Applied numerical modeling of saturated / unsaturated flow and ...

1.3m
0.3m
as a representative for highly soluble salts,
naphthalene for moderately sorbing and
phenanthrene for strongly sorbing organic
compounds. Contaminant leaching from the
DW to the groundwater surface is studied
with six different characteristic subsoil
units of Germany (BGR, 2006) to be able to
compare the influence of hydraulic and
basic physico-chemical soil properties on
contaminant attenuation. Fig. 14 presents
the conceptual model for the road dam.
Here coarse grained DW is used as unbound
base / subbase layers below the
asphalt surface of the road. According to
German road construction regulations, the
base / subbase layers are covered by low
and high permeable soil layers along the
embankment (Fig. 14).
RCB
Körnung 0/32
b horizon
c horizon
symmetry axis
10m 1.5m
asphalt layer
impermeable low permeable soil
3%
4%
12%
1:1. 5
2.3m
10 cm high
permeable soil
2m
groundwater surface
(point of compliance)
Fig. 14: Road construction with demolition
waste recycled in base / subbase layers
(Beyer et al., 2007b [EP 6]).
The simulation strategy for this study
combines the Eulerian and Lagrangian
frameworks of transport modeling. Unsaturated
flow, i.e. the hydraulics of the constructions,
is modeled with GeoSys /
Rockflow using standard FEM. Fig. 15
shows that the two-dimensional scenarios
exhibit complex flow patterns under unsaturated
conditions. The velocity vectors at
the element nodes of the FEM mesh for the
road dam presented in Fig. 15 indicate
unhindered infiltration of water from the
low permeable soil on top of the embankment
into the DW material. Along the
sloped material boundary with high permeable
soil on top of the coarse DW, however,
strong capillary barrier effects are observed.
These cause a concentration of the water
flux on top of the DW and generation of
lateral runoff, almost completely bypassing
the DW.
Fig. 15: Element nodes and velocity vectors
of unsaturated water flow in a section of the
model domain (Beyer et al., 2007b [EP 6]).
Reactive transport of the model compounds
is simulated using the stream tube concept
of the SMART code, to take full advantage
of the reactive process models implemented
in SMART (degradation, intraparticle
diffusion kinetics, sorption, etc.). Coupling
between GeoSys / Rockflow and SMART is
achieved through the travel time pdf. These
are generated by simulation of conservative
tracer breakthrough curves by GeoSys /
Rockflow using the FEM. The pdf are used
as input for the reactive transport simulations
with SMART. The model output of
SMART for the three model substances at
the groundwater surface represents concentrations
integrated along the cross-section
of the contaminant transport path only.
These breakthrough curves for the tracer in
the road dam are displayed in Fig. 16 as
grey curves. The three black curves
represent the same tracer breakthrough
concentrations but are integrated along the
groundwater surface of the overall model
domain. Hence, they account for dilution by
uncontaminated seepage water which
bypasses the actual transport path due to the
capillary barrier.
Breakthrough curves in relative concentrations
C/C0 [-] are given here for three of
the six soils regarded in this study, i.e. for a
cambisol, a podzol and a chernozem. Comparing
the concentration breakthrough for
the transport path (grey curves), it is found
that the earliest breakthrough time for the
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