The Growing pressures on land for food, fibre and fodder in addition to industrial
expansion and consequent need for infrastructure facilities has eventually given
rise to ever increasing population, which intern has given rise to competing
and conflicting demands on land and water resources where its occurrence is
also a finite one. About 175 mha of land in India, constituting about 53% of
her total geographical area, suffers from deleterious effect on soil erosion
and other forms of land degradation. Keeping in view of the exploding population
and a need for food security of the future generation, it is realized that the
water and land resources need to be developed and managed in an integrated and
comprehensive manner. It has been already realized that the soil and water conservation
measures carried out on a watershed basis play a prominent role in this strategy
of comprehensive land and water management. Hattermann et
al. (2004) has presented a paper in integrated catchment model and a
method with which it is possible to analyze local water table dynamics inside
sub basins along with river flow in the regional scale. Moon
et al. (2004) have analysed hydrographs and water-table fluctuation
statistically to estimate groundwater recharge.
Many authors like Narasimhan (1965), Balasubramanian
and Sastri (1989) Sathyamoorthy et al. (1997)
and Aravindan et al. (2005) have computed aquifer
parameter in different formations. Groundwater modeling studies helped to evolve
quantitative understanding of vertical flow and for better quantification of
spatio temporal variations of geohydrological variables (Thangarajan,
2004; Aravindan and Manivel, 2005). The present
study was an attempt to study the water table fluctuation there by occurrence
of possible recharge conditions and to determine the groundwater flow direction.
MATERIALS AND METHODS
Study area: The study area (Fig. 1) sedimentary part of
Gadilam river basin lies between Latitudes 11°1900 to 11°4000
N and Latitudes of 79°1900 to 79°4750 E. Public Works Department
(Government of Tamil Nadu), during 1992 and under United Nation Development
Program has carried out studies in the entire Gadilam river basin covering both
sedimentary and hard rock aquifers in the east while composite South Arcot district
of Tamil nadu, India.
|| Location Map of the study area
The overall climate of the area is warm and dry. Average temperature in the area is around 28°C. The maximum temperature attained is around 38°C in summer (May) and minimum temperature is around 25°C in January. The annual rainfall ranges from 995 to 1300 mm from west to east. Generally the rainfall is highest along the coast and less in the uplands of west. The mean monthly rainfall of 185 mm occurs during November.
The drainage pattern in the area is found to be sub dendritic. Study areas northern boundary is bounded by the drainage divide of major Ponniyar River. The study area is characterized by gently undulating topography with low relief sand stone and lateritic hills.
Methodology adopted for achieving the above mentioned objective is as follows. There are three major components in this study they are a) Field data collection including geological map; b) Water level data was collected from Public Work Division (PWD); c) Remote sensed Landsat TM satellite imagery along with SRTM satellite image was geometrically rectified with reference to the Survey of India (SOI) topographic sheets in the scale of 1: 50,000 using ERDAS IMAGINE 8.7 software. The drainage pattern was initially derived from SOI toposheets and later updated using linearly stretched False Colour Composite (FCC) of Landsat TM satellite data and from other image processed outputs were used to interpret drainage and geomorphology of this region. Water Table and water level fluctuation map is prepared by using spatial analyst tools of Arc/GIS (9.3) software.
RESULTS AND DISCUSSION
Geology: Gadilam River enters the sedimentary part of the basin from
the Archaeon- sedimentary contact at Thirunavallur in Ulundurpet Taluk and traverses
via Thiruvamur where Malattar confluences the main Gadilam before confluencing
at Bay of Bengal. Uppanar a back water stream joins Main Gadilam East of Dhevanampatnam
in Cuddalore. The Sountern part of the study area is drained by Manimutharu
and Main Vellar River. The study area is also covered by rocks of the recent
age. Central part of the study area was found to have river alluvium derived
from tertiary rocks which comprises of lignite deposit and coastal alluvium
adjacent to coast. As it is the sedimentary basin, hydraulic boundary was extended
upto the two river boundaries in the south. The Fig. 2 shows
the Geological map of the study area. The study area comprises of two major
hydrogeologic environments (a) Recent Alluviam and (b) Sandstone found in Lateritic
terrain. The average depth to water level ranges from 5 to 45 m. The recharge
areas constitute about 50% of the basin. The productive aquifers are met at
a depth of 60 m below ground level depending on the topography and rock type.
Geomorphology: In recent years the increasing use of satellite remote sensing has made it easier to define the spatial distribution of different ground water prospective classes on the basis of geomorphology and other associated features. The delineation of the geomorphic units is based on the interpretation of remotely sensed satellite data along with field observations made like topography, relief and other associated features on soil and vegetational covers.
Land forms: The present day landforms are irregular outline of ongoing
different geomorphic and related neotectonic process. The rate of deposition
and erosion is never found uniform because of the occurrence of uneven outcropping
ridges and hills.
|| The Geology map in the sedimentary part of Gadilam river
|| Geomorphology of the study area
By careful observation of various outcrop patterns with the help of Landsat
imageries, the characteristic land form is recognized. The following are the
landforms present in the study area (Fig. 3).
Pediments: In many arid and semi-arid regions long, smooth rocks cut
plains extends out at varying distances up to several miles from mountain fronts.
These plains are called as pediments. The pediments are the plains of degradation.
