介绍在地质大学电法科研组“高密度电阻率法正、反演软件”的基础上,研制高密度电阻率法2.5维电阻率成像系统;讨论不平地形条件下高密度电阻率法2.5维电阻率成像的算法和软件,可用于九种常用电极装置观测结果的电阻率成像。对理论和实测数据的成像结果表明,该成像系统程序运行稳定,成像效果很好。     

土壤—地下水系统非水相液体污染区的电性特征及电阻率法探测

从土壤-地下水系统被非水相液体污染后引起的导电性变化、污染区地电模型演化、电阻率异常认识及量化解释等方面分析了国内外利用电阻率法探测这一问题的研究现状.已有研究表明,污染区电性随污染程度、污染时间变化显示一定的动态性;污染区地电模型建立是个复杂过程,不仅反映污染物的扩散过程,还要考虑地质、地下水、土层含水量等影响因素,并随外部条件和时间变化显示一定的动态性;这也决定了实测电剖面异常特征的复杂性.由于基于电测数据的真电阻率获取及污染土电性基本模型建立都尚未解决,利用电性探测结果进行污染量化分析的研究还没有形成统一的认识和结论,这也是未来很长时间内需要攻克的难题.     

Experimental study of fly-ash migration by using magnetic method

Several studies have shown that magnetic measurements can be used in assessing soil contamination due to atmospheric deposition of pollutants. Reliable spatial mapping of magnetic susceptibility of soils assumes high temporal stability of deposited particles, accumulated in top-soil horizons. One of the main methodological concerns is whether the migration of deposited anthropogenic ferrimagnetic particles may bias the measured values. Measurements carried out on high-porosity (sandy) soils, or on soils with a very variable water regime may yield inconsistent values of top-soil magnetic susceptibility as the indicator of contamination. This study focuses on the laboratory examination of migration of fly ashes from a coal-burning power plant in sands of different porosity and under a simulated rain regime. Columns of sand of different grain sizes, placed in plastic cylinders, were contaminated on the surface by the fly ash. The vertical migration of magnetic particles was monitored using measurements of magnetic susceptibility with an SM400 Kappameter. Calibration measurements in the water environment showed an erroneous performance and resulted in the technical improvement of the used susceptibility meter (Model 2009). Our results show that the vertical distribution of flyash particles deposited on fine sand is very stable even after repeated rain simulation. The peak value of magnetic susceptibility is located in a stable position a few millimeters under the surface. Hence, standard top-soil magnetic mapping is in such a case reliable and fully representative. Contrary to that, in case of coarse sand, the peak value of magnetic susceptibility migrates by more than 10 cm. The results will be further used for numerical modeling of contaminant transport in porous media.     

Multifluid flow in bedded porous media: laboratory experiments and numerical simulations

Understanding light nonaqueous-phase liquid (LNAPL) movement in heterogeneous vadose environments is important for effective remediation design. We investigated LNAPL movement near a sloping fine- over coarse-grained textural interface, forming a capillary barrier. LNAPL flow experiments were performed in a glass chamber (50 cm×60 cm×1.0 cm) using two silica sands (12/20 and 30/40 sieve sizes). Variable water saturations near the textural interface were generated by applying water uniformly to the sand surface at various flow rates. A model LNAPL (Soltrol® 220) was subsequently released at two locations at the sand surface. Visible light transmission was used to quantitatively determine water saturations prior to LNAPL release and to observe LNAPL flow paths. Numerical simulations were performed using the Subsurface Transport Over Multiple Phases (STOMP) simulator, employing two nonhysteretic relative permeability–saturation–pressure (k–S–P) models. LNAPL movement strongly depended on the water saturation in the fine-grained sand layer above the textural interface. In general, reasonable agreement was found between observed and predicted water saturations near the textural interface and LNAPL flow paths. Discrepancies between predictions based on the van Genuchten/Mualem (VGM) and Brooks–Corey/Burdine (BCB) k–S–P models existed in the migration speed of the simulated LNAPL plume and the LNAPL flow patterns at high water saturation above the textural interface. In both instances, predictions based on the BCB model agreed better with experimental observations than predictions based on the VGM model. The results confirm the critical role water saturation plays in determining LNAPL movement in heterogeneous vadose zone environments and that accurate prediction of LNAPL flow paths depends on the careful selection of an appropriate k–S–P model.     

Migration of Petroleum Products Through Sandy Hydrogeologic Systems

Laboratory column experiments were carried out to study the migration of petroleum products through a sandy porous medium. It was found that the oil pressure needed to displace water from the pores of the sand medium increased with depth below the top of the column. While oil under negative pressure displaced water through the variably unsaturated zone, a significant vertical column of oil was needed to displace water from the pores at the water table. These results indicate that oil penetration to and below the water table will occur only if the porous medium is highly conductive and the rate of oil leak is high. For small to moderate leak rates and hydraulic conductivities, oil would preferentially spread laterally through the zones above the water table. This process of spreading could serve as a natural barrier to severe aquifer contamination by petroleum products.
A simplified procedure was developed to give an order-of-magnitude estimate of the preferred subsurface migration pathways of leaked petroleum products. This procedure utilizes the main drainage and wetting curves for oil and water and the interfacial pressure between these two fluids. This approach could be useful in guiding exploratory investigations, reducing both the risk for further spreading of the contaminants and the investigative cost.     

