Environmental Engineering Reference
In-Depth Information
[13.10]
vv
=
+
v
+
v
advection
dispersion
diffusion
vCuDCd CCuDC
=−∇−∇=−∇
'
m
where D' gathers together the contributions from diffusion and dispersion.
13.3. Pollutant retention phenomena
In the transient regime, storage − i.e. the pollutant retention in the soil − varies
with respect to time. It is therefore important to identify the retention phenomena
and to model them. Several phenomena are likely to overlap. In addition to the
pollutant itself, several phases are present and contribute to pollutant retention:
- the solid matrix, soil or rock, regarded as immobile here, may fix or release the
pollutant;
- the mobile pore water that flows in the pores and cracks that are large enough.
We will note the quantity of pollutant stored in the mobile water of a soil
volume unit S p , expressed in kg/m 3 . The change of pollutant in mobile water is a
variation in concentration and can be simply written as:
S p &
& =
C
[13.11]
- the almost immobile water trapped in the rather impermeable pores and cracks,
but in contact with the fluid, allowing pollutant exchanges by diffusion.
Pollutant storage is possible in each of the three phases.
Various physical and chemical phenomena are likely to be mobilized and to
govern the local evolution of pollutant concentration [BEA 87, BIV 93, ING 98]:
- radioactive decay;
- balance or transient chemical or biochemical relations;
- balance or transient sorption relations.
The balance equations and out-of-balance relationships must be written,
according to the individual cases and physical processes.
To illustrate the complexity of the phenomena involved, consider Figure 13.2,
which shows that several schemes may successively appear. Chemical adsorption
involves chemical links between the pollutant and the solid, while physical
Search WWH ::




Custom Search