Ganga-Meghna Brahmaputra || West Bengal || Bangladesh || Middle Ganga Plain, Bihar || Uttarpradesh
Jharkhand || North-East Hilly States || Rajnandgaon, Chattisgarh || Behala, Kolkata, WB || As toxicity- Homeopathic Treatment
Effectiveness & Reliability - As Field Testing Kits || Utility Of Treatment Plant
Causes, Effects & Remedies - Groundwater As Calamity || References

Our study on
GROUNDWATER ARSENIC CONTAMINATION IN WEST BENGAL - INDIA (20 years study)

Reported work done by SOES

Brief background
Arsenic crisis in India dates back to as early as 1976 when a preliminary survey on arsenic in dugwells, hand pumps and spring water from Chandigarh and different villages of Punjab, Patiala, Haryana and Himachal Pradesh in northern India was reported in Lancet by Dr. D.V Datta 
Officially, arsenic poisoning in West Bengal was first diagnosed by a dermatologist K.C. Saha of School of Tropical Medicine (STM), Calcutta (Kolkata was previously known as Calcutta) to an outdoor patient of village Ramnagar of Baruipur police station in the district of South 24-Parganas on 6th July, 1983. Later it came out that many arsenic patients existed in many villages well before 1983 but they could not be clinically diagnosed, so were not highlighted. Garai et al. reported the first scientific paper published on Arsenic toxicity in West Bengal where he had warned of malignancy of the hyperkeratotic spots and liver if diagnosis is delayed. The School of Environmental Studies (SOES), Jadavpur University joined the arsenic work at the beginning of 1988.

In 1988 when we commenced arsenic survey in West Bengal, we knew about 22 affected (As > 50 µg/L) villages in five districts now according to our latest survey the number of affected villages increased to 3417 in 111 blocks in eleven districts excluding Kolkata because it has no blocks. During last 19 years with every additional survey we noticed an increasing number of contaminated villages and more affected people.

Summary of Groundwater arsenic contamination situation in West Bengal
Location and demography
West Bengal is one of the 29 states in India. It extends to the east longitude 85^o50^’E and 89^o50^’E and north latitude 21^o10^’N and 27^o38^’N. The area of West Bengal is 89193 sq. km having a population of about 80.1 million. Its administrative structure consists of several districts: each district has several blocks/police stations; each block has several Gram Panchayets (GPs), which are cluster of villages. There are 19 districts, 341 blocks and 37910 villages in West Bengal. We have covered all 19 districts covering 241 of 341 total blocks, and 7823 villages from West Bengal India.

Regional Geological setting
The State of West Bengal can be divided into four physiographic parts:

1. Districts of Darjeeling, Jalpaiguri and Koochbihar in the Himalayan Region.
2. District of Purulia and Western part of the districts of Barddhaman, Medinipur, Birbhum and northern part of Bankura occupy the eastern fringe of Chotanagpur Plateau.
3. Sundarban area of the South 24 Parganas and small part of North 24 Parganas form the Deltaic zone.
4. Remaining areas of the State being plains.

150-250m thick granular zone occurring as alluvial fans in the extreme northern part of West Bengal acts as the recharge zone for the unconfined aquifers with high permeability. This zone receives on an average 3000mm of rainfall annually. This granular zone gets separated in most of the areas by 2 to 10 m thick clay layers within a depth of about 300m where confined groundwater occurs. These aquifers at depth to the south of the fan zone are hydraulically connected to the recharge zone and contain groundwater mildly affected by arsenic. The recent flood plain deposits of Malda district, however, recorded high concentration of groundwater arsenic.
The subsurface geological picture of the southern part of West Bengal to the east of the Achaeans shield area is nearly similar to its northern counterpart except the absence of cobbles and pebbles in the sequence and the Pleistocene sediments covering almost one-half of the area to the east of the shield area. Eastward Holocene deltaic sediments that by nature are characterized by frequent change in facieses from sand to clay and vice-versa at short distances both laterally and vertically follow it. At the delta head located in Murshidabad and Nadia districts, 150-250m thick granular zone containing groundwater with high concentration of arsenic under unconfined condition occurs. It forms the recharge zone for the deeper aquifers down south. Like the Northern part, here also this thick granular zone gets separated by several clay layers, the thickness of which gradually increases southward. Beside, a clay layer appears at the top of the sequence with thickness gradually increasing southward from 2 to 30m precluding direct rainfall recharge to the group of aquifers below the top clay. These aquifers constitute the confined aquifer system receiving water from the recharge area to the north as well as to the west formed by the weathered sections within the crystalline rocks in the shield area.
In and around Kolkata beside the top clay layer, another 20-30m thick clay layers occurs at around 150m depth, the thickness of which increases to 50-60m further south. It is followed by alternating sequence of sand and clay layers down to a depth of about 300m.
In the delta and flood plains due to attenuation of intervening clay layers, the group of aquifers at depth gets interconnected at some places giving avenues for polluted groundwater to travel at deeper depths.

Overall contamination situation in West Bengal 
According to the latest available data, we analyzed 1,40,150 hand tubewell water samples for arsenic in all 19 districts covering 241 of 341 total blocks from West Bengal India. Table I shows an overview of arsenic contamination situation of West Bengal upto December 2005.
Table II shows the distribution of tubewells from each of the 19 districts of West Bengal. Out of 1,40,150 samples analyzed, 48.1% had arsenic above 10µg/L (the WHO guide line value) and 23.8% above 50µg/L (the Indian standard value). Importantly, 3.3% of the analyzed tubewells had arsenic concentrations above 300µg/L (the concentration predicting overt arsenical skin lesions (Rahman et al, 2001). A total of 187 (0.13%) hand tube-wells were highly contaminated (>1000 µg/L). The maximum arsenic concentration (3700 µg/L) was found in Ramnagar village of GP Ramnagar II, Baruipur block, in South 24 Parganas district. This tubewell was a private one and all the nine members of the owners’ family had arsenical skin lesions and seven of them who had severe arsenical skin lesions, had already died, five of them died within age range below 30 years.

Categorization of arsenic contamination in West Bengal
Figure I (map) sums up groundwater arsenic contamination status of all 19 districts of West Bengal. Based on the arsenic concentrations found in the 19 districts of West Bengal we have classified them into three categories: Severely affected, mildly affected, and arsenic safe.
Nine districts (Malda, Murshidabad, Nadia, North-24-Parganas, South-24-Parganas, Bardhaman, Howrah, Hoogly and Kolkata), where more than 300 µg/L arsenic concentrations were found in tubewells are categorized as severely affected. Out of 1,35,555 samples analyzed from these districts 67,306 (49.7%) had arsenic concentrations above 10µg/L and 33,470 (24.7%) above 50 µg/L.
The five districts (Koch Bihar, Jalpaiguri, Darjiling, North Dinajpur and South Dinajpur) where the contaminated tubewells show arsenic concentrations mostly below 50µg/L (only a few above 50µg/L but none above 100 µg/L), termed as mildly affected. We analyzed 2,923 water samples from these districts, 285 (9.8%) had arsenic concentration between 4 and 10µg/L, 163 (5.7%) above 10µg/L and 6(0.2%) above 50 µg/L.

Table-1: Present Groundwater Arsenic Contamination Status of West Bengal, India