			Home About us Contact

Matrix Diffusion (matrix + diffusion)

Distribution by Scientific Domains

Engineering	75%

Selected Abstracts

Retardation of Organic Contaminants in Natural Fractures in Chalk

GROUND WATER, Issue 3 2002
A. Wefer-Roehl
Transport of a conservative compound and two sorbing compounds through fractured chalk was studied using flow-through columns consisting of chalk cores with a single subvertical fracture. Two types of chalk matrix were compared, an oxidized white chalk with low organic carbon content (0.2%), and a gray chalk with a higher organic carbon content (1.3%). Initial rapid breakthrough followed by a delayed approach to a relative concentration of unity for the conservative compound (2,6,difluorobenzoic acid [DFBA]) was clear evidence for diffusion into the porous chalk matrix. Matrix diffusion of DFBA was apparently much greater in the gray chalk columns than in the white chalk columns. Breakthrough curves (BTCs) of the sorbing compounds (2,4,6,tri-bromophenol [TBP] and ametryn [AME]) were retarded in all cases as compared to the conservative compound. Sorption retardation was far greater in the gray chalk as compared with the white chalk, in good agreement with results from batch sorption experiments. BTCs for the conservative compound were relatively nonhysteretic for both white and gray chalk columns. In contrast, BTCs for the sorbing compounds were hysteretic in all cases, demonstrating that sorption was not at equilibrium before desorp-tion began. These experiments suggest that on a field scale, transport of contaminants through fractures in chalk and other fractured porous media will be attenuated by diffusion and sorption into the matrix. [source]

Analysis of a Vertical Dipole Tracer Test in Highly Fractured Rock

GROUND WATER, Issue 5 2002
William E. Sanford
The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 ,m) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (,50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters. [source]

Simultaneous analyses and applications of multiple fluorobenzoate and halide tracers in hydrologic studies

HYDROLOGICAL PROCESSES, Issue 14 2005
Qinhong Hu
Abstract An analytical method that employs ion chromatography has been developed to exploit the use of fluorobenzoic acids (FBAs) and halides more fully as hydrologic tracers. In a single run, this reliable, sensitive, and robust method can simultaneously separate and quantify halides (fluoride, chloride, bromide, and iodide) and up to seven FBAs from other common groundwater constituents (e.g. nitrate and sulphate). The usefulness of this analytical method is demonstrated in both field and laboratory tracer experiments. The field study examines the hydrologic response of fractures and the matrix to different flow rates and the contribution of matrix diffusion in chemical transport. Laboratory tracer experiments with eight geologic media from across the USA,mostly from Department of Energy facilities where groundwater contamination is prevalent and where subsurface characterization employing tracers has been ongoing or is in need,reveal several insights about tracer transport behaviour: (1) bromide and FBAs are not always transported conservatively; (2) the delayed transport of these anionic tracers is likely related to geologic media characteristics, such as organic matter, pH, iron oxide content, and clay mineralogy; (3) use of iodine as a hydrologic tracer should take into account the different sorption behaviours of iodide and iodate and the possible conversion of iodine's initial chemical form; (4) the transport behaviour of potential FBA and halide tracers under relevant geochemical conditions should be evaluated before beginning ambitious, large-scale field tracer experiments. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Closing the mass balance at chlorinated solvent sites: Sources and attenuation processes

REMEDIATION, Issue 2 2010
Roopa Kamath
Using detailed mass balance and simple analytical models, a spreadsheet-based application (BioBalance) was developed to equip decision makers with a predictive tool that can provide a semiquantitative projection of source-zone concentrations and provide insight into the long-term behavior of the associated chlorinated solvent plume. The various models were linked in a toolkit in order to predict the composite impacts of alternative source-zone remediation technologies and downgradient attenuation processes. Key outputs of BioBalance include estimates of maximum plume size, the time frame for plume stabilization, and an assessment of the sustainability of anaerobic natural attenuation processes. The toolkit also provides spatial and temporal projections of integrated contaminant flux and plume centerline concentrations. Results from model runs of the toolkit indicate that, for sites trying to meet traditional, "final" remedial objectives (e.g., two to three orders of magnitude reduction in concentration with restoration to potable limits), "dispersive" mechanisms (e.g., heterogeneous flow and matrix diffusion) can extend remedial time frames and limit the benefits of source remediation in reducing plume sizes. In these cases, the removal of source mass does not result in a corresponding reduction in the time frame for source remediation or plume stabilization. However, this should not discourage practitioners from implementing source-depletion technologies, since results from the toolkit demonstrate a variety of measurable benefits of source remediation. Model runs suggest that alternative, "intermediate" performance metrics can improve and clarify source remediation objectives and better monitor and evaluate effectiveness. Suggested intermediate performance metrics include reduction in overall concentrations or mass within the plume, reduction of flux moving within a plume, and reduction in the potential for risk to a receptor or migration of a target concentration of contaminant beyond a site boundary. This article describes the development of two key modules of the toolkit as well as illustrates the value of using intermediate performance metrics to evaluate the performance of a source-remediation technology. © 2010 Wiley Periodicals, Inc. [source]