Groundwater Contamination (1984) / Chapter Skim
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2. Movement of Contaminants in Groundwater: Groundwater Transport -- Advection and Dispersion
2. Movement of Contaminants in Groundwater: Groundwater Transport -- Advection and Dispersion
Pages 37-45
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From page 37... ...
For example, if a spot of dye is injected into porous material through which groundwater is flowing, the spot will enlarge in size as it moves downgradient. More specifically, in a three-dimensional Cartesian coordinate system where the average groundwater velocity is parallel to the x axis, a sphere of dye moving horizontally along the x axis will undergo longitudinal spreading or dispersion parallel to the x axis and transverse dispersion parallel to the y and ~ axes.
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From page 38... ...
, and serious efforts at applying modified forms of this theory to field studies involving the transport of contaminants in groundwater have been under way since the early 1970s. However, there is still considerable uncertainty concerning methods for quantifying dispersion and for measuring dispersion in the field.
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From page 39... ...
After the initial development period, that is when dispersion has become Fickian, the concentration distribution and concentration-time profiles should behave according to particular solutions of Eq.
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From page 40... ...
DEFINITION OF DISPERSIVITY Contaminant transport models usually have been applied to existing waste-disposal sites where a contaminant plume had been identified during a field monitoring program. The standard modeling procedure has been to adjust values of dispersivity until the model correctly reproduces the observed concentration distribution.
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From page 41... ...
derived a variety of expressions for evaluating the asymptotic longitudinal and transverse dispersivities. In their analyses, the asymptotic dispersivity is expressed in terms of various statistical properties of the hydraulic conductivity distribution.
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From page 42... ...
They conclude that, "A better mathematical formulation of the transport process in porous and fractured media, valid for all time, seems necessary." More specifically, according to Gillham and Cherry (1982~: "The present challenge is to develop a physically based transport model that incorporates spatially and/or temporally variable dispersion parameters that can be determined in a practical manner and with an acceptable degree of certainty." SUMMARY AND DISCUSSION The discussions of dispersion in the preceding sections focused on applications to continuous porous media; dispersion in fractured porous media will be considered in a later section of this chapter. The state of the art for quantifying dispersion in continuous porous media is summarized below.
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From page 43... ...
concluded that "longitudinal dispersion can be represented asymptotically by a Fickian equation, with dispersivity much larger than porescale dispersivity," but given the uncertainties involved in defining the hydrogeologic system a stochastic approach is necessary and "the traditional approach of predicting solute concentrations by solving deterministic partial differential equations is highly questionable in the case of heterogeneous formations." The conclusion drawn from these studies is that although there has been considerable progress within the past 5 years in understanding the nature of macroscopic dispersion in porous media, to date, a credible, practical, and reliable model for analyzing contaminant transport near the source has not been identified. However, theoretical studies by several researchers suggest that for large times (or large travel distances)
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From page 44... ...
(1981) derived an analytical solution of the advection-dispersion equation for one-dimensional contaminant transport with longitudinal dispersion in the fracture, coupled to a solution of a model representing diffusion of solute frown the fracture into the rock matrix.
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From page 45... ...
. Contaminant transport in fractured porous media: Analytical solutions for a system of parallel fractures, Water Resour.
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