User's guide of Proceessing Modflow 5.0

114 Processing Modflow
negligible compared to the mechanical dispersion (see below) and is only important when
groundwater velocity is very low.
< Longitudinal dispersivity " [L], horizontal transverse dispersivity " [L] and vertical
L TH
transverse dispersivity " [L] describe the spreading of the solute concentration in
TV
groundwater caused by the irregular shape of the interconnected pore space and the velocity
variations at the microscopic level. The velocity of groundwater varies according to the size
of the pores and water moves faster at the internal points between soil grains than on the
solid surface. This spreading is often referred to as mechnical dispersion and it occurs in all
three spatial directions. The coefficient of mechnical dispersion is defined by " @ v , where
i i
" is the dispersivity and v is the average linear velocity in the i direction. The sum of
i i
mechanical dispersion and molecular diffusion is called hydrodynamic dispersion.
R ' 1 % D b
n @ K d
3.6.2 MOC3D
Values of dispersivity used for simulations generally dependent on the scale of a
concentration plume being considered. While a plume “grows”, it will not only undergo the
microscopic mechnical dispersion but also the dispersion caused by macroscopic
heterogeneities. This results to a trend of increasing dispersivity values with the scale of
observation. Summaries of the scale-dependent dispersivity values are provided by Anderson
(1979, 1984) and by Gelhar et al. (1985, 1992).
< Retardation factor [-]: For a linear isotherm the retardation factor R is independent of the
concentration field. R is calculated by
where n is the porosity of the porous medium.
Fig. 3.36 The Dispersion / Chemical Reaction (MOC3D) dialog box
(3.35)