Geology Reference
In-Depth Information
Geology and Hydrogeology
The study area exhibits varied physiographic features with elevation ranging
from 1.0 to 11.5 m above MSL. The ground surface slopes towards the east
and west from the central elevated portion. The region which lies towards
the west of the Buckingham Canal has topographic elevation varying between
1.0 and 8.0 m above MSL. Western boundary of the study area has the
highest elevation of 8.0 m above MSL. Geologically, the area comprises
unconsolidated sandy formation of different depositional environments
belonging to Quaternary age. The alluvial deposits are comprised of
interlayered clay, silt, sand, gravel and pebble beds. These formations overlie
the charnockites of Archean age. Charnockites occuring below this sandy
formation function as impermeable strata or bed rock. The total depth of the
aquifer system ranges between 3 and 23 m (Gnanasundar and Elango, 2000).
The hydraulic conductivity of the aquifer lying west of the Buckingham
Canal has values between 25 and 35 m/day, while the sub basin along the
east of the canal has high hydraulic conductivity with values ranging between
45 and 75 m/day. The specific yield of the aquifer ranges from 0.17 to 0.23.
GROUNDWATER MODELLING
The computer software programme MODFLOW, Groundwater Modelling
System (GMS), (McDonald and Harbaugh, 1998) developed by the United
States Geological Survey was used for the present study to give input data
and process the model output. This equation describes the groundwater flow
under non-equilibrium and anisotropic medium, provided the principal axes
of hydraulic conductivity are aligned with x-y Cartesian coordinates axes.
The computer programme uses finite-difference technique and block-centered
formulation to solve the groundwater flow equation for the three dimensional
steady and transient flow in heterogeneous media of the South Chennai
aquifer.
Model Formulation
The conceptual model of the hydrogeologic system was derived from a
detailed study of the geology, borehole lithology and water level fluctuations
in wells. Groundwater of the study area was found to occur in both alluvial
formations and in the underlying weathered rocks. The measured groundwater
head in monitoring wells, screened in the alluvial and weathered formation,
is about the same. Based upon this information, the alluvial and the weathered
fractured rocks can be considered as a single, unconfined layered system.
Grid Design and Boundary Conditions
The scanned map of the study area was imported in the MAP module of
GMS with proper geographical registration. Boundaries and canal of this
