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

concentration maximum is for the cambisol,
followed by the podzol and the chernozem.
In general, however, breakthrough times are
very similar for the three soils. Also the
maximum concentrations observed are of
comparable magnitude. As the tracer is conservative,
concentration reductions of about
70 % can be attributed to the dispersion
process. Integration of breakthrough concentrations
along the overall lower model
boundary (black curves) yields a further
reduction of concentrations, as about 30 %
of the infiltration bypasses the contaminant
transport path.
Fig. 16: Tracer breakthrough curves at the
groundwater surface below the road dam
for three different subsoil types (Beyer et
al., 2007b [EP 6]).
For naphthalene and phenanthrene, contaminant
transport is regarded with and
without degradation. For the cases with
degradation, a simple first order kinetics is
used. Sorption of both compounds is
modeled by a linear isotherm, where the
equilibrium sorption coefficient is derived
from the soil organic carbon content and the
distribution coefficient between organic
carbon and the aqueous phase (Beyer et al.,
2007b [EP 6]). Sorption kinetics are quantified
using the intraparticle diffusion model
(eq. 18). Concentration breakthroughs are
presented in Fig. 17. For the moderately
sorbing naphthalene without decay the influence
of soil organic carbon Corg is clearly
20
visible. The cambisol, which is almost free
of Corg (0.01 %), shows the earliest breakthrough
of the three soils. For the podzol
with a little higher Corg (0.21 %), the maximum
concentration breakthrough is slightly
retarded. The latest breakthrough time is
observed for the chernozem, which is the
soil with the highest Corg (0.88 %) regarded
here. For the strongly sorbing phenanthrene
this behaviour is even more characteristic.
For the chernozem, the maximum concentration
breakthrough is not yet observed
within 275 a of simulated contaminant leaching.
In contrast to the tracer, for which
source concentrations are depleted within a
few years, retardation within the DW causes
naphthalene and phenanthrene leachate
concentrations to stay on an almost
unreduced level throughout the simulation
period of 275 a (Beyer et al., 2007b [EP 6]).
As high contaminant concentrations are
constantly delivered from the source
material, dispersive concentration reductions
remain ineffective. Hence for the
transport path maximum concentration
breakthrough between 70 and 90 % of C0 is
observed. As for the tracer, these are
reduced by about 30 % when integrated
along the overall lower model boundary.
In comparison to the other type scenarios
(parking lot, noise protection dam) studied
in Beyer et al. (2007b [EP 6]), the contaminant
residence times in the road dam are
rather short, as high amounts of runoff
water from the road asphalt which infiltrate
along the embankment result in comparably
high flow velocities. The short residence
times reduce the effectiveness of degradation.
Hence, naphthalene and phenanthrene
concentrations are reduced by factors
between 2.5 and 5, respectively, while for
the other two type scenarios concentration
reductions by factors between 10 and 150
were observed. From this type scenario
modeling study the relevant transport and
attenuation processes for contaminant
leachate from DW used in road constructions
could be identified and quantified for
the assumed model structure.