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1.
A 3D ERT study of solute transport in a large experimental tank   总被引：2，自引：0，他引：2  
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.  相似文献   

2.
This study investigates and quantifies the influence of physical heterogeneity in granular porous media, represented by materials with different hydraulic conductivity, on the migration of nitrate, used as an amendment to enhance bioremediation, under an electric field. Laboratory experiments were conducted in a bench‐scale test cell under a low applied direct current using glass bead and clay mixes and synthetic groundwater to represent ideal conditions. The experiments included bromide tracer tests in homogeneous settings to deduce controls on electrokinetic transport of inorganic solutes in the different materials, and comparison of nitrate migration under homogeneous and heterogeneous scenarios. The results indicate that physical heterogeneity of subsurface materials, represented by a contrast between a higher‐hydraulic conductivity and lower‐hydraulic conductivity material normal to the direction of the applied electric field exerts the following controls on nitrate migration: (1) a spatial change in nitrate migration rate due to changes in effective ionic mobility and subsequent accumulation of nitrate at the interface between these materials; and (2) a spatial change in the voltage gradient distribution across the hydraulic conductivity contrast, due to the inverse relationship with effective ionic mobility. These factors will contribute to higher mass transport of nitrate through low hydraulic conductivity zones in heterogeneous porous media, relative to homogeneous host materials. Overall electrokinetic migration of amendments such as nitrate can be increased in heterogeneous granular porous media to enhance the in situ bioremediation of organic contaminants present in low hydraulic conductivity zones.  相似文献   

3.
Accurate quantification of in situ heterogeneity and flow processes through fractured geologic media remains elusive for hydrogeologists due to the complexity in fracture characterization and its multiscale behavior. In this research, we demonstrated the efficacy of tracer-electrical resistivity tomography (ERT) experiments combined with numerical simulations to characterize heterogeneity and delineate preferential flow paths in a fractured granite aquifer. A series of natural gradient saline tracer experiments were conducted from a depth window of 18 to 22 m in an injection well (IW) located inside the Indian Institute of Technology Hyderabad campus. Tracer migration was monitored in a time-lapse mode using two cross-sectional surface ERT profiles placed in the direction of flow gradient. ERT data quality was improved by considering stacking, reciprocal measurements, resolution indicators, and geophysical logs. Dynamic changes in subsurface electrical properties inferred via resistivity anomalies were used to highlight preferential flow paths of the study area. Temporal changes in electrical resistivity and tracer concentration were monitored along the vertical in an observation well located at 48 m to the east of the IW. ERT-derived tracer breakthrough curves were in agreement with geochemical sample measurements. Fracture geometry and hydraulic properties derived from ERT and pumping tests were further used to evaluate two mathematical conceptualizations that are relevant to fractured aquifers. Results of numerical analysis conclude that dual continuum model that combines matrix and fracture systems through a flow exchange term has outperformed equivalent continuum model in reproducing tracer concentrations at the monitoring wells (evident by a decrease in RMSE from 199 to 65 mg/L). A sensitivity analysis on model simulations conclude that spatial variability in hydraulic conductivity, local-scale dispersion, and flow exchange at fracture-matrix interface have a profound effect on model simulations.  相似文献   

4.
Groundwater sampling from open boreholes in fractured‐rock aquifers is particularly challenging because of mixing and dilution of fluid within the borehole from multiple fractures. This note presents an alternative to traditional sampling in open boreholes with packer assemblies. The alternative system called ZONFLO (zonal flow) is based on hydraulic control of borehole flow conditions. Fluid from discrete fractures zones are hydraulically isolated allowing for the collection of representative samples. In rough‐faced open boreholes and formations with less competent rock, hydraulic containment may offer an attractive alternative to physical containment with packers. Preliminary test results indicate a discrete zone can be effectively hydraulically isolated from other zones within a borehole for the purpose of groundwater sampling using this new method.  相似文献   

