The story unfolds some 20 million years ago when the Indian Plate began to collide with the stable Asian Plate. The collision produced heat and as an outcome some rocks were enriched in arsenic. The collision also produced the loftiest and the youngest mountains in the world, the Himalayas. Naturally slopes were formed and it is common sense that water flows in the direction of slope. Water from the mountains began to flow down the slope. The rivers thus developed began to flow from the Himalayas towards the sea.
The Ganga and the Brahmaputra are two major rivers which drain a major part of Himalaya and Tibet and culminate in the Bay of Bengal, forming the giant Ganga-Brahmaputra Delta (GBD). These rivers painstakingly carry grains of rock produced by their erosion prowess to the Indian Ocean. During monsoon the carrying capacity of the rivers rise many fold. As the rivers become sluggish on reaching their destination ' the Sea, a large part of these grains is deposited making the flood plains. Over the millennia the nature has been busy creating layers of the transported grains on the plains. