Bulletin of the Geological Society of Malaysia, 2008
Geophysical surveys in particular ground penetrating radar (GPR), electrical resistivity tomograp... more Geophysical surveys in particular ground penetrating radar (GPR), electrical resistivity tomography (ERT) and vertical resistivity probe (VRP) were used in mapping subsurface geological structures and groundwater contaminants at Sungai Kandis, Klang to identify the approximate boundaries of contaminant plumes and to provide stratigraphic information at this site. The study area was formerly an illegal dumping site of hydrocarbon and toxic waste. A good correlation exists between GPR signatures, ERT layers, vertical resistivity probe and the contaminated soil. The presence of contaminant plumes as well as the water table are also observed in the GPR and ERT sections at depths approximately of 0.5 to 1 m. In this study, a total of 16 GPR traverses and 10 ERT lines with lengths from 30 to 100 m were established. VRP measurements were conducted in 14 shallow boreholes with a maximum depth of about 1m. The VRP results show high apparent resistivity values ranging from 200 to 10000 Ωm associated with oil contaminated layer. The presence of this layer was also detected in the 2D resistivity sections as a thin band of high resistivity values ranging from 60 to 200 Ωm. In the GPR section, the oil contaminated layer exhibits discontinuous, subparallel and chaotic high amplitude reflection patterns.
Experimental evidences on the scaling behavior of a sandy porous media
Various authors treated the scaling of the hydrodispersive parameters in porous media. Neverthele... more Various authors treated the scaling of the hydrodispersive parameters in porous media. Nevertheless several studies and reports finding in literature on this matter, specifically on the dispersivity increases with scale of measurement, are based on a statistical or experimental approach (Neuman, 1990; Wheatcraft & Tylor, 1988; Clauser, 1992; Schulze-Makuch, 2005). Following this last approach, we analysed the scale behaviour of a sandy porous media, the grain size of which was characterized carefully in laboratory. For this aim we carried out several tracer tests on three cylindrical samples of the considered sandy soil. The diameter of these samples was the same, equal to 0.0635 m, and the lengths respectively equal to 0.15 m, 0.30 m and 0.60 m. At the bottom and the top of the cylindrical sample two membrane were located to allow the water flow. The water arrived in the sample from the bottom by a plastic tube and came out from the top to exclude the presence of air in the soil sa...
The importance of estimating spatially variable aquifer parameters such as transmissivity is wide... more The importance of estimating spatially variable aquifer parameters such as transmissivity is widely recognized for studies in resource evaluation and contaminant transport. A useful approach for mapping such parameters is inverse modeling of data from series of pumping tests, that is, via hydraulic tomography. This inversion of field hydraulic tomographic data requires development of numerical forward models that can accurately represent test conditions while maintaining computational efficiency. One issue this presents is specification of boundary and initial conditions, whose location, type, and value may be poorly constrained. To circumvent this issue when modeling unconfined steady-state pumping tests, we present a strategy that analyzes field data using a potential difference method and that uses dipole pumping tests as the aquifer stimulation. By using our potential difference approach, which is similar to modeling drawdown in confined settings, we remove the need for specifying poorly known boundary condition values and natural source/sink terms within the problem domain. Dipole pumping tests are complementary to this strategy in that they can be more realistically modeled than single-well tests due to their conservative nature, quick achievement of steady state, and the insensitivity of near-field response to far-field boundary conditions. After developing the mathematical theory, our approach is first validated through a synthetic example. We then apply our method to the inversion of data from a field campaign at the Boise Hydrogeophysical Research Site. Results from inversion of nine pumping tests show expected geologic features, and uncertainty bounds indicate that hydraulic conductivity is well constrained within the central site area.
The study of electrical signals associated with groundwater flow is a powerful method to determin... more The study of electrical signals associated with groundwater flow is a powerful method to determine, in a non-invasive way, the distribution of hydraulic heads in aquifers, hence the distribution of the hydraulic conductivity and storativity. Experimental hydraulic heads and electrical (self-potential, SP) signals associated with a pumping test in an unconfined aquifer were reproduced with a numerical model based on the finite-difference method. For simplicity, we assumed axial symmetry around the pumping well. We assessed the equivalent hydraulic conductivity and specific storage of the aquifer by fitting the piezometric levels obtained in the course of the pumping against synthetic hydraulic heads produced by the model. The current coupling coefficient is obtained from the best fit between the experimental and modelled SP signals at the end of the pumping phase. Then, keeping the previously determined hydraulic parameters, the relaxation of both the hydraulic heads and the SP signals were modelled and found to be consistent with the measured values. The data show almost linear relationships between the hydraulic heads in the aquifer and the electrical signals recorded at the ground surface. These results show that SP signals allow monitoring subsurface flow in the course of pumping experiments.
