Environmental Engineering Reference
In-Depth Information
It is worth stressing that the simplified equivalency criteria can only give reliable
indications on a case-by-case basis referring to specific conditions related to both
time and space domains (see [ROW 98] for further details).
Referring to the steady state landfill model illustrated above, it is still interesting
to observe that there is a minimum in the aquifer relative concentration
versus
thickness of soil layers separating the bottom barrier from the groundwater surface
relationship [OLI 00]. This is due to the combination of a positive contribution in
terms of diffusive transport when the attenuation layer is thickened, and a
corresponding decreasing efficiency in terms of advective transport at a given
hydraulic gradient. This is because the hydraulic conductivity (
k
a
) value of the
attenuation layer is in general higher than the hydraulic conductivity (
k
b
) of the
landfill liner.
Figure 16.23 shows the ratio between the thickness of attenuation layer (
L
a
) and
barrier system (
L
b
) that minimizes the groundwater concentration in the aquifer
versus
the Peclet number of the barrier evaluated assuming unitary hydraulic
gradient (
P
b1
=
k
b
/
n
b
⋅
D
b
). Current ranges of variation of the main parameters have
been considered, particularly the equivalent hydraulic conductivity and diffusion
coefficients ratios between the barrier and attenuation layer (
k
b
/
k
a
;
n
b
D
b
/
n
a
D
a
).
Figure 16.23 has been plotted assuming a hydraulic leachate head (δ
h
) and a
barrier thickness equal to 0.3. Nevertheless, the plots of Figure 16.23 can also be
used with other δ
h
/
L
b
ratios by simply evaluating the permittivity and diffusivity
parameters (equations [16.4] and [16.5]) of an equivalent barrier. These parameters
include part of the attenuation layer or of an attenuation layer that includes part of
the barrier in order to obtain a total barrier thickness. For the actual value of δ
h
, the
total barrier thickness is consistent with the ratio δ
h
/
L
b
= 0.3. Even though it is
unlikely that, in the case of a thick and unsaturated attenuation layer, a steady-state
flow will occur within the time of interest for the considered landfill, Figure 16.23 is
useful for calibrating and optimizing the position of the landfill bottom (Figure
16.23), avoiding over-thickening the attenuation layer without getting appreciable
advantages in terms of minimization of pollutant impact on the groundwater.
It is interesting to observe that using the current parameters for modern
composite liners (i.e.
P
b
≅ 1), the optimized attenuation layer thickness ranges from
3 to 10 m. This corresponds to the thickness of attenuation layers or the minimum
distance between the landfill bottom and groundwater surface indicated by the
regulations of some European countries.
