H21G-01 INVITED 08:00h
Non-invasive methods to study flow and transport at the soil core and lysimeter scale
Non-invasive methods offer a great potential to study flow and transport processes at the core to the field and regional scale. In this contribution we will focus on the application of selected techniques such as MRI (Magnetic Resonance Imaging), X-Ray-Tomography (X-RT), MERIT (Magnetic Electrical Resistivity Imaging Technique), GPR (Ground Penetrating Radar) and Spectral Induced Polarisation (SIP) at the core to lysimeter scale. MRI is a powerful tool to derive local scale transport parameters. Based on the imaging of the 3-D temporal evolution of the spatial moments of a solute transport in a soil core, the local scale dispersivity of the soil can be derived. We also use MRI to image the root distribution inside a packed soil column. We employ the effect that the transverse relaxation time of water in the porous medium is considerably smaller than in the root tissue of rizinus communis. Different MRI pulse sequences were tested showing that the best contrast is obtainable by the strongly T2* weighted method CISS. X-RT provides information on the structure of the porous media. By parametrizing this structural information we may obtain an improved description of solute transport in undisturbed soil cores. GPR allows to map the spatial and temporal distribution of soil moisture in large undisturbed lysimeters. Combined with outflow data, this provides unique information to evaluate and improve mathematical models. New developments like MERIT are on their way which additionally exploits the magnetic information inherent in Electrical Resistivity Tomography-experiments to improve the spatial distribution of solute concentrations at lysimeter scale. SIP methods may be used to derive local scale pore size distribution and hydraulic conductivity. The single relaxation times, deduced from a measured phase spectrum either via multi-Cole-Cole-fits or as a whole relaxation time distribution, are a function of the relaxation length, which is connected to pore space respectively particle size distribution. From this information hydraulic conductivity may be derived using theoretical considerations.
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H21G-02 INVITED 08:20h
Joint geophysical and hydrological inversion of a mesoscale laboratory experimental dataset
A core goal of hydrogeophysics is mapping of geophysical data into temporal and spatial hydrological information. A fundamental question in this effort is the uncertainty associated with this mapping, and the effect and benefits of integrating point measurements of hydrological information such as moisture content in the geophysical inversion. An infiltration experiment was performed at INEEL to address this question. The goal of this experiment was to construct the general patterns of the wetting front and spatial distribution of the moisture content from the inverted resistivity field. 3D timelapse ERT data and 20 point measurements of moisture change over the experiment as well as the infiltration time history were collected automatically. The point samples of moisture content data were translated into electrical resistivities as prior information for the ERT inversion. This inversed resistivity field was translated into moisture content map used as prior information for hydrological inversion. The 3-D hydrologic inversion model was able to derive 3-D distribution of unsaturated hydraulic conductivity and moisture release curves for water movements. The output of the inverted moisture content map was translated into an electrical resistivity map as prior information for the ERT inversion, and these steps were repeated iteratively until no further improvement on the inversed resistivity field, moisture content distribution and saturated hydraulic conductivity field were obtained. Set up of analysis of this data through a web based interface in which data and models are accessible through a relational database allows for both research reproducibility and "what if" scenarios in which the effect on the end result of removing specific data can be easily investigated and evaluated.
H21G-03 INVITED 08:40h
4D Geoelectrical Tomography for Hydrogeological Applications: Lab-scale and Field Experiments in Southern Apennine Chain (Italy).
Electrical Resistivity and Self-Potential tomographic techniques have been recently employed as low-cost and powerful tools for a wide spectra of geological and environmental applications. In the frame of a national project mainly devoted to apply new and emerging electromagnetic technologies for hydrogeological hazard monitoring, we focused our attention on the 4D geoelectrical tomography. The time-lapse analysis of high-resolution electrical images of near-surface can give a relevant contribute for describing the complex processes related to water movements in the vadose zone. Active (resistivity) and passive (self-potential) geoelectrical measurements have been carried out in both field and lab-scale conditions. The field experiments have been planned for studying the shallow water fluxes in landslide areas located on Southern Apennine chain. The controlled experiments were performed by using a sand box (6.0 x 4.0 x 2.0 meters) which allows to reproduce a medium-large scale natural environment. Electrical resistivity and self-potential data were recorded continuously by using unpolarizable electrodes connected to a multiplexed high impedance and resolution data-logger. Novel tomographic techniques have been applied for obtaining time series of high resolution electrical images during water infiltration experiments under controlled conditions. A full exploitation and analysis of data coming from lab-scale experiments are presented and discussed. Finally, the possible implications of laboratory results with the interpretation of geophysical field experiments in landslide areas are analysed.
H21G-04 09:00h
Sandbox Experiments of Self-Potential Signals Associated With Pumping Tests
The flow of water in a charged porous material is the source of an electrical field called the self-potential. The origin of this coupling is associated with the drag of the excess of charge contained in the vicinity of the pore water interface by the pore fluid flow. Sandbox experiments were undertaken in order to understand the relationship between self-potential data and the shape of the water table during pumping experiments. The experiments were performed in a thin Plexiglas tank filled with a well-calibrated sand. The electrical potential distribution was measured passively at the top of the tank with a set of 27 Pb/PbCl2 "Petiau" electrodes. During the experiment the piezometric level was measured with a set of 6 piezometers. The results of these pumping tests are the measurements of a detectable electrical field produced at the top surface and analyzed with analytical solutions of the coupled hydroelectric problem.
