The problem of water pollution with arsenic is maximum in

Concept

Arsenic reaches drinking water mainly by natural geological release, not by industrial discharge. In young, water-saturated river alluvium the arsenic held on iron-oxyhydroxide coatings of the sediment grains is set free into the groundwater once the aquifer turns reducing (oxygen-poor). Shallow tube wells sunk into such Holocene alluvial aquifers can therefore yield water above the drinking-water limit of 0.01 mg/L (10 ppb) laid down by WHO and by BIS IS 10500, and long exposure produces arsenicosis — pigmentation, keratosis and cancers.

Application

The stem asks where this contamination is greatest in India, so the deciding factor is where the alluvium is thickest, youngest and most strongly reducing. That is the Ganga–Brahmaputra–Meghna delta, and the Bengal basin lies at its heart. Groundwater arsenic poisoning on a mass scale was first documented in India here in the early 1980s, and this region still carries the largest arsenic-affected population and the widest spread of affected districts — Murshidabad, Nadia, North 24 Parganas, Malda, Bardhaman and Howrah among them.

Contrast

• The middle Ganga plain of Bihar (Bhojpur, Buxar, Bhagalpur) is affected too, but the belt is narrower and hugs the river meander zone upstream of the delta.

• Occurrences in Uttar Pradesh lie further upstream still, in eastern Ganga-side districts such as Ballia and Ghazipur.

• Madhya Pradesh rests on Deccan basalt and Vindhyan hard rock rather than young alluvium, so its characteristic groundwater-quality problems are fluoride and nitrate.

Conclusion

The arsenic water-pollution problem is therefore at its maximum in West Bengal.