Explanation:

• In India, seven states namely- West Bengal, Jharkhand, Bihar, Uttar Pradesh in the flood plain of the Ganga River; Assam and Manipur in the flood plain of the Brahamaputra and Imphal rivers and Rajnandgaon village in Chhattisgarh state have so far been reported affected by Arsenic contamination in groundwater above the permissible limit of 10 µg/L
• Arsenic is an odorless and tasteless metalloid that's widely found in the Earth's crust. Arsenic and its compounds are found in three forms - crystalline, powder, and amorphous or vitreous forms. Trace amounts of it can be found in rocks, soil, water and even in the air we breathe. It is the 26th most abundant element.
• Because of arsenic's hygroscopic nature, it is readily soluble in water, forming an acidic solution - Arsenic acid. This acid is a weak acid and the salts are called arsenates.
• In Bihar, India, the source of the arsenic in the groundwater came from the Himalayas, washed down in the form of arsenopyrite, a conjugate of arsenic and iron. It settled in the riverbeds along the Hindu-Gangetic River basin as silt. And no, it didn't happen last year or even several years ago.
• And over the centuries, as rivers changed course and human settlements sprang up on the floodplains, people used the river water for drinking and irrigation. However, in more recent times, bacterial contamination has complicated the whole arsenic contamination issue.
• Another reason is the growing dependence on well water. But that couldn't explain the fact that some wells had been in use for centuries. One explanation could be the excessive amounts of water from wells being used during the dry seasons.
• Excessive withdrawals now lower the water table during the dry season, exposing arsenic-bearing minerals to air, which converts normally insoluble salts to soluble oxides. Then, during the next rainy season, as the aquifers refill with water, the soluble arsenic oxides mix with it and are pumped out in the drinking water. Almost 90 million people are drinking this water.
• In the Ganges Delta, the affected wells are typically more than 20 meters and less than 100 meters deep. Groundwater closer to the surface typically has spent a shorter time in the ground, therefore likely absorbing a lower concentration of arsenic; water deeper than 100 m is exposed to much older sediments which have already been depleted of arsenic.
• The arsenic found in the basin is most certainly man-made, even though natural levels do occur, the amounts reported by the research team are indicative of pesticide use and mining operations.