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Parts of Tumkur District, Karnataka
Groundwater studies
Aquifer systems exist in both shallow weathered formations and in deeper fractured systems. They have been explored up to a depth of 200m.
The overburden thickness map shown below is based on casing depths. It is indicative of weathered zone thickness less than 10m in the central part and increasing towards north-west as well as south-east. It goes up to a maximum of 35 m in the south-east.
Figure 2.15: Overburden thickness
The aquifers are mostly under unconfined condition in the northern part. Semi-confined to confined conditions exists in the deeper fracture zone aquifers. The hydrogeological conditions vary widely over a short distance due to large scale heterogeneities. In general, 2 sets of fracture zones are encountered in boreholes, one in 60-75 m depth range with yield less than 3.6 m3/hr and the other in 140-150 m depth range with yield about 3.6 m3/hr. High yielding fractures have also been encountered.
Figure 2.16: Hydrogeology map
Water level
In pre-monsoon period the water table goes deep, down to a maximum depth of 48m at places, making the weathered zone dry. The only source of groundwater is then the deep seated saturated fracture zones.
The depth to water level in post monsoon period is around 10 metres below ground level (m bgl) m in the northern part of the area and goes beyond 20 m bgl in the southern parts and in limited areas in the north. There are small patches with water level around 5 m bgl.
Three major directions of groundwater flow viz., SW-NE, SE-NW and SSW-NNE are observed in the area.
Figure 2.17: Depth to water level map in September 2011.
Geophysical studies
Geophysical surveys have been conducted to decipher weathered zone and fractured zones thicknesses and assess the aquifer characteristics of the weathered zone.
By combining the borehole data and resistivity sounding data, a more precise overburden thickness map can be prepared.
Exploratory drilling conducted so far by CGWB in the area up to a maximum depth of 204 m reveals the presence of a number of yielding fractures at depth ranging from 18 to 185 m bgl. Generally, fractures occur mostly at depth ranges of 60 to 75 m bgl and 140 to 150 m bgl.
The yield of exploratory wells drilled varies from 1 to 40m3/hr. It is mostly around 3 to 4 m3/hr from boreholes drilled to a depth of 90 m.
Groundwater quality
The chemical quality of water samples collected from monitoring wells under the pilot study show that:
pH of ground water varies from 6.8 to 8.4, indicating that the water is alkaline in nature.
EC is mostly between 500 to and 1500 ┬ÁSiemens/cm, except at a few places where higher values are recorded, up to 2700 ┬ÁSiemens/cm.
The ground water quality is mostly within potable range.
Figure 2.18: Electrical conductivity map.
Groundwater resources
The dynamic ground water resources for the area were assessed in March 2009. The entire study area belongs to the watershed 4D3D8, which falls under the Krishna basin and Vedavati sub-basin.
For the watershed 4D3D8, the total recharge is 2799 ha.m, with natural losses at 279.9 ha.m .
Therefore, net ground water availability is 2519.1 ha.m. The existing ground water draft for irrigation and domestic and industrial uses are 3812.47 and 174 ha.m respectively. Hence, the net ground water availability is zero.
The stage of ground water development is 158% in the watershed 4D3D8 and it falls in over exploited category.