IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002
deeper levels. Lithological constraints dictate that groundwater
is present in the pore spaces of vesicular basalt and in fractured
portion of massive part of basalt flows. But the groundwater in
study area is restricted to the zones of secondary porosity
developed due to fractures, joints and weathering.
3.2 Hydrogeology
In Deccan Basalt terrain groundwater occurs under phreatic
conditions in the exposed lave flows and under semiconfined
conditions in the flows at deeper level. Lithological constraints
dictate that groundwater is present in the pore spaces of the
vesicular basalt and in the jointed and fractured portions of
massive parts of the flows. The primary porosity in the basalts
is associated with the vesicles, which are the pore spaces
developed due to the escape of volatile and gases when the lava
erupts on the surface as a lava flow. This primary porosity in
the basalt is naturally limited and related to the quantum of
gasses/volatile in the eruptive phase, which resulted in the
basalt flow. The groundwater in the study area therefore is
restricted mostly to the zones of secondary porosity developed
in these rocks due to fractures, joints and weathering.
From the hydrogeological point of view, the frequency and
extent of jointing, fracturing and the flow contacts and
weathering along them are the most significant parameters
imparting permeability and porosity for forming suitable
groundwater reservoirs in the Deccan Basalt terrain. The
vesicular zones occurring in the upper parts of flows or units,
though porous, are not permeable, as the vesicles are not
interconnected. Secondly, the vesicles are generally filled with
amygdules, green earth, glassy material etc. The red bole layer,
flow breccias with secondary mineral development and the
massive parts of the flow, with non-interconnected joints, are
impervious. The secondary porosity (Joints and fractures)
generally reduces with depth and hence the near surface
(unconfined) aquifer system rarely, extends below 30 m depth
(Agashe R M, 1990).
32 HYDROGEOMORPHOLOGICAL STUDIES
The geological, structure drainage characteristics,
hydrogeological and well inventory data was integrated with
the satellite information for the finalisation of
hydrogeomorphological map (Fig.1).
In the present study the geomorphic surfaces of the study area
were classified into different morphounits or landforms on the
basis of physiographic characteristics, morphological features,
relief, slope, drainage density and lithology supplemented with
satellite imagery. Use of profile study, other topographical
characteristics and visual field studies (erosion intensity and
siltation in major rivers) was also made in arriving at the
classification and in delineating the boundaries of these
morphounits.
Geomorphic surfaces of the study area have been classified into
7 landform units. The hydrogeomorphological characteristics of
the landforms are described below:
a) Younger Alluvial Plains: This geomorphic unit occurs on
either side of the major rivers and their tributaries covering
about 10% of the total geographical area of the basin. The
alluvial plain features recorded in present study include point
bar and channel bar deposits. The younger alluvial plain has an
elevation range from 378 to 400 m above msl. The
characteristic features of the younger alluvial plain of study
area are of extremely low slope angle (0° to 2°) with almost flat
surfaces, low drainage density (< 1 km/km?), low stream
frequency (1to 1.5 streams/km?) and low relative relief (< 5%).
The younger alluvial plain comprises thick to fairly thick (10 to
25 m) alluvium. It consists of uncalcified silty sand and grey
brown silty clay with occasional presence of gravels and
pebbles of basalt, quartz and chalcedony. The groundwater
potential is from good to very good. These units are developed
along the Purna, Dudhana, Karpara rivers and their tributaries
and such places can be exploited for groundwater through deep
bores. The water table is shallow i.e. < 1 m bgl observed during
post-monsoon. The water table fluctuation is found to be from
0.5 to 1.2m (Table 1). Bore wells are giving the average yield
of 520 lpm.
b) Older Alluvial Plain: This unit is seen as irregular patches
within the younger floodplains along the river channel of
Dudhana, Purna and Karpara Rivers. It covers 7 % of the total
area of the basin. Features associated with this unit are point bar
deposits, severe gully erosion in the form of parallel gullies and
meander scars. This morphounit is located at the foot of the
pediments with the elevation range between 380 and 420 m
above msl. The morphometric attributes of older alluvial plain
are gently sloping (0° to 2°), to nearly horizontal surfaces, low
drainage density (< 1 km/km?), low stream frequency (1 to 1.5
streams/km?), low relative relief (i.e. between 5% and 10%) and
low channel gradient.
The older alluvial plain is fairly thick (15 to 20 m). This unit is
covered by soils which are very thick dark greyish brown to
dark brown coloured, calcareous, moderately well to ill drained
alluvium with high moisture retentive capacity. The
groundwater in these units is brackish on account of leaching of
salts and calcareous material i.e. calcretes. The water table is
shallow i.e. 0.5 to 3.2 m bgl during post monsoon period in
both dug and bore wells The water table fluctuation is found to
be from 0.7 to 2.1m in dug wells and 1 to 3 m in bore wells.
Dug wells are giving the average yield of about 450 lpm, while
bore wells have average yield of 515 lpm (Table 1).
Table 1. Well inventory and yield particulars of wells in Purna River basin
Sr. Geomorphic Observation Range of Range of depth to Water Range of Av.
No Unit wells depth (m) water (m bgl) table yield Yield
Type | Nos. Pre Post fluctuation (1pm) (Ipm)
monsoon | monsoon (m)
1 Younger DW 7 8.2 - 10.5 1.1-2.5 0.2 - 0.9 0.5 - 1.2 430-615 | 520
alluvial plain
2 Older alluvial | DW 5 7.8 - 11.5 1.6 - 3.5 0.5 - 1.6 0.7 - 2.1 380-560 | 450
plain BW 4 21.2-40.2 | 2.5-6.5 1.3-32 1.0 - 3.0 410-580 | 515
520