Monitoring soil contaminations using a contactless conductivity probe1

In order to investigate the possibility of using low-frequency electromagnetic waves to detect and monitor oil contamination of soils, a series of laboratory measurements were performed. A new measurement system to monitor the resistivities of soil and sand samples while samples are being contaminated by diesel oil is presented. The frequency used in measurements is 100 kHz. Since the measurement system is composed of coil-type transmitters and receivers, there is no need for electrodes to be in contact with samples. The contamination process is simulated using diesel oil dripping on top of soil and sand samples. The conductivity distributions in samples along the sample length are recorded as a function of time. Water-wet sand and soil samples were measured during diesel oil contamination. The measured data show that the conductivities of soil and sand samples change during the contamination process. The change in resistivity for measured samples before and after diesel oil contamination is in the range of 20% to 50%, giving a reflection coefficient change in the low-frequency limit of 4.7% to 7%. This amount of change in the reflection coefficient makes it very challenging to detect and monitor oil contamination based on EM reflection from the contaminants. The results suggest that EM methods based on propagation and induction, such as tomography and borehole induction, could be used for this purpose.     

Predicting Vertical LNAPL Distribution in the Subsurface under the Fluctuating Water Table Effect

The present study proposes a methodology for predicting the vertical light nonaqueous-phase liquids (LNAPLs) distribution within an aquifer by considering the influence of water table fluctuations. The LNAPL distribution is predicted by combining (1) information on air/LNAPL and LNAPL/water interface elevations with (2) the initial elevation of the water table without LNAPL effect. Data used in the present study were collected during groundwater monitoring undertaken over a period of 4 months at a LNAPL-impacted observation well. In this study, the water table fluctuations raised the free LNAPL in the subsurface to an elevation of 206.63 m, while the lowest elevation was 205.70 m, forming a thickness of 0.93 m of LNAPL-impacted soil. Results show that the apparent LNAPL thickness in the observation well is found to be three times greater than the actual free LNAPL thickness in soil; a finding that agrees with previous studies reporting that apparent LNAPL thickness in observation wells typically exceeds the free LNAPL thickness within soil by a factor estimated to range between 2 and 10. The present study provides insights concerning the transient variation of LNAPL distribution within the subsurface and highlights the capability of the proposed methodology to mathematically predict the actual LNAPL thickness in the subsurface, without the need to conduct laborious field tests. Practitioners can use the proposed methodology to determine by how much the water table should be lowered, through pumping, to isolate the LNAPL-impacted soil within the unsaturated zone, which can then be subjected to in situ vadose zone remedial treatment.     

Comparison of relative permeability-saturation-pressure parametric models for infiltration and redistribution of a light nonaqueous-phase liquid in sandy porous media

To test and evaluate the ability of commonly used constitutive relations for multifluid flow predictions, results of numerical flow and transport simulations are compared to experimental data. Three quantitative experiments were conducted in 1-m-long vertical columns. The columns were filled with either a uniform sand, a sand with a broad particle-size distribution, or with a layered system where a layer of a course-textured uniform sand was placed in an otherwise finer-textured uniform sand. After establishing variably water-saturated conditions, a pulse of a light nonaqueous-phase liquid (LNAPL) was injected uniformly at a constant rate. Water and LNAPL saturations were measured as a function of time and elevation with a dual-energy gamma-radiation system. The infiltration and redistribution of the LNAPL were simulated with nonhysteretic and hysteretic parametric relative permeability-saturation-pressure (k-S-P) models. The models were calibrated using two-phase air-water retention data and an established scaling theory. The nonhysteretic Brooks-Corey k-S-P model, which utilizes the Burdine relative permeability model, yielded predictions that closely matched the experimental data. Use of the nonhysteretic and hysteretic k-S-P models, based on the van Genuchten S-P relations and k-S relations derived from the Mualem relative permeability model, did not agree as well with the experimental data as those obtained with the Brooks-Corey k-S-P model. Explanations for the differences in performance of the three tested parametric k-S-P models are proposed.     

Measuring the effect of structural connectivity on the water dynamics in heterogeneous porous media using speedy neutron tomography

The temporal and spatial distribution of water within a porous medium is affected by the medium’s structure, i.e., the spatial arrangement of its constituents. To analyze structural effects on the fluid dynamics, we measured the 3D water content distribution in a heterogeneous sand column during two drainage-wetting cycles using neutron transmission tomography. The sample with a volume of 105 cm³ contained 101 cubes of fine and 49 cubes of coarse sand with particles ranging from 0.01 to 0.05 and 0.03 to 0.09 cm, respectively. The pressure at the lower boundary was determined by the water reservoir positioned between 7 and 39 cm below the top of the column. The duration of one complete 3D scanning with a spatial resolution of 127 μm was 56 s. The signal to noise ratio of the measurements was low due to the short exposure time in the neutron beam, but it was possible to quantify the water content in the individual cubes and hence the effect of structure on macroscopic water distribution. Continuous structures of coarse sand drained faster than coarse sand without connection to the upper boundary. During the initial wetting phase, cubes of coarse sand material completely embedded in the fine material remained water unsaturated due to air entrapment. The effect of the coarse sand connectivity was analyzed in two-dimensional numerical simulations based on Richards equation. In contrast to the measurements, no effect of structure connectivity was found. The coarse sand cubes embedded within the fine matrix drain as quickly as the coarse sand cubes arranged in a continuous channel due to the model assumption of a continuous air phase.     

