Hydrogeologically
the region has very diverse conditions since it is occupied by rocks ranging in age from
Archean to Recent.
The occurrence and
movement of ground water in the various formations found in the region is as follows.
Archeans:
These are composed of old
metamorphics, graintes, gneisses and schiests. They are hard and compact formations with
low primary permeability forming poor aquifers. Ground water occurs in these only in the
weathered mantle and underlying fractured zone. Dugwells in this formation have depths of
5-30 m with water levels between 3-17 mbgl. Where thickness of aquifer is considerable
specific capacity ranges from 20-200 lpm/m of drawdown. Hydraulic conductivity is
generally less than 1 m/d and specific yield less than 5%. The yields of open wells ranges
between 40-135 m3/d.
Cuddapahs and
Vindhyans
These are composed of
sandstone, shale and limestone. The sandstone and shale are hard and compact and form poor
aquifers. Ground water occurs in these in the weathered mantle and fractured zone below.
The limestone is different in its hydrogeological properties having large solution
cavities which give rise to immense secondary permeability. The wells in limestone
formations yield 100-500 m3/day for 3 m drawdown. Specific capacity ranges from 100-300
lpm/m of drawdown, hydraulic conductivity lies between 5-15 m/d and specific yield ranges
from 5-15% in good karstic zones.
Gondwanas:
The Gondwanas are
sedimentary formations rich in granular zones and form good aquifers. They support both
dugwells and tubewells capable of yielding 100-500 m3/d for drawdowns of 6-10 m. The
specific capacity ranges between 75-250 lpm/m of drawdown, hydraulic conductivity lies
between 10-25 m/d and specific yield is from 10-15%.
Infra Trappeans:
The Bagh and lameta beds
and Nimar sandstone are also sedimentary formations but have a limited extent and poor to
moderate permiabilities. The limestone and calcareous clays when karstified from
productive aquifers. The coralline limestone, the marls and nodular limestone are hard and
compact having poor permeability. The Nimar sandstone has interaranular porosity. Joints,
fracture, bedding planes which give moderate scope for ground water movement. The depth to
wells varies from 3-13 m and depth to water level between 2-12 mbgl.
Deccan Traps:
These form the most
important aquifers in the region. Being the most extensive. The weathered, fractured,
jointed and vesicular units of basalts form moderate to good aquifers. These formations
have highly variable yields ranging from 10-750 m3/d. Dugwells range in depth from 4-20 m
with water levels varying between 2-14 mbgl. The specific capacity ranges from 50-150
lpm/m of drawdown, hydraulic conductivity varies between 5-15 m/d and the specific yield
is 5-10%.
The Deccan Traps
formations can be tapped by dug-cum-bore and drilled wells. It is observed that the yield
increases by 5-10 time when 10-15 cm bores extendig down to the lower vesicular zone are
drilled at the base of dugwells. Yields of 400-600 m3/d can be obtained in this way. In
some areas the control of dolerite dykes on occurrence of ground water was observed. Wells
located on the upstream side of these dykes gave better yields. Also wells located on
tectonic lineaments gave better yields.
Alluvium:
It comprises of
uncosolidated gravel, sand, silt, clay in various proportiions and has primary
intergranular porosity and permeability. Hence it is the most promising formation for
ground water development. The thickness of alluvium varies from 10-318 m with aquifer
thickness from 10-160 m being more in the Ganga basin than in the Narmada basin. The depth
to water ranges from 5-30 mbgl. The transmissivities are high in the Ganga basin being
upto 3000 m3/d. In the Narmada basin transmissivities range between 83-283 m2/d. The
yields vary from 30-50 m3/hr for shallow wells and 30-200 m3/hr for deep wells in the
Ganga basin. In the Narmada basin the yields are of the order of 15-30 m3/hr in the
phreatic zone and 70-200 m3/hr in the deeper zone. In the wainganga sub basin the wells
yield 60-600 m3/d. In the Tapi basin the yields are 25-95 m3/d. |
HYDROGEOLOGICAL STUDIES - Hydrogeological
Conditions