5.
Biostimulation is increasingly used to accelerate microbial remediation of recalcitrant groundwater contaminants. Effective application of biostimulation requires successful emplacement of amendment in the contaminant target zone. Verification of remediation performance requires postemplacement assessment and contaminant monitoring. Sampling‐based approaches are expensive and provide low‐density spatial and temporal information. Time‐lapse electrical resistivity tomography (ERT) is an effective geophysical method for determining temporal changes in subsurface electrical conductivity. Because remedial amendments and biostimulation‐related biogeochemical processes often change subsurface electrical conductivity, ERT can complement and enhance sampling‐based approaches for assessing emplacement and monitoring biostimulation‐based remediation. Field studies demonstrating the ability of time‐lapse ERT to monitor amendment emplacement and behavior were performed during a biostimulation remediation effort conducted at the Department of Defense Reutilization and Marketing Office (DRMO) Yard, in Brandywine, Maryland, United States. Geochemical fluid sampling was used to calibrate a petrophysical relation in order to predict groundwater indicators of amendment distribution. The petrophysical relations were field validated by comparing predictions to sequestered fluid sample results, thus demonstrating the potential of electrical geophysics for quantitative assessment of amendment‐related geochemical properties. Crosshole radar zero‐offset profile and borehole geophysical logging were also performed to augment the data set and validate interpretation. In addition to delineating amendment transport in the first 10 months after emplacement, the time‐lapse ERT results show later changes in bulk electrical properties interpreted as mineral precipitation. Results support the use of more cost‐effective surface‐based ERT in conjunction with limited field sampling to improve spatial and temporal monitoring of amendment emplacement and remediation performance.  相似文献   

6.
ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium (Walloon Region of Belgium) to find suitable locations for new water wells in zones with little hydrogeological data. Since boreholes information needed to be representative of the area, large fractured zones were searched for the drillings. Large ERT profiles (320 to 640 m) allowed us to image the resistivity distribution of the first 60 m of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) indexes and sensitivity models were analyzed in order to avoid a misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to complement our electrical results. Some negative anomalies possibly related to preferential flow pathways were detected. A drilling campaign was conducted according to geophysical results. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling program. We noticed that all the wells placed in low resistivity zones associated with SP anomalies provide very high yields and inversely, wells drilled in resistive zones or outside SP anomalies are limited in terms of capacity. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.  相似文献   

7.
A groundwater recharge process of heterogeneous hard rock aquifer in the Moole Hole experimental watershed, south India, is being studied to understand the groundwater flow behaviour. Significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams. In order to localize groundwater recharge zones and to optimize implementation of boreholes, a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone. Magnetic resonance soundings (MRS) have been performed to characterize the aquifer and measure groundwater level depletion. The results of MRS are consistent with the observation in boreholes, but it suffers from degraded lateral resolution. A better resolution of the regolith/bedrock interface is achieved using electrical resistivity tomography (ERT). ERT results are confirmed by resistivity logging in the boreholes. ERT surveys have been carried out twice—before and during the monsoon—across the stream area. The major feature of recharge is revealed below the stream with a decrease by 80% of the calculated resistivity. The time‐lapse ERT also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow. However, in this area time‐lapse ERT does not match with borehole data. Numerical modelling shows that in the presence of a shallow water infiltration, an inversion artefact may take place thus limiting the reliability of time‐lapse ERT. A combination of ERT with MRS provides valuable information on structure and aquifer properties respectively, giving a clue for a conceptual model of the recharge process: infiltration takes place in the conductive fractured‐fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

8.
Forced gradient tracer tests between two boreholes can be used to study contaminant transport processes at the small field scale or investigate the transport properties of an aquifer. Full depth tests, in which tracer samples are collected just from the discharge of the abstraction borehole, often give rise to breakthrough curves with multiple peaks that are usually attributed to different flow paths through the aquifer that can rarely be identified from the test results alone. Tests in selected levels of the aquifer, such as those between packer‐isolated sections of the boreholes, are time consuming, expensive; and the identification of major transport pathways is not guaranteed. We present a method for simultaneously conducting multiple tracer tests covering the full depth of the boreholes, in which tracer sampling and monitoring is carried out by a novel multilevel sampling system allowing high frequency and cumulative sampling options. The method is applied to a tracer test using fluorescein conducted in the multilayered sandstone aquifer beneath the city of Birmingham, UK, producing six well‐defined tracer breakthrough curves.  相似文献   

9.
Fractured rocks have presented formidable challenges for accurately predicting groundwater flow and contaminant transport. This is mainly due to our difficulty in mapping the fracture‐rock matrix system, their hydraulic properties and connectivity at resolutions that are meaningful for groundwater modeling. Over the last several decades, considerable effort has gone into creating maps of subsurface heterogeneity in hydraulic conductivity (K) and specific storage (Ss) of fractured rocks. Developed methods include kriging, stochastic simulation, stochastic inverse modeling, and hydraulic tomography. In this article, I review the evolution of various heterogeneity mapping approaches and contend that hydraulic tomography, a recently developed aquifer characterization technique for unconsolidated deposits, is also a promising approach in yielding robust maps (or tomograms) of K and Ss heterogeneity for fractured rocks. While hydraulic tomography has recently been shown to be a robust technique, the resolution of the K and Ss tomograms mainly depends on the density of pumping and monitoring locations and the quality of data. The resolution will be improved through the development of new devices for higher density monitoring of pressure responses at discrete intervals in boreholes and potentially through the integration of other data from single‐hole tests, borehole flowmeter profiling, and tracer tests. Other data from temperature and geophysical surveys as well as geological investigations may improve the accuracy of the maps, but more research is needed. Technological advances will undoubtedly lead to more accurate maps. However, more effort should go into evaluating these maps so that one can gain more confidence in their reliability.  相似文献   

