In groundwater flow

Molecular diffusion (or self-diffusion) is the process by which molecules show a net migration, most commonly from areas of high to low concentration, as a result of their thermal vibration, or Brownian motion. The majority of reactive transport models are designed to simulate the distribution of reactions in groundwater flows and, as such, the accounting for molecular diffusion is lumped with hydrodynamic dispersion, in the definition of the dispersivity. 

At the same time, reaction modeling is now commonly coupled to the problem of mass transport in groundwater flows, producing a subfield known as reactive transport modeling. Whereas a decade ago such modeling was the domain of specialists, improvements in mathematical formulations and the development of more accessible software codes have thrust it squarely into the mainstream. 

Figure 6.15. Illustration of dispersion in groundwater flow at various scales.

Dispersion Coefficients in Groundwater Flow. In a uniform media of particles, the longitudinal dispersion coefficient, Dl, and the transverse dispersion coefficient, Dt, are both functions of the grain diameter and velocity. (In our previous example, Dl... 

Control volume method Finite element method Boundary element method and analytic element method Designed for conditions with fluxes across interfaces of small, well-mixed elements - primarily used in fluid transport Extrapolates parameters between nodes. Predominant in the analysis of solids, and sometimes used in groundwater flow. Functions with Laplace s equation, which describes highly viscous flow, such as in groundwater, and inviscid flow, which occurs far from boundaries. 

Younger, P. L. 2000. Predicting temporal changes in total iron concentrations in groundwaters flowing from abandoned deep mines a first approximation. Journal of Contaminant Hydrology, 44, 47-69. 

In combination with the flux equations and appropriate equations of state, (5) and (6) constitute the full set of model equations accounting for chemical osmosis in groundwater flow and solute transport. 

LaRoche, J., Nuzzi, R., Waters, R., Wyman, K., Falkowski, P.G, and Wallace, D.W.R. (1997) Brown tide blooms in Long Island s coastal waters linked to interannual variability in groundwater flow. Global Change Biol. 3, 397-410. 

To analyze the transport and retention of chemical contaminants in groundwater flowing through soils, experimental and theoretical studies generated several reliable models. Diverse numerical methods have been applied to solve the governing equations efficiently. Some computer models include the simulation of physical and chemical processes. 

Engesgaard, P., and Kipp, K. L., 1992, A geochemical transport model for redox-controlled movement of mineral fronts in groundwater flow systems A case of nitrate removal by oxidation ofpyrite Water Resources Research, v. 28, p. 2829-2843. 

M3TD Groundwater MT3D is a transport model that simulates advection, dispersion, source/sink mixing, and chemical reactions of contaminants in groundwater flow systems in either two or three dimensions. 

Figure 11.15 Changes in Eh, pH, dissolved oxygen and HS" in the direction of ground-water flow in the Lincolnshire Limestone aquifer, U.K. Distances are measured from the outcrop area, which is oxidizing, to deeper, confined portions of the formation downdip, where the groundwater becomes anaerobic. After D. R. Champ and J. Gulens, Oxidation-reduction sequences in groundwater flow systems. Can. J. Earth Sci. 16 12-23. Copyright 1979 by NRC Research Press. Used by permission.

Chemical reactions can cause porosity and permeability changes, which in turn cause changes in groundwater flow. Similarly, chemical reactions can cause changes of density, which in turn will change the flow field. 

In glaciers and ice sheets or in groundwater flow systems, water may be moved significant distances and elevations from the site where it fell. This is complicated on ice sheets by changes over time in the ice-sheet thickness (Cuffey 2000), and near ice sheets by isostatic effects of the changing ice load. 

Transport in Groundwater Flows and Associated Health Effect.427... 

BRIEF DESCRIPTION OF THE ROLE OF INORGANIC COLLOIDAL FINES-ASSOCIATED CONTAMINANT TRANSPORT IN GROUNDWATER FLOWS AND ASSOCIATED HEALTH EFFECT... 

When the transport problem is multidimensional, even if the flow system remains one-dimensional, a solute plume originating from a point source will disperse both longitudinally and transversely to mean flow direction. Transverse dispersion is caused by the fact that the flow paths can split and branch out to the side to bypass soil grains as a fluid flows through a porous medium. This will occur even in the laminar flow conditions that are prevalent in groundwater flow. 

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