Saturday, June 2, 2012

Hydrogeology of Rajarhat New Town

The project area and its surrounding parts are underlain by huge thickness of quaternary sediments laid down by the southerly flowing Bhagirathi river and its tributaries. The major area is occupied by recent alluvium comprising of gray sticky clay, fine and medium sand mainly. At a depth of 120 metre or more there is a brown clay horizon of sub recent age. Average thickness of the clay layer near to the surface is about 25 metres.
Groundwater occurs in several granular zones deposited by the river system. Sand horizons of different grades textures and colours constitute the main aquifers. In Rajarhat area due to the presence of thick clay at the top the upper aquifer gives a semi confined character. The drilling data available from the tube well drilled at Ecospace, Bengal Ambuja Complex shows that there is a thick gray clay bed upto a depth of 10 metres. From 10 to 42 metres there are several layers of fine sand often mixed with clay. This is the upper low yielding aquifer of the area. Below this there is a dark gray clay bed between 42 metre and 55 metres.  From 55 metres to 84 metres there is a bed of fine sand. Below this medium sand occurs  upto a depth of 100 metres (16m thickness). Below this upto a depth of  135metres there is fine sand.
The litho log and assembly is given in plate I
The zone between 55 metre and 135 metre constitutes a potential aquifer in this area and most of the groundwater is withdrawn from this area.
Groundwater level in Rajarhat block is regularly monitored. The post monsoon Groundwater level (2011) of 7 monitoring stations in and around the project area is given in the table1. With the help of the data a depth to water level zonation map is prepared which is shown in plate II.

Table 1. Post Monsoon depth to water level of Rajarhat area. (Source: SWID)
Sl NoMouza
Type of well
Post monsoon DWL mbgl
1              Patharghata      
Village centre
Tube well
2              Chandpur           
Panchayet office
Tube well
3              Gopalpur            
Health Centre
Tube well
4              Rajarhat              
Bishnupur Health Adm unit
Tube well
5              Ghuni   
Primary School
Tube well
6              Jagadishpur       
Primary School
Tube well
7              Sukantanagar Salt Lake 
Near Club
Tube well

It appears that the post monsoon depth to water level of the project area is between 8 to 9 metres below ground level. Water level is also collected from a deep tube well of the Tinkanya project site. The depth to water level of that well on25 January 2012 is 8.29 mbgl. The tube well tapped the aquifer between the depth 80m bgl to 95 m bgl.

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