Environmental Engineering Reference
In-Depth Information
ln 2
t
=
with a half-live of
1
2
. A more complex form of chemical evolution can
λ
also be taken into account. We must then consider the chemical components one by
one and their balance or out-of-balance relationships.
If we extract water by pumping with a rate
Q
water
, we also extract pollutant
according the relationship:
&
S
=−
QC
[13.15]
p
water
Conversely, water table feeding with polluted water at a rate of
Q
water
at
concentration
C
r
gives a storage level of:
&
SQC
=
[13.16]
p
water
r
In highly permeable and very porous soils or rocks (e.g. drinking water
reservoirs) the flows pass through preferred channels, leaving other areas in
relatively immobile conditions. This is particularly the case with fractured rocks.
Two porosities overlap: crack porosity and common intergranular porosity. The
former relates to interstices that are much larger than the latter, giving rise to
preferential flow. This type of flow at different scales is common. Areas with low
porosity, while contributing little to general flow with very low Darcy's velocities,
are communicating with the channels of preferential flow. They are therefore able to
store pollutant through diffusion. By simulating the diffusion through a small block
of rock with low permeability, it appears that this phenomenon may, at first
approximation, be modeled under the form:
&
S
=
α
(C - C
)
[13.17]
immobile
immobile
Globally, storage results of a set of mechanisms including: sorption at the
surface of the solid matrix, diffusion in the immobile water, chemical, biological or
radioactive decrease, etc. Synthetically, we write:
&
(
)
S= fctC
CC
,
,
[13.18]
solid
immobile
If this relationship is linear, we have:
&
S C C C
=+ +
[13.19]
p
solid
immobile
