Hydrogeology of Rajarhat New Town

Geology

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.

Hydrogeology:

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The zone between 55 metre and 135 metre constitutes a potential aquifer in this area and most of the groundwater is withdrawn from this area.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

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	Location	Type of well	Post monsoon DWL mbgl
1              Patharghata	Village centre	Tube well	5.5
2              Chandpur	Panchayet office	Tube well	0.85
3              Gopalpur	Health Centre	Tube well	11.25
4              Rajarhat	Bishnupur Health Adm unit	Tube well	5.53
5              Ghuni	Primary School	Tube well	9.9
6              Jagadishpur	Primary School	Tube well	7.7
7              Sukantanagar Salt Lake	Near Club	Tube well	12.3

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.

ARSENIC AND FLUORIDE IN GROUNDWATER

Arsenic Contamination
Groundwater in a large part of the Bengal Basin contains high levels of arsenic. Large number of people of India and Bangladesh are compelled to drink water having arsenic concentration above the permissible limit.

At present, the primary source of drinking water in rural areas is the private domestic tube wells, which withdraws water from shallow aquifers (50 to 200 ft). Data from different sources indicate high arsenic content in water from such shallow aquifers in extensive areas in the delta. However, in many areas the deeper (> 500 ft) aquifers too show arsenic content above the permissible limit of 0.05 mg/litre according to the Indian standards.

Several countries in the world suffer from arsenic contamination of groundwater. The highest magnitude of this contamination is seen in Bangladesh and West Bengal. But in other parts of the world, e.g. Mongolia, Brazil, United States, Argentina, Ghana, Nepal, Pakistan etc are worst effected. In India, arsenic contamination is reported from Bihar, Uttar Pradesh, Madhya Pradesh, Tripura and Assam.

Arsenic Toxicity
Acute: severe abdominal pain, fever, cardiac arrhythmia.
Chronic: muscle weakness and pain, gross edema, Gastrointestinal disturbances, liver and kidney damage, swelling of peripheral nerves (neuritis), paralysis, liver injury, jaundice, peripheral vascular disease - blackfoot disease. Chronic drinking water exposure causes cancer (skin, lung and other organs).

Fluoride Contamination
Fluoride is another toxic material generally present in ground water. Prolonged drinking of water containing high fluoride may cause degeneration of bone and teeth. This is called fluorosis. About 300 people from the village of Nasipur (with a population of about 2000) in Birbhum, have been paralysed after drinking water contaminated with fluoride. According to media report, the local residents claim that the number is more than 1200. Investigations have proved that the fluoride content in Nasipur's water is 14 ppm, against the permissible limit of 1 to 1.5 ppm. There are other districts in West Bengal where fluoride is also found in groundwater.

Fluoride contamination in groundwater is not new in India. Fluoride above toxic limit is found in groundwater of many states like Jharkhand, Assam, Meghalaya, Madhya Pradesh, Orissa, Chhattishgarh, Tamil Nadu, and Andhra Pradesh. In West Bengal presence of fluoride in groundwater has been detected in the year 1998 by the State Water Investigation Directorate, West Bengal. It was found that in a single village of Birbhum district about 2000 people fell victim of the contamination.

Fluoride is readily incorporated into the crystalline structure of bone, and accumulates over time. The condition of fluoride poisoning is called skeletal fluorosis which increases risks of bone fracture. Skeletal fluorosis is a bone and joint condition associated with prolonged exposure to high concentrations of fluoride. Fluoride increases bone density and causes changes in the bone that lead to joint stiffness and pain.

Arsenic and fluoride contamination in groundwater are geogenic in nature. Detection of contamination through hydrogeological investigation is the only preventive measure.