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Chandrabhaga Watershed , Nagpur District, Maharashtra
 
Groundwater studies
Hydrogeology
In Deccan traps, groundwater occurs in the pore spaces of weathered and vesicular basalt units and in the joints and fractures in the massive basalt of each basalt flow. Ground water occurs under phreatic conditions in the exposed lava flows and in semi-confined to confined state in the subsurface flows.
The secondary porosity developed due to weathering, fracturing and jointing helps storage and movement of groundwater and forms groundwater repositories. Weathering not only produces granular materials and loosens the matrix of massive rock but also widens the fractures, joints and shear zones. It also develops typical spheriodal weathering in the top layers providing wide scope for infiltration of water through gaps and weak planes to recharge the aquifer system.
The vesicular and weathered basalts are highly productive.
Basalts being underlain by Gondwana sediments and at places by granite gneisses, 4 types of subsurface hydrogeological conditions are observed down to the explored depth of 200m. They are (a) the entire 200m depth section of Deccan basalt, (b) Deccan basalts underlain by Gondwana sediments at depth, c) Deccan basalts underlain by granite-gneiss, and (d) the entire 200m depth section of Gondwana sediments (eastern part of watershed). The granular zones in the underlying Gondwana sediments, comprising medium to coarse grained friable sandstones, form good aquifers with moderate to high yield.
Depth to water level in general varies between 10 and 20 m in pre-monsoon period. Groundwater is developed mostly by dugwells and also by borewells. High yielding dug wells are in weathered and fractured basalt and in the Gondwanas, occurring in topographic depressions. Yields vary from 20 to 380 m3/hr. The boreholes drilled on an average to a depth of 40 to 70m yield a maximum of about 10 m3/hr. The borehole drilled by CGWB at Katol towards west of the area, had a yield of around 138 m3/hr from the zones within 219.5 m depth. At Kalmeswar, the borehole yield was around 65m3/hr from zones within 107 m depth. The deepest groundwater zone in the depth range of 287.73-296.57 m was encountered towards north of the area at Silewara in Saoner Taluk.
 
Figure 3.13: Weathered zone in the upper reaches of Chandrabhaga watershed, Nagpur.
 
Water Level
To decipher the depth to water level, 50 key wells were established in the watershed following a grid pattern. A total of 9 wells were established in Gondwana Formations and 41 wells in Deccan Trap basalt. The depth to water levels in November 2011 ranged between 3.65 (Pardi) and 9.50 mbgl (Tondakhairi) in the Gondwana formation whereas in the basaltic terrain it is between 1.0 (Vasboli) and 13.82 (WadhanaBuzrug) mbgl. The perusal of the Post monsoon map indicates that in southern and western part of the watershed the water levels are shallow and ranging from 0 to 2 mbgl and 2 to 5 m bgl. The water level in the range of 5 to 10 m bgl is observed in the north eastern part of the watershed, whereas the deeper water levels more than 10 m bgl has been recorded in village Wadhone BK in northern part and Brahmni in central part of the area.
 
Figure 3.14: Post monsoon Depth to Water Level, Nov 2011.
 
Figure 3.15: Pre monsoon Depth to Water Level, May 2012.
 
The depth to water levels in May 2012 ranged between 6.90 (Gowari) and 16.10 mbgl (Tondakhairi) in the Gondwana formation. In basaltic terrain, it is ranging between 2.50 (KotwalBardi) and 15.65 mbgl (Sawangi). The perusal of the fig.8 indicates that in shallow water levels in the range of 3 to 6 m bgl are observed as isolated patches in southern and northern parts of the watershed. Moderate water levels ranging from 6 to 9 mbgl are observed in the major part of the watershed, in the central and western part, and in the south eastern fringe areas. Deeper water levels in the range of 9 to 12 m bgl and more than 12 mbgl are observed in north eastern and south-central-eastern parts of the watershed.
 
Geophysical Studies
The focus of the aquifer mapping study is the delineation of thin low resistivity layers (vesicular basalt and fracture zones in massive basalt) at a depth underlying highly resistive (massive basalt) layer and the deeper moderately resistive (Gondwana sandstone) layer.
The focus of the aquifer mapping study is the delineation of thin low resistivity layers (vesicular basalt and fracture zones in massive basalt) at a depth underlying highly resistive (massive basalt) layer and the deeper moderately resistive (Gondwana sandstone) layer.
 