The importance of estimating spatially variable aquifer parameters such as transmissivity is wide... more The importance of estimating spatially variable aquifer parameters such as transmissivity is widely recognized for studies in resource evaluation and contaminant transport. A useful approach for mapping such parameters is inverse modeling of data from series of pumping tests, that is, via hydraulic tomography. This inversion of field hydraulic tomographic data requires development of numerical forward models that can accurately represent test conditions while maintaining computational efficiency. One issue this presents is specification of boundary and initial conditions, whose location, type, and value may be poorly constrained. To circumvent this issue when modeling unconfined steady-state pumping tests, we present a strategy that analyzes field data using a potential difference method and that uses dipole pumping tests as the aquifer stimulation. By using our potential difference approach, which is similar to modeling drawdown in confined settings, we remove the need for specifying poorly known boundary condition values and natural source/sink terms within the problem domain. Dipole pumping tests are complementary to this strategy in that they can be more realistically modeled than single-well tests due to their conservative nature, quick achievement of steady state, and the insensitivity of near-field response to far-field boundary conditions. After developing the mathematical theory, our approach is first validated through a synthetic example. We then apply our method to the inversion of data from a field campaign at the Boise Hydrogeophysical Research Site. Results from inversion of nine pumping tests show expected geologic features, and uncertainty bounds indicate that hydraulic conductivity is well constrained within the central site area.
In the framework of various studies to characterize the aquifer at the groundwater experimental f... more In the framework of various studies to characterize the aquifer at the groundwater experimental field near Montalto Uffugo, Italy, the present work estimates the spatial distribution of hydraulic conductivity of the unconsolidated deposits that underlie the area, by applying the geostatistical technique of kriging with external drift to electrical-resistivity and hydraulicconductivity data. The reliability of the estimation method was tested by implementing a model, based on the method of cells, that simulates groundwater flow, with the estimated values of hydraulic conductivity. The results obtained indicate that the estimation method used has an acceptable degree of reliability. Résumé Dans le cadre de plusieurs études réalisées pour caractériser l'aquifère sur le site expérimental près de Montalto Uffugo (Italie), ce travail analyse la distribution spatiale de la conductivité hydraulique de formations non consolidées présentes dans cette région. La technique géostatistique du krigeage a été appliquée, avec une dérive externe, aux données de résistivité électrique et de conductivité hydraulique. La fiabilité de la méthode d'estimation a été testée en établissant un modèle, basé sur la méthode des cellules, qui simule l'écoulement souterrain, à partir des valeurs estimées de la conductivité hydraulique. Les résultats obtenus indiquent que cette méthode d'estimation présente un degré de fiabilité acceptable. Resumen Dentro del marco de los estudios destinados a la caracterización del acuífero del campo experimental de aguas subterráneas cercano a Montalto Uffugo (Italia), el presente trabajo estima la distribución espacial de la conductividad hidráulica de los depósitos no consolidados subyacentes. El estudio se ha llevado a cabo mediante la técnica geoestadística del krigeado con deriva externa de los datos de resistividad eléctrica y de conductividad hidráulica. Se ha verificado la fiabilidad de la estimación mediante un modelo de simulación del flujo de aguas subterráneas, basado en el método de las celdas, que utiliza los valores estimados de la conductividad hidráulica. Los resultados obtenidos indican que el método de estimación empleado posee un grado de fiabilidad aceptable. Key words hydraulic properties 7 heterogeneity 7 geophysical methods 7 geostatistics 7 Italy
Dispersion is a key process controlling transport of solutes and gases in porous media. Performin... more Dispersion is a key process controlling transport of solutes and gases in porous media. Performing tracer tests for dispersion measurements with solutes is generally much more time consuming compared to used gases. The goal of this study is to estimate the solute dispersion based on gas dispersion measurements to reduce time consumption. Solute (chloride ion) breakthrough curves were measured at different fluid velocities, covering the identical Reynolds range used in a previous study on gas dispersion. Commercial gravel, consisting of solid particles, has been used as a porous medium. Chloride and oxygen dispersion coefficients were determined based on the measured breakthrough data and in turn used to calculate solute and gas dispersivities as a function of the particle size (D m) and particle size range (R). Solute dispersivity increased with increasing particle size range, in agreement with the previous study. Data showed that dispersivity increases more gradually with decreasing mean diameter.
