Civil Engineering Reference
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deposits. It is a flow-related characteristic of groundwater vulnerability.
For example, if a layer of solid fractured rock with relatively thin fractures occurs
on the contaminant migration pathway, the contaminant front with a relatively
high concentration may rapidly reach the underlying aquifer by separate fractures,
and theoretically its concentration can be measured in separate points near the
fracture outlets. However, the whole average infiltration velocity
w
(or the down-
ward flow rate) below the rock layer remains low because of the very thin fracture
network and corresponding low layer vertical hydraulic conductivity. Thus the
layer screening ability and its related protection potential remain high, which cor-
responds to a low contamination potential
P
.
Relative mapping of the predicted contamination potential for an aquifer
over the given area can be done by relative contaminant concentration,
p = c
(
z*
,
t*
)
∙w
(
z*
).
In our case of a 1D vertical model for the groundwater vulnerability
assessment, the vertical contaminant concentration in groundwater,
C
(
z,t
)
,
is
obtained from the solution of the initial-boundary problem for the advection-
dispersion partial differential equation representing the 1D case of equation
(1.14) [
Gladkiy et al
., 1981]:
∂
∂
z
Dz
C
∂
∂
wC Cn
C
t
−=
∂
()
−
λ
,
(4.1)
z
∂
∂
∂
C
z
CtCe
(,)
0
=
−
λ
t
,
=
0
,
(4.2)
0
zL
=
where
t
is time,
C
0
is initial concentration at the surface (in the infiltrating water),
w
(m/day) is infiltration velocity,
D
(
z
) (m
2
/day) is the dispersion coefficient,
λ =
ln 2
/T
is the contaminant decay constant (
T
contaminant half-life period),
n
is the dimensionless storage coefficient,
n
=+
θ
k
d
,
(4.3)
accounting for porosity or moisture content (in the unsaturated zone)
θ
and
dimensionless distribution coefficient
k
d
that relates the contaminant concentra-
tions (in Bq/dm
3
) in the liquid phase
C
and solid phase
N,
respectively:
NkC
=⋅
.
(4.4)
Equation (4.4) represents the linear isotherm (Henry's law) for an equilibrium
sorption process that is applicable for our case of relatively low contaminant
concentrations in the porous media [
Bochever and Oradovskaya
, 1972;
Shestopalov
, 2002].
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