Figure 3.16: VES sites locations.
 
The table below shows the resistivity ranges and inferred lithology.
Resistivity range in ohm .m Lithology Thickness range in. m
3.2 181 Top Soil 0.2 8
1 6 Clay 1.1 27.7
1.8 5 Shale 1.1 82.9
6 9 Sandy Clay 3 11.3
11 34 Sand 4.3 11.3
10 14 Sandstone + Shale 2.3 114
16 87 Sandstone 3.5 79.3
152 1343 Compact sandstone 0.9 84
3.5 35 Weathered Basalt 0.6 39.5
30 43 Moderately Weathered Basalt 3.5 8.6
40.5 58 Vesicular Basalt 1.1 125
6.7 40 Fractured Basalt 1.3 144
More than 60 Massive Basalt 1.1 102
12 Fracture Granitic Gneiss    
More than 554 Massive Granitic Gneiss    
10 Lameta bed 5  
 
Based on the VES results, it is inferred that the thickness of the basalt is increasing from west to east. It is approximately 60m around Yerla in the western part, approximately 130m around Dorli, and more than 150 m in the eastern and northern parts. However, Gondwanas are also inferred at shallow depths, approximately 30 to 40m depths around Rawalgaon. This shows the undulating nature of the paleo-topography on which Deccan lavas were poured out.
The Gondwana formations are extending more than 150m depth in the NE part of the area. The Gondwanas are absent in the SW part of the area; this was confirmed from the existing borehole data at Yerla. In this area, the Achaeans are encountered below basaltic formation at depths around 60 mbgl. In the Northern and western parts of the area, the thickness of the top basaltic formation is more than 150m.
 
Exploration
CGWB has drilled 6 exploratory wells, 1 observation well and 3 Piezometers in the watershed. The depth of the wells ranges from 40 to 202m bgl. Water bearing zones are encountered in these wells in the depth range of 7 to 15m bgl and 183.5-186.5m bgl. The discharge of the wells ranges from negligible to 50 m3/h.
 
Figure 3.17: Location of exploratory wells and piezometers.
 
At Dorli site in the western part of the watershed, the Gondwana Sandstone is encountered at 123 and 111m bgl in the exploratory and observation wells respectively. On the other hand, the same sandstone is encountered at very shallow depth i.e., 27m bgl in eastern part of the watershed at Jhunki (Shindi). At the Borgaon (Khurd) and Pardi piezometer sites, sandstone is encountered at ground level, suggesting a thickening of the Basalt layer toward the west.
 
Groundwater Quality
The chemical quality of water samples collected from exploratory wells drilled by CGWB in shallow aquifers is good except for nitrate, which is found above permissible limits of 45 mg/l.
Industrial pollution is also report at places in the watershed.
With regard to pH, its values for shallow groundwater are recorded between 7.5 and 8.7 indicating that the ground water is slightly alkaline in nature.
EC values are between 640 and 2580 µSiemens/cm . A major part of the watershed shows moderate mineralisation with EC values ranging from 1000 to 1500 µSiemens/cm. EC values of less than 1000 µSiemens/cm are observed in the extreme eastern and western parts of the watershed, whereas high EC values (over 1500 µSiemens/cm ) are observed in central and extreme north eastern part of the watershed.
 
Figure 3.18: Electrical conductivity in the area.
 
Groundwater Resources
The estimation of ground water resources in this Pilot study area of the Chandrabhaga watershed (WGKKC-2), based on the 2007-08 data, indicates that there is a moderate ground water development in the area. Details of the resources, estimated by CGWB and Ground water Development Agency (GSDA), are as follows :
Assessment Unit Assessment sub-unit Net annual groundwater availability (ham/yr.) Annual ground water draft (ham/yr.) Stage of groundwater development
( % )
Irrigation Domestic and industrial uses
WGKKC – 2 Command 767.04 967.50 37.41  
Non-command 5018.28 2645.10 196.28  
Total 5785.32 3612.60 233.69 66.48
Source: Report on the Dynamic Ground Water Resources of Maharashtra (2008-2009)
 
The ground water resources of the watershed were evaluated in 2009. Based on the stage of ground water development and the long-term water level trend, the watershed is categorized as “Safe”.