Environmental Engineering Reference
In-Depth Information
19.2.2 Advection
Advection refers to the transport of contaminants by the bulk movement of ground-
water and it often represents the most important process driving the down-gradient
migration of dissolved contaminants in aquifer systems. The motion of ground-
water can be described by the experimentally derived Darcy's law, which, in
three-dimensions, can be expressed as:
K
ij
∂
h
q
i
=−
(19.4)
∂
x
i
where
q
i
[L T
−
1
] is the specific discharge or Darcy velocity (discharge per unit cross
sectional bulk area),
K
ij
[L T
−
1
] is the hydraulic conductivity tensor,
x
i
is the spatial
coordinate and
h
[L] is the hydraulic head. The average linear velocity of the flow
can be defined as the flux of water across the unit cross sectional area of pore space
and is described as the ratio between the specific discharge and the porosity:
q
i
n
v
i
=
(19.5)
The average linear velocity should not be confused with the local (microscopic)
velocity of the fluid or with the average velocity at which water molecules travel
along a flow path, which is greater than the average linear velocity because of the
tortuosity of the porous medium.
Contaminant transport by advection alone is often described as plug flow and
yields a sharp contaminant concentration front advancing along the groundwater
flow direction. This concept is illustrated in Fig.
19.2
where the one-dimensional
advective transport from an instantaneous and a continuous contaminant release is
depicted. After a time
t
1
the contaminant front reaches the distance
x
1
, whereas at a
later time
t
2
the front has moved to a farther distance
x
2
.
Fig. 19.2
Relative concentration as a function of distance in the flow direction, illustrating the
advective movement of a contaminant from an instantaneous (
left
) and a continuous release (
right
).
The position
x
1
is reached at time
t
1
and the position
x
2
is reached at time
t
2
19.2.3 Hydrodynamic Dispersion
Hydrodynamic dispersion is the process whereby a contaminant plume spreads
out from the main direction of groundwater flow and results in dilution with an
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