Mass transport is the process responsible for the movement of solutes and suspended biomass in the saturated aquifer. It consists of two major processes: advection and dispersion. Advective transport is a function of the average linear velocity (or pore water velocity):

(2.2)

Where:	V = pore water velocity (L/T)
	q = darcy velocity (L/T)
	n = effective porosity

The pore water velocity is the specific discharge divided by porosity. In turn, the specific discharge is a function of hydraulic gradient and hydraulic conductivity:

(2.3)

Where:	q = specific discharge (L3/L2·T)
	K = hydraulic conductivity (L/T)
	dh = change in hydraulic head (L)
	dl = change in length (L)

Hydraulic conductivity values range from 1.0 to 10^-13 m/s for gravel to unfractured metamorphic rocks respectively (Freeze and Cherry, 1979).

Dispersive transport is a mixing process that is a function of spatial concentration gradients. The dispersion coefficient in Equation (2.1) is a function of two processes: molecular diffusion and mixing due to pore water velocity variations. The equations calculating the longitudinal and transverse dispersion coefficients are:

	(2.4)
	(2.5)

Where:	Dl = dispersion coefficient in principal direction of flow (longitudinal dispersion coefficient) (L2/T)
	Dt = dispersion coefficient perpendicular to direction of flow (transverse dispersion coefficient) (L2/T)
	a l = longitudinal dispersitivity coefficient (L)
	a l = transverse dispersitivity coefficient (L)
	D* = effective diffusion coefficient (L2/T)
	V = pore water velocity (in principle direction) (L/T)

The effective diffusion coefficient, D*, is the molecular diffusion coefficient in water adjusted for porous media effects. The dispersitivity coefficients are a measure of mixing effects caused by heterogeneity in the hydraulic flow field and are typically a function of scale (Fetter, 1993).