10.
The effect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of Haut Glacier d'Arolla, Switzerland, was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters. Electrical conductivity profiles were also measured within borehole water columns to identify the water sources driving water level changes, and to determine patterns of water circulation in boreholes. Prior to channel formation, boreholes showed idiosyncratic and poorly coordinated behaviour. Diurnal water level fluctuations were small and driven by supraglacial/englacial water inputs, even when boreholes were connected to a subglacial drainage system. This system appeared to consist of hydraulically impermeable patches interspersed with storage spaces, and transmitted a very low water flux. Drainage reorganization, which occurred around 31 July, 1993, in response to rapidly rising meltwater and rainfall inputs, seems to have involved the creation of a connection between an incipient channel and a well-established channelized system located further down-glacier. Once a major channel existed within the area of the borehole array, borehole water level fluctuations were forced by discharge-related changes in channel water pressure, although a diversity of responses was observed. These included (i) synchronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating inverse/lagged responses. Synchronous responses occurred in boreholes connected directly to the channel, while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system. Pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed, producing inverse and alternating patterns of water level response. © 1998 John Wiley & Sons, Ltd.  相似文献   

11.
Hydraulic tomography (HT) is a method for resolving the spatial distribution of hydraulic parameters to some extent, but many details important for solute transport usually remain unresolved. We present a methodology to improve solute transport predictions by combining data from HT with the breakthrough curve (BTC) of a single forced‐gradient tracer test. We estimated the three dimensional (3D) hydraulic‐conductivity field in an alluvial aquifer by inverting tomographic pumping tests performed at the Hydrogeological Research Site Lauswiesen close to Tübingen, Germany, using a regularized pilot‐point method. We compared the estimated parameter field to available profiles of hydraulic‐conductivity variations from direct‐push injection logging (DPIL), and validated the hydraulic‐conductivity field with hydraulic‐head measurements of tests not used in the inversion. After validation, spatially uniform parameters for dual‐domain transport were estimated by fitting tracer data collected during a forced‐gradient tracer test. The dual‐domain assumption was used to parameterize effects of the unresolved heterogeneity of the aquifer and deemed necessary to fit the shape of the BTC using reasonable parameter values. The estimated hydraulic‐conductivity field and transport parameters were subsequently used to successfully predict a second independent tracer test. Our work provides an efficient and practical approach to predict solute transport in heterogeneous aquifers without performing elaborate field tracer tests with a tomographic layout.  相似文献   

12.
Techniques for characterizing the hydraulic properties and groundwater flow processes of aquifers are essential to design hydrogeologic conceptual models. In this study, rapid time series temperature profiles within open‐groundwater wells in fractured rock were measured using fiber optic distributed temperature sensing (FO‐DTS). To identify zones of active groundwater flow, two continuous electrical heating cables were installed alongside a FO‐DTS cable to heat the column of water within the well and to create a temperature difference between the ambient temperature of the groundwater in the aquifer and that within the well. Additional tests were performed to examine the effects of pumping on hydraulic fracture interconnectivity around the well and to identify zones of increased groundwater flow. High‐ and low‐resolution FO‐DTS cable configurations were examined to test the sensitivities of the technique and compared with downhole video footage and geophysical logging to confirm the zones of active groundwater flow. Two examples are presented to demonstrate the usefulness of this new technique for rapid characterization of fracture zones in open boreholes. The combination of the FO‐DTS and heating cable has excellent scope as a rapid appraisal tool for borehole construction design and improving hydrogeologic conceptual models.  相似文献   

