Geoscience Reference
In-Depth Information
unsaturated zones or are carried away by the runoff. An effective porosity
of 0.2, longitudinal dispersivity of 30 m and transverse dispersivity of 10 m
were uniformly assumed for the entire area. The mass transport model was
calibrated in two stages: steady state and transient state. This transient
state model was used for prognostic model. It was also assumed that TDS
do not influence the density and viscosity values, which may affect the
groundwater flow and pollutant migration. As various other parameters
(collected in the field) were assigned to the corresponding nodes.
4. Steady State Calibration
TDS concentration “
C
” was calculated at all node points for September
1988, a date up to which the system was assumed to be in a steady-state
condition. There was a mismatch between observed and computed values of
“
C
”. Therefore, efforts were made to obtain a reasonably better match by
modifying the magnitude and distribution of the background concentration
and pollutant load. However, the situation could not be improved much.
This may be due to a variety of factors; the most important which are the
lacunae and inaccuracies in the database. To get the real representation
of the aquifer system, field data (January 2001) was considered for other
steady-state condition and it also run to visualize the mass transport model.
The computed versus observed
C
was given good result.
5. Transient Calibration
As the steady-state model could not reproduce the observed data at all the
points, a time variant simulation was carried out. This was done for
the period January 1988 to July 1995 based on available of PWD data.
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The pollution load was reaching the groundwater system at various clus-
ters during this period. The computed “
C
” for five PWD-wells is higher
than the observed values. These values, however, could not be rectified, as
there was no basis for modifying either model parameters or the pollutant
load in the absence of any actual field estimates. It should be mentioned
here that the present model is only to illustrate the feasibility of applying
modelling techniques to study this problem and to use it for prediction of
system behavior for some future scenarios.
6. Sensitivity Analysis
The impact of varying conductivity, dispersivity, and C
/
W (TDS pollu-
tion load at the source) was studied. The variation is caused in the TDS
