Environmental Engineering Reference
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infiltrating recharge water assumed to be identical to the clean water coming into
the aquifer from the left boundary of the model domain. The result of one simulated
scenario with 100 mm/year recharge is shown in Fig. 19.8b . In comparison to the
base scenario, the effect of recharge determines a smaller toluene plume due to
the establishment of a second reactive fringe on the top of the plume where the
infiltrating water comes into contact with the dissolved contaminant. This second
reactive fringe also causes a deeper infiltration of the toluene plume in the aquifer.
As shown in the analytical formula and empirical correlation reported above
(Eqs. ( 19.19 ) and ( 19.20 )), the steady state plume length also depends on the thick-
ness of the contamination source. An increase of the source thickness results in
higher mass fluxes in the aquifer and thus in longer steady state plumes as can be
seen in Fig 19.8c where a source thickness of 3.5 m was used instead of the 2 m
thickness of the base scenario.
Simulations have been performed starting from the base scenario using different
source thicknesses in a range of 1.5-3.5 m and with transverse dispersivity values of
2
10 3 m. The results confirmed the dependency of the steady-state
plume length on the square of the source thickness as can be observed in Fig. 19.11 .
This plot shows the steady state plume length as a function of source thickness for
different values of transverse dispersivity, at a double-logarithmic scale.
The simulations performed above assumed a homogeneous aquifer system.
However, aquifer heterogeneity, in particular physical heterogeneity, characterizes
many contaminated sites and strongly influences contaminant plumes migration.
A considerable research effort has been dedicated on including the complex het-
erogeneity of many natural porous media into either deterministic or stochastic
contaminant transport models. For the present reactive transport modeling study,
just the effect of flow focusing in well-defined high permeability inclusions was con-
sidered. The focusing of flow in high-permeability zones in heterogeneous porous
media causes the streamlines to converge and diverge depending on the permeability
10 3 and 5
×
×
Fig. 19.11 Steady state plume length as a function of source thickness for different values of
transverse dispersivity
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