13.
The assessment of hydraulic conductivity of heterogeneous aquifers is a difficult task using traditional hydrogeological methods (e.g., steady state or transient pumping tests) due to their low spatial resolution. Geophysical measurements performed at the ground surface and in boreholes provide additional information for increasing the resolution and accuracy of the inverted hydraulic conductivity field. We used a stochastic joint inversion of Direct Current (DC) resistivity and self-potential (SP) data plus in situ measurement of the salinity in a downstream well during a synthetic salt tracer experiment to reconstruct the hydraulic conductivity field between two wells. The pilot point parameterization was used to avoid over-parameterization of the inverse problem. Bounds on the model parameters were used to promote a consistent Markov chain Monte Carlo sampling of the model parameters. To evaluate the effectiveness of the joint inversion process, we compared eight cases in which the geophysical data are coupled or not to the in situ sampling of the salinity to map the hydraulic conductivity. We first tested the effectiveness of the inversion of each type of data alone (concentration sampling, self-potential, and DC resistivity), and then we combined the data two by two. We finally combined all the data together to show the value of each type of geophysical data in the joint inversion process because of their different sensitivity map. We also investigated a case in which the data were contaminated with noise and the variogram unknown and inverted stochastically. The results of the inversion revealed that incorporating the self-potential data improves the estimate of hydraulic conductivity field especially when the self-potential data were combined to the salt concentration measurement in the second well or to the time-lapse cross-well electrical resistivity data. Various tests were also performed to quantify the uncertainty in the inverted hydraulic conductivity field.  相似文献   

14.
Heterogeneity of hydraulic properties in aquifers may lead to contaminants residing in lower‐permeability zones where it is difficult to deliver remediation amendments using conventional injection processes. The focus of this study is to examine use of a shear‐thinning fluid (STF) to improve the uniformity of remedial amendment distribution within a heterogeneous aquifer. Previous studies have demonstrated the significant potential of STFs for improving remedial amendment delivery in heterogeneous aquifers, but quantitative evaluation of these improvements from field applications is lacking. A field‐scale test was conducted that compares data from successive injection of a tracer in water followed by injection of a tracer in an STF to evaluate the impact of the STF on tracer distribution uniformity in the presence of permeability contrasts within the targeted injection zone. Data from tracer breakthrough at multiple depth‐discrete monitoring intervals and electrical resistivity tomography (ERT) showed that inclusion of STF in the injection solution improved the distribution of the injected fluid within the targeted treatment zone. One improvement was a reduction in the movement of injected fluids through high‐permeability pathways, as evidenced by slower breakthrough of tracer at monitoring locations where breakthrough in baseline tracer‐only injection data was faster. In addition, STF‐amended injection solutions arrived faster and to a greater extent in monitoring locations within low‐permeability zones. ERT data showed that the STF injection covered a higher percentage of a two‐dimensional cross section within the injection interval between the injection well and a monitoring well about 3 m away.  相似文献   

15.
A new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.  相似文献   

16.
Identifying flows into, out of, and across boreholes is important for characterizing aquifers, determining the depth at which water enters boreholes, and determining the locations and rates of outflow. This study demonstrates how Single Borehole Dilution Tests (SBDTs) carried out under natural head conditions provide a simple and cheap method of identifying vertical flow within boreholes and determining the location of in‐flowing, out‐flowing, and cross‐flowing fractures. Computer simulations were used to investigate the patterns in tracer profiles that arise from different combinations of flows. Field tracer tests were carried out using emplacements of a saline tracer throughout the saturated length of boreholes and also point emplacements at specific horizons. Results demonstrated that SBDTs can be used to identify flowing fractures at the top and bottom of sections of vertical flow, where there is a change in vertical flow rate within a borehole, and also where there are consistent decreases in tracer concentration at a particular depth. The technique enables identification of fractures that might be undetected by temperature and electrical conductance logging, and is a simple field test that can be carried out without pumping the borehole.  相似文献   

17.
Biased monitoring of fresh water-salt water mixing zone in coastal aquifers   总被引：2，自引：0，他引：2  
In coastal aquifers, significant vertical hydraulic gradients are formed where fresh water and underlying salt water discharge together upward to the seafloor. Monitoring boreholes may act as "short circuits" along these vertical gradients, connecting between the higher and the lower hydraulic head zones. When a sea tide is introduced, the fluctuations of both the water table and the depth of the mixing zone are also biased due to this effect. This problem is intensified in places of long-screen monitoring boreholes, which are common in many places in the world. For example, all approximately 500 boreholes of the fresh water-salt water mixing zone in the coastal aquifer of Israel are installed with 10 to 50 m long screens. We present field measurements of these fluctuations, along with a three-dimensional numerical model. We find that the in-well fluctuation magnitude of the mixing zone is an order of magnitude larger than that in the porous media of the actual aquifer. The primary parameters that affect the magnitude of this bias are the anisotropy of the aquifer conductivity and the borehole hydraulic parameters. With no sea tide, borehole interference is higher for the anisotropic case because the vertical hydraulic gradients are high. When tides are introduced, the amplitude of the mixing zone fluctuation is higher for the isotropic case because the overall effective hydraulic conductivity is greater than the conductivity in the anisotropic case. In the aquifer, the fresh water-salt water mixing zone fluctuations are dampened, and tens of meters inland from the shoreline, the fluctuations are on the order of few centimeters.  相似文献   

