Geoscience Reference
In-Depth Information
second and third-order drainage.
16
,
17
The average annual rainfall is about
915.5 mm from a period of 1971-2001.
18
Geologically, the area is occupied with Archaean granites and gneisses,
intruded by dykes. Black cotton soil and red sandy soil (thickness: 0.52-
5.35 m) predominate in the area but thickness of weather varies from 3.1
to 26.6 m.
16
Charnokite rock is found in the extreme southern and south-
eastern part of the Sirumalai hill. Groundwater is extracted through dug
well, dug-cum-bore wells and bore wells for different purposes.
19
The gen-
eral trend of groundwater motion under shallow aquifer is in north and
northwest directions.
20
3. Approach for Numerical Modeling
As geometry and boundary conditions in the aquifer are generally so com-
plex, because aquifer is in hard area.
21
Here the main stages are followed for
mass transport modeling. They are: (1) solving the groundwater flow equa-
tion using finite-difference method, (2) estimation of fluid velocities at each
node, and (3) solving the mass-transport equation using finite-difference
technique and method of characteristics using the flow velocities.
22
Grid design: The area is divided into a series of grid blocks or cells
23
(size
of 250
250 m
2
, total grids = 3,342, area = 209 km
2
) and the groundwater
heads will be computed at the center of each grid block (Fig. 1). The layer
is unconfined condition and corresponds to a layer type 1 in MODFLOW.
Hydraulic conductivity values as well as specific yield values were assigned
from the field data. The actual values of the ground surface elevation and
bottom elevation of the bedrock were entered at the model.
No-flow boundary: It has been set in the southern part of the basin.
Head-dependent boundaries: The northern boundary of the area will be
simulated through Generalized Head Boundary (GHB). Other important
boundaries: (1) the weathered part of aquifer will be considered as a porous
one, (2) areal recharge and pumpages will be assigned at random, and
(3) wherever dykes and exposures are present, transmissivity values will be
adjusted and assigned as per its direction and length.
TDS in the surface euents was more than 30,000 mg/l during the
period September 1988 to February 2002. The quantity of euents seeping
to the groundwater system was assumed to be 30% of the surface eu-
ents. It was also assumed that on a conservative basis the solvent reaching
the water table has a solute concentration, which is 30% of that present
at the surface. The remaining 70% of the solutes may get absorbed in the
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