H21G-05 09:15h
Complex Conductivity Measurements of Sand-Clay Mixtures with Varying Grain Size, Clay Content and Solution Chemistry
We have measured the electrical-impedance response (10-3 Hz to 103 Hz) of sand-clay mixtures, with varying grain size, clay content, and pore solution chemistry in an attempt to reproduce and expand upon the classic experiments of Klein and Sill (Geophyscis, 1982). Klien and Sill used a series form of the empirical Cole-Cole model to invert their data for a central relaxation time and a chargeability parameter. A major result of their study was that the relaxation time was generally observed to increase with the size of the sand grains (or glass beads): where, and the relaxation time was also observed to be dependent upon the solution conductivity and the amount of clay in the mixture. These results, however, were somewhat inconsistent with the model of Madden and Marshall (1959) and subsequent models that are based on diffusive polarization mechanisms which theoretically predict a value of a=2. Similar to Klein and Sill we found the relaxation time to increase with the size of the sand grains and to be a function of the solution conductivity and clay content. Using the original form of the Cole-Cole model, however, we obtained a larger value for the power law exponent:, which is more consistent with the predictions of the theoretical models. Lastly, in trying to reproduce Klein and Sill's experiments we found that the method of sample preparation significantly affects the complex conductivity response of the sand-clay mixture. In particular, the distinct relaxation peak observed by Klein and Sill is not reproduced unless the sand-clay mixtures are oven dried and then vacuum saturated. Samples made from "wet" sand-clay slurries do not have distinctive relaxation peaks - the measured phase of the sample does not approach zero degrees even at 10-3 Hz. NMR measurements of the samples indicate that the "wet" sand-clay mixtures have a much broader effective pore size distribution than the oven-dried and vacuum saturated sand-clay mixtures.
H21G-06 09:30h
Spectral Induced Polarization of Saturated and Unsaturated Triassic Sandstone
Over the past decade there has been a significant increase in interest in the use of induced polarization (IP) for hydrogeological investigations. This has been driven by the realization that the IP response is potentially linked to hydraulic properties of a porous medium, since lithology, together with chemical and physical characteristics of the pore fluid interface control the IP processes. Whilst attempts have been made to demonstrate empirical and theoretical relationships between saturated hydraulic conductivity of soils and IP behavior, the focus here is on the link between pore size distribution and the frequency dependence of IP. Such a link may reveal a correlation of parameters describing moisture retention and relaxation of electrical impedance, with obvious potential value for characterization of unsaturated flow parameters. Furthermore, the relationship between spectral IP and moisture content in soils has been largely overlooked and yet may offer useful insight into hydraulic processes operating in the vadose zone. To address these issues, a study of spectral IP in samples of saturated and unsaturated Triassic sandstone has been carried out. Impedance spectra have been analyzed using macroscopic relaxation models and compared with physical measurements of rock samples (pore size distribution, particle size distribution, SEM, hydraulic conductivity). Electrical data were acquired over the frequency range 0.01 Hz to 1000 Hz on plugs drilled horizontally and vertically in samples of core with lithological contrasts. The spectra exhibit Cole-Cole type relaxations with distinct phase angle peaks. The relaxation curves are seen to vary with pore size distribution, in particular the relaxation time constant appears correlated with a measure of characteristic pore throat size. Under varying levels of water saturation, changes in relaxation are observed. Most notable is the decrease in relaxation time constant with reducing levels of saturation, caused by the variation in dominant wetted pore throat size under different degrees of saturation. Furthermore, coarse-grained samples reveal much steeper changes in time constant with saturation than fine-grained samples. The results add further evidence of useful links between IP measurements and key hydraulic properties, the value and limitations of which are discussed.
H21G-07 09:45h
Effects of Precipitation and Particle Size on Low Frequency Electrical Properties of Zero Valent Iron
Observational methods are required to monitor the long-term efficiency of permeable reactive barrier (PRB) installations used to remediate hydrocarbon and heavy metal contaminated groundwater. Our previous studies investigated the relationship between induced polarization (IP) and zero valent iron (Fe0) surface area and electrolyte activity. In this continuous research, we performed experiments on mixtures of Fe0 and Ottawa sand to study (1) the relationship between IP and zero valent iron particle size, and (2) the impact of precipitation induced on the Fe0 surface. The latter experiment addresses the issue of whether reduction in PRB performance can be inferred from electrical measurements. The effect of iron particle size on IP was studied by running background solutions (0.01 C 0.1M NaCl) through samples of the mixture of sand with 5% of different sizes of iron (diameter 1-0.2 mm). Precipitation experiments were conducted by running samples with different solutions to induce precipitation on the iron surface. This involved: (1) long term execution (about 1 year) of four identical samples (5% Fe0 mixture with sand) continuously flushed with four different solutions (0.1 M NaHCO3, 0.1 M Na2SO4, 0.1 M Na2HPO4 and pure water); (2) a short term experiment (in days) with high percentage of Fe0 (30%, 70%) run with high highly basic solutions (0.1M NaOH and 0.1 M Na2CO3). The results show that the IP response is correlated with iron particle size: First, the IP magnitude is inversely proportional to iron particle diameter principally due to the inverse relation between particle diameter and specific surface area. Second, the dominant relaxation time of the polarization is inversely proportional to the particle diameter. The short term precipitation experiment revealed two significant observations: (1) the IP magnitude increased after induced precipitation on Fe0 surface which we tentatively attribute to an increase in the surface area due to an irregular coating of metallic precipitants (FeCO3, Fe(OH)2, FeOOH, Fe(OH)3) on the Fe0 (2) a clear increase in the time constant, which we attribute to an increase in the effective particle size due to the precipitation. Our results to date suggest that IP may be a viable method for long-term monitoring of reactive iron barriers.