18.
Merokarst aquifers — relatively thin (<1–2 m) karstified carbonate units interbedded between mudstone, shale, or sandstone — constitute a significant proportion of carbonate terrain and underlie a large portion of the west- and south-central USA, yet few advances have been made in our understanding of porosity development and flow-path generation in these complex systems in decades. Toward this end, we used a multi-geophysical approach at the well-studied Konza Prairie Biological Station (KPBS), a part of the larger Flint Hills (25,734 km2), underlain by thin limestone units (1–2 m thick) interbedded with mudstone/shale units (2–4 m thick), to elucidate hydrologic connectivity and potential controls on known groundwater flow directions. We combined electrical resistivity tomography (ERT), surface and borehole nuclear magnetic resonance (NMR), and ground penetrating radar (GPR) measurements across a low order catchment where over 25 boreholes and groundwater wells sampling perched aquifers could be used to constrain interpretation of lithology, potential flow paths, and permeability. Data revealed that groundwater export may be an unappreciated component of lateral-flow-dominated models used to represent merokarst in that: (a) potentiometric surfaces from two limestone units showed groundwater flows toward a hydrologic depression, opposite the direction of stream flow, in the upstream portion of the catchment, (b) long term measures of groundwater levels revealed a greater variance and overall water storage in this same upstream area compared to wells near the outlet, and (c) ERT and NMR results indicate pronounced lateral heterogeneity within a given unit, suggestive of a greater degree of vertical hydrological connectivity than usually considered for horizontally-layered merokarst. Our data suggest vertical connectivity can shunt water to depth in these “sandwiched” merokarst aquifers, yielding atypical groundwater flow directions and unrealized deep export of weathering solutes and carbon.  相似文献   

19.
A new tracer experiment (referred to as MADE‐5) was conducted at the well‐known Macrodispersion Experiment (MADE) site to investigate the influence of small‐scale mass‐transfer and dispersion processes on well‐to‐well transport. The test was performed under dipole forced‐gradient flow conditions and concentrations were monitored in an extraction well and in two multilevel sampler (MLS) wells located at 6, 1.5, and 3.75 m from the source, respectively. The shape of the breakthrough curve (BTC) measured at the extraction well is strongly asymmetric showing a rapidly arriving peak and an extensive late‐time tail. The BTCs measured at seven different depths in the two MLSs are radically different from one another in terms of shape, arrival times, and magnitude of the concentration peaks. All of these characteristics indicate the presence of a complex network of preferential flow pathways controlling solute transport at the test site. Field‐experimental data were also used to evaluate two transport models: a stochastic advection‐dispersion model (ADM) based on conditional multivariate Gaussian realizations of the hydraulic conductivity field and a dual‐domain single‐rate (DDSR) mass‐transfer model based on a deterministic reconstruction of the aquifer heterogeneity. Unlike the stochastic ADM realizations, the DDSR accurately predicted the magnitude of the concentration peak and its arrival time (within a 1.5% error). For the multilevel BTCs between the injection and extraction wells, neither model reproduced the observed values, indicating that a high‐resolution characterization of the aquifer heterogeneity at the subdecimeter scale would be needed to fully capture 3D transport details.  相似文献   

20.
We consider the effect of randomly heterogeneous hydraulic conductivity on the spatial location of time-related capture zones (isochrones) for a non-reactive tracer in the steady-state radial flow field due to a pumping well in a confined aquifer. A Monte Carlo (MC) procedure is used in conjunction with FFT-based spectral methods. The log hydraulic conductivity field is assumed to be Gaussian and stationary, with isotropic exponential correlation. Various degrees of domain heterogeneity are considered and stability and accuracy of the MC procedure is examined. The location of an isochrone becomes uncertain due to heterogeneity, and it is strongly influenced by hydraulic conductivity variance. The probability that a particle released at a point in the aquifer is pumped by the well within a given time is identified. We propose a new expression for the probabilistic spatial distribution of isochrones, which is formally similar to the analytical solution for a uniform medium and takes into account the effects of heterogeneity.  相似文献   

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