Civil Engineering Reference
In-Depth Information
From the viewpoint of modeling, the groundwater vulnerability and protect-
ability assessments for the confined aquifers are less difficult tasks as compared
to the upper groundwater. This is caused by less complicated and easier predictable
physical processes of groundwater flow and migration in the saturated medium,
as compared to the moisture transport in the unsaturated zone.
There are a lot of well-developed modeling methods and computer codes for
the prediction of groundwater flow and transport in the stored groundwater
aquifers. Among them the most known are MODFLOW and MT3D [
McDonald
and Harbaugh
, 1988;
Zheng
, 1990;
Ciang and Kinzelbach
, 2001].
On the other hand, because of the higher depth of the confined groundwater,
the lack of data for lithology and permeability of water-bearing deposits at depths
of confined aquifers can be more significant than for the upper groundwater. In
this relation, significant variations exist in the assessments of flow and transport
parameters of geological media such as hydraulic conductivity, dispersion, and
distribution coefficients. In conditions of insufficient degrees of study of the flow
and transport parameters for the confined aquifers, it is possible to apply for their
vulnerability and protectability assessment a simplified methodology similar to
the one described above for the unconfined upper groundwater. The methodology
is based on combining the zoning methods and 1D modeling of the contaminant's
downward migration from the previously assessed upper aquifer (see Section 4.1)
into the given confined aquifer through the low-permeable confining bed. The
corresponding 1D model is described by the same general formulation (4.1)-(4.4).
As the upper model boundary for the confined aquifer model, the
characteristic depth
z
1
*
of the previously performed assessment of the upper
aquifer is taken. The model is formulated in terms of the relative dimensionless
concentration in such a way that the upper boundary relative concentration value
at depth
z
1
*
is taken to be 1. In such a way the protection ability of the whole
overlying deposit from the contaminated surface, including the previously
assessed upper aquifer, is taken into account.
The vertical downward flow velocity in the confining bed between the first
and second (confined) aquifers,
w
(m/day), is determined in accordance
with assumptions of the vertical flow between the aquifers by the formula
[
Polubarinova-Kochina
, 1977;
Ciang and Kinzelbach
, 2001]
k
m
HH
0
w
=
(
−
),
(4.7)
1
2
0
where
k
0
(m/day) and
m
0
(m) are vertical hydraulic conductivity and thickness of
the confining bed, respectively, and
H
1
and
H
2
are groundwater heads in the first
(unconfined) and second (confined) aquifers, respectively. These parameters are
determined by available literature data, observations in the monitoring wells, and
results of modeling.
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