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

A high resolution, cross-borehole, 3D electrical resistivity tomography (ERT) study of solute transport was conducted in a large experimental tank. ERT voxels comprising the time sequence of electrical images were converted into a 3D array of ERT estimated fluid conductivity breakthrough curves and compared with direct measurements of fluid conductivity breakthrough made in wells. The 3D ERT images of solute transport behaviour were also compared with predictions based on a 3D finite-element, coupled flow and transport model, accounting for gravity induced flow caused by concentration differences.The tank (dimensions 185×245×186 cm) was filled with medium sand, with a gravel channel and a fine sand layer installed. This heterogeneous system was designed to complicate solute transport behaviour relative to a homogeneous sand tank, and to thus provide a challenging but insightful analysis of the ability of 3D ERT to resolve transport phenomena. Four ERT arrays and 20 piezometers were installed during filling. A NaCl tracer (conductivity 1.34 S/m) was injected and intensively monitored with 3D ERT and direct sampling of fluid chemistry in piezometers.We converted the bulk conductivity estimate for 250 voxels in the ERT imaged volume into ERT estimated voxel fluid conductivity by assuming that matrix conduction in the tank is negligible. In general, the ERT voxel response is in reasonable agreement with the shape of fluid conductivity breakthrough observed in six wells in which direct measurements of fluid conductivity were made. However, discrepancies occur, particularly at early times, which we attribute to differences between the scale of the image voxels and the fluid conductivity measurement, measurement errors mapped into the electrical inversion and artificial image roughness resulting from the inversion.ERT images revealed the 3D tracer distribution at 15 times after tracer injection. The general pattern and timing of solute breakthrough observed with ERT agreed with that predicted from the flow/transport modelling. However, the ERT images indicate a vertical component of tracer transport and preferential flow paths in the medium sand. We attribute this to transient vertical gradients established during tracer injection, and heterogeneity caused by sorting of the sand resulting from the filling procedure. In this study, ERT provided a unique dataset of 250 voxel breakthrough curves in 1.04 m³. The use of 3D ERT to generate an array of densely sampled estimated fluid conductivity breakthrough curves is a potentially powerful tool for quantifying solute transport processes.     

Finite-element analysis of the transport of water and solutes in title-drained soils

The finite-element method based on a Galerkin technique was used to formulate the problem of simulating the two-dimensional (cross-sectional) transient movement of water and solute in saturated or partially saturated nonuniform porous media. The numerical model utilizes linear triangular elements. Nonreactive, as well as reactive solutes whose behaviour can be described by a distribution coefficient or first-order reaction term were considered. The flow portion of the model was tested by comparison of the model results with experimental and finite-difference results for transient flow in an unsaturated sand column and the solute transport portion of the model was tested by comparison with analytical solution results. The model was applied to a hypothetical case involving movement of water and solutes in tile-drained soils. The simulation results showed the development of distinct solute leaching patterns in the soil as drainage proceeded. Although applied to a tile drainage problem in this study, the model should be equally useful in the study of a wide range of two-dimensional water and solute migration problems.     

LNAPL Distribution in a Cohesionless Soil: A Field Investigation and Cryogenic Sampler

Lighter-than-water Non-Aqueous Phase Liquids (LNAPLs), such as jet fuels or gasolines, are common contaminants of soils and ground water. However, the total volume and distribution of an LNAPL is difficult to accurately determine during a site investigation. LNAPL that is entrapped in the saturated zone due to fluctuating water table conditions is particularly difficult to quantify. Yet, the amount of entrapped product in the saturated zone is theoretically higher, per volume of soil, than the residual product in the unsaturated zone, and small amounts of LNAPL in the saturated zone can contaminate large volumes of ground water.
The only method currently available to quantify the amount of LNAPL is direct soil-core sampling combined with laboratory analysis of the fluid extracted from the soil cores. However, direct sampling of saturated ground water systems with conventional samplers presents a number of problems. In this study, a new sampler was developed that can be used to retrieve undisturbed soil and pore fluid samples from below the water table in cohesionless soils. The sampler uses carbon dioxide to cool the bottom of a saturated soil sample in situ to near freezing. Results of a field study where a prototype sampler was tested demonstrate the usefulness of a cryogenic sampler and show that the amount of LNAPL entrapped below the water table can be a significant part of the total LNAPL in the soil.