Environmental Engineering Reference
In-Depth Information
Looking at the results in terms of R c and J h /( c 0 q h 0 ), the following comments can
be made:
− a lining system comprising a CCL plus an attenuation layer can lead to better
long-term performance than a GCL plus an attenuation layer of the same total
thickness;
− the higher contaminant concentration and flux shown by the GCL is mainly
due to parameters Γ GCL and Γ CCL , i.e. due to the higher hydraulic conductivity and
then to the advective transport, whereas the two diffusive contributions are fully
comparable;
− the advective transport is largely the prevailing contribution to contaminant
migration, referring to the barriers considered in the example. Therefore, in this case
a further reduction of the diffusion coefficient of both the CCL and GCL is
ineffective.
In summary, it seems that the permittivity ( k / L ) is still the critical issue for the
GCL, looking in particular at compatibility problems with leachates. On the other
hand, diffusive transport is reduced by the contribution of the attenuation layer and
by good performance of the GCLs. In the following part of this chapter, it will be
shown that the reduction in advective pollutant transport by a geomembrane placed
on the top of these mineral barriers can significantly change the conclusions of the
comparison shown in the previous example.
Finally, we obtain a preliminary indication about the performance of the barriers
shown in Figure 16.22 under transient conditions (i.e. finite mass of contaminant
and decay phenomena). It is possible, in a first approximation, to simply compare
the following parameters taking into account that a lower value leads to a better
performance in terms of both advective and diffusive transport:
; Γ Λ
ΩΩ
where:
RL
ρ
K
i
i
Ω=
R
=
i
i
i
L
n
i
i
with:
R i : retardation factors of the i-th layer;
ρ I : dry density of the i-th layer;
K di : distribution coefficients of the i-th layer.
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