n =

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1) Forces resisting the motion of fluid in a porous medium are

2) Acceleration of the fluid is small, hence

Darcy's Law in 3D shows that the driving force is a function fo the PRESSURE DISTRIBUTION and the WEIGHT of the fluid (in z direction), not a function of the pressure gradient alone.

In the general case:

1) Porous medium is assumed to be

therefore the specific discharge is assumed to be in the

as the resultant force on a volume of fluid. The specific discharge vector is assumed to be

to the head contours even in heterogenous media (preferential flow; the head contours become quasi-parallel, reflecting head losses due to friction from preferential movement), unless there are other forces at work that Darcy's Law can't account for (e.g. Coriolis effect, if the scale of the problem is large enough for this to have effect)..

In isotropic medium; specific discharge is expressed in terms of the ratio of two quantities;

the intrinsic PERMEABILITY of the medium (m2), and

dynamic VISCOSITY of the fluid (Ns/m2)

Unlike other hydrological components, groundwater data is complicated by the fact that flow is

, the data is

because it's underground,

to collect because it is difficult to reach, and must be considered over

due to the slow speed of infiltration and groundwater movement. Thus, data needs to be collected over longer periods of time, and it can be very difficult to predict what is happening. Additionally, aquifers stretch over

which contributes to data being sparse.

Rate of flow in rocks affected by, 1)

between solid particles; larger means liquid is able to flow with less interference. 2)

; shorter means it will get there faster. 3)

Water in pores further away are less affected by friction than those in contact with the solids.

Fluid particles disperse as they move; occurs in

(in direction of flow) and

directions. Where flow is very slow like in groundwater, lateral dispersion is often the same or similar magnitude as longitudinal dispersion.

Extremely important to identify

because by the time the contaminant is picked up by water sampling it can have spread extremely far. Cumulative effects are often the biggest issue.