Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 806

0 0

-1 -1

-2 -2

-3 -3

-4 -4

-4 -3 -2 -1 0

0 0.1 0.2 0.3 0.4 0.5

pressure head, h (m)

water content, θ

Fig. 18.10 Numerically calculated pressure head ( left ) and water content ( right ) versus depth for
a homogeneous silt, assuming a water flux q of 2 10 − 6 m/s. Solid line is for t = ×

0, while the
distributions at other times (1, 5, 10, 20, 30, 40, 50, 60, 70 h) are indicated by dashes of decreasing
length

18.2.8.2 Two-Layer Soil

Heterogeneous soil profiles are the rule rather than the exception. In general, two
or more layers (soil horizons) are present parallel to the surface. Although water
flow is not necessarily one-dimensional in such cases, one can often approximate
the infiltration process by a one-dimensional model.
The first example considers a sand over silt layer. Hydraulic properties, initial
and boundary conditions are those used for the single-layer soil described above. For
the water content, the initial condition displays now a discontinuity at the interface
between both layers (Fig. 18.11 ). The pressure head, on the other hand, is continuous
across the interface. This behaviour follows from the Buckingham-Darcy equation
(Eq. 18.9 ) which has a finite q only when d h /d z is finite, or when h is continuous
everywhere. Up to t 30 h, the infiltration into the sand behaves identically as in the

homogeneous sand. The pressure head behind the infiltration front is

=

− 0.46 m and

10 − 6 m/s. Owing to the continuity
of the pressure head across the interface, the silt soil has a conductivity of 1

the corresponding conductivity in the sand is 2 ×

10 − 6
m/s, which is two times smaller than in the sand. As a result, water flow is retarded
in the silt and the water content above the interface increases. This in turn leads
to a higher K in the silt layer. At steady-state, a unit gradient condition is present

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