Civil Engineering Reference
In-Depth Information
during this period a significant (or even maximum) concentration at the given
assessment depth can be reached.
The
attainment time
necessary for a given contaminant concentration to be
reached at a given depth can be used as a criterion (or determining parameter) for
the groundwater vulnerability or protectability assessment. From a modeling
approach, assessment by this criterion can be detailed by an analysis of the value
and variation in time of the predicted contaminant concentration at characteristic
depth
z*
of the assessed aquifer. It is noted [
Zwahlen
, 2004] that from the view-
point of groundwater customers (municipal services, farms, individual users) the
groundwater vulnerability assessment should provide answers to three questions:
1. How long will it take for the contaminant to reach the groundwater in the
case of activation of one or more surface contaminant sources?
2. What contaminant concentration will be reached in the groundwater used,
for example, in the groundwater pumping wells?
3. How long contamination above the maximum contaminant level will last?
The parametric assessment by the
concentration attainment time
or the more
conservative assessment by the
water percolation time
necessary for infiltrating
water to reach the assessed aquifer from the surface answers only the first of these
three questions, whereas the modeling assessment allows the accurate prediction
of the characteristic value or even variation in time (during a definite period of
interest) of the contaminant concentration at a depth of the assessed aquifer or
in the water intake location.
In the a priori assessment of groundwater vulnerability, it is not known in
advance what initial concentration will occur in the potential contamination
source (at the day surface) or only some approximate values of the concentration
can be supposed. For this reason, in the modeling assessments the relative
dimensionless contaminant concentration is often considered, determined by the
ratio of the assessed concentration at the given location to the source or initial
source contaminant concentration. In our 1D modeling of vertical downward
contaminant transport from the surface, we used relative dimensionless concen-
tration
c
(
z
,
t
) determined by the ratio of the dimensional predicted concentration
at the assessed depth to initial surface contaminant concentration (in the infil-
trating water)
C
0
:
c
(
z
,
t
)
= C
(
z
,
t
)
/C
0
.
The predicted value of this relative concentration for the characteristic
prediction time
t*
at a given depth
z*
in the aquifer
c
(
z*
,
t*
) characterizes the
migration permeability
of covering deposits, which can in turn serve as a measure
of the
cover vulnerability
of the given aquifer to surface contamination, that is,
aquifer vulnerability related to the protective role of its overlying (covering) bed
of rocks and deposits.
The inverse (reciprocal) relative concentration
c
−1
(
z*
,
t*
)
= C
0
/C
(
z*
,
t*
) may
serve as an indicator of protection ability (or protection potential) of covering
deposits and a measure of groundwater protectability (with respect to surface
contamination) of the given aquifer.
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