Ground water flow associated with pumping and injection tests generates self-potential signals th... more Ground water flow associated with pumping and injection tests generates self-potential signals that can be measured at the ground surface and used to estimate the pattern of ground water flow at depth. We propose an inversion of the self-potential signals that accounts for the heterogeneous nature of the aquifer and a relationship between the electrical resistivity and the streaming current coupling coefficient. We recast the inversion of the self-potential data into a Bayesian framework. Synthetic tests are performed showing the advantage in using self-potential signals in addition to in situ measurements of the potentiometric levels to reconstruct the shape of the water table. This methodology is applied to a new data set from a series of coordinated hydraulic tomography, self-potential, and electrical resistivity tomography experiments performed at the Boise Hydrogeophysical Research Site, Idaho. In particular, we examine one of the dipole hydraulic tests and its reciprocal to sh...
The goal of this work is to de ne a set of guidelines for \validating" mathematical models that a... more The goal of this work is to de ne a set of guidelines for \validating" mathematical models that are to be used in real-world problems of subsurface ow and contaminant transport. Groundwater models today are being used to study increasingly complex problems involving large temporal and spatial scales and with many uncertainties inherent in the data and in the models themselves. It becomes important therefore to establish procedures and benchmark tests with which to assess a given model's adequacy for simulating a speci c groundwater problem. In the GULLIVER project we propose to create a public library of phenomenological schemes (PS), each one satisfactorily solved by an established model. A user with a speci c application and model at hand can select the PS that most closely corresponds to his application, and corroborate his model against the established solution. The user's model is considered validated when the two numerical solutions match according to speci ed criteria. In this paper we describe this methodology and present its application to a 2D saturated ow and transport test case based on data from a eld study site in Calabria, Italy.
Study of Hydrocarbon Saturation by Geophysical Methods
The contamination of soil and groundwater by LNAPLs is a problem that has been treated over sever... more The contamination of soil and groundwater by LNAPLs is a problem that has been treated over several years, because, they are used in the industrial worldwide. Its location and behavior in the subsurface has been monitored through invasive methods that disturb the environment and they are costly. In this sense, we integrated a noninvasive hydrogeophysical method as Ground Penetrating Radar (GPR) with chemical analysis (Infrared Spectrophotometry) to localize contaminant distribution and quantify TPH concentrations . For this purpose, we made a 2D hydrogeophysical experiment in a sand box saturated by diesel in which we validated this methodology.
Relating solute and Gas Dispersion in Granite at Different Transport Velocities
Poromechanics V, 2013
ABSTRACT Dispersion is a key process controlling transport of solutes and gases in porous media. ... more ABSTRACT Dispersion is a key process controlling transport of solutes and gases in porous media. Performing tracer tests for dispersion measurements with solutes is generally much more time consuming compared to used gases. The goal of this study is to estimate the solute dispersion based on gas dispersion measurements to reduce time consumption. Solute (chloride ion) breakthrough curves were measured at different fluid velocities, covering the identical Reynolds range used in a previous study on gas dispersion. Commercial gravel, consisting of solid particles, has been used as a porous medium. Chloride and oxygen dispersion coefficients were determined based on the measured breakthrough data and in turn used to calculate solute and gas dispersivities as a function of the particle size (Dm) and particle size range (R). Solute dispersivity increased with increasing particle size range, in agreement with the previous study. Data showed that dispersivity increases more gradually with decreasing mean diameter. Keywords: solute dispersion, gas dispersion, dispersivity ratio, porous medium, Reynolds number, particle size range.
The Importance of Reduced-Scale Experiments for the Characterization of Porous Media
The Handbook of Environmental Chemistry, 2013
In order to reduce threats to the quality of groundwater resources, prevention and control are th... more In order to reduce threats to the quality of groundwater resources, prevention and control are the most important activities to carry out. In general, these activities require the ability to model the flow and solute transport phenomena in the aquifer. Thus, it is essential to collect information about potential contamination sources, boundary conditions, hydrological forcing, and the magnitudes and spatial distributions of the hydrodynamic and hydrodispersive parameters of the porous media. Measurements of such parameters, available only at a finite number of locations, are often obtained by means of different techniques and can be representative of various measurement scales.
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