A pediment may or may not have an alluvial veneer over it but it is basically
rest over bed rock surface. The pediment is located as an intermediate landform
between inselberge and shallow pediments. The groundwater condition in pediments
is expected to vary depending upon the type of underlying folded structures,
fracture systems and degree of weathering. Groundwater potentiality in pediments
can be considered to be normal to poor (Sankar et al.,
1998) but presence of lineaments or fractures can provide some scope for
movement of groundwater and hence may be the prospective zones for groundwater
exploration. Bore wells located in shallow and buried pediments have deep water
table condition when compared to those in pediments. In the study area pediments
occupies the northwestern and southeastern part of the study area. These units
are mainly covered in between NW and SE (the central part) of the study area
Buried pediments: Comprises of weathered sandstone (mottled) at considerable depths. Groundwater prospect is found to be good in buried pediments because tube wells are located in buried pediments which have shallow water table condition when compared to pediments, as recharge of groundwater is found to be high through fraiable and mottled formations. Burried pediments occupy the north eastern and south western part (Fig. 3).
Water table: Ground water level data has been collected from 25 observation wells in the study area. It is a well established fact that the occurrence and movement of groundwater depends upon lithology, landforms and the structure. If these factors are favorable then the aquifer condition holds good for good groundwater potentiality. A good aquifer is one which can be recharged during the period of monsoon when rain water gets infiltrated and recharged. This means that during the pre monsoon period the aquifer used to have a deep water table condition compared to post monsoon period. The difference in water table can be calculated once the water table of both seasons is recorded. From the water level data, existing 25 wells were considered for both pre and post-monsoon season (Fig. 4). Wells were found to be fairly distributed throughout the study area. Water table conditions of pre and post monsoon season are shown in Table 1.
Pre-Monsoon water table: Water level data of pre monsoon period was collected for the year 2007. Depth to water table from ground surface was measured with the help of measuring tape. Altitude is measured with the help of GPS. These water level data is converted to mean sea level referenced data by using altitude value. After getting the MSL referenced to water level data, water table contour map is prepared with the help of Arc GIS 9.1 (Fig. 5). The pre monsoon water table contour varies between -45 m (bmsl) to 75 m (AMSL). Shallow water tables are found in eastern part; whereas deeper water values are found in the southern and northern part (Fig. 5).
|| Well location map of the study
|| Water table data in the sedimentary part of Gadilam river
|| Water Table contour map of the study area (pre-monsoon)
Average water table depth from ground is 60 m is water in tertiaring formation were found to be in confined conditions a results they were at deeper depth and found to be occurring below.
Post monsoon water table: Post monsoon water level data was collected
for January 2008. Depth to water table is measured from the respective observation
wells. Similar procedures were adopted as similar to pre monsoon. Post monsoon
water table map was carried out using Arc GIS for post monsoon water table contours
were prepared (Fig. 6). Post monsoon water table varies in
the range of 60 m (BMSL) to 70 m (AMSL). Similar to pre monsoon water level
eastern part shows shallow water table where as, southern part shows deeper
water table (Fig. 6).
|| Water Table contour map of the study area (post-monsoon)
Reason is water in recent formation was found to be in unconfined condition
as a result they were at shallow depth as they are found to be in a plain country.
Overall average water table depth from ground is 60 m.
Water table fluctuation: Water table fluctuations map was prepared using
the difference in level of pre and post monsoon water level data. Water level
fluctuation map is prepared using spatial analysis of Arc GIS software (Fig.
7). Water table fluctuation is one of the important map to study the movement
of groundwater. From the above map we could find the cone of depression or abstraction
is more in south central part of the study area due to abstraction in Neyveli
mines. As a result ground water flow is towards the Neyveli mines (Fig.
7). From the fluctuation map, it is also found that maximum recharge in
water level is in the Northern and Southern part of the study area in between
vellar and Neyveli mines with less than 6 mAMSL of water level.
|| Water level fluctuation map of the study are
Northeastern and Northern part of the study area shows minimum rise in water
level with less than -0.5 m (Fig. 7).
Generally water table has a configuration similar to that of land surface; however, depth to water levels are deeper in uplands (recharge areas) rather than in the area adjacent to the river valley and discharge areas.
Water level data of pre and post monsoon season was collected and water table
contour map is prepared with the help of Arc GIS 9.1 as shown in Fig.
5 and 6. Shallow water tables are found in eastern part,
where as deeper water tables are found in the southern and central part. Average
water table depth from ground is 60 m below the ground level. Post monsoon water
table varies I the range of-51 m (BMSL) to 70 m (AMSL). Even during post monsoon
water level in southern part shows deeper water table where as, central and
western part shows elevated water table (Fig. 6). In post
monsoon period it is found that maximum recharge in deeper water level is found
in the central and southern part of the study area. North-eastern and northern
part of the study area shows minimum rise of water level of less than 6 m. As
they were found in plain country and the reason is water in recent formation
were to be under unconfined condition (Fig. 2 and 6).
From the above study it is clear that for ground water fluctuation it is controlled
by terrain morphology, under lying formation and the quantum of groundwater
abstraction. An important finding of the above study is ground water movement/flow
is toward Neyveli mines where cone of depression was created due to huge abstraction
to depressurize the aquifer. Maximum fluctuation of 40 m is found within 6 months
to confirm the recovery after monsoon season (Fig. 5-7).
So, it is very clear apart from periodic recovery of deep bore wells of confining
formation interaction between the connate water and naturally recharged rain
water cannot be ruled out.
In the study area pediments occupies the north western and south eastern part
(Fig. 3). Bore wells located in pediments have deep water
table when compared to those in buried pediments. Ground water prospects will
be found to be good in buried pediments because tube wells located in buried
pediments have shallow water table in the east (Fig. 3, 5
and 7), when compared to pediments as the recharge of ground
water is found to be high through friable and mottled sandstone. Above formations
were found to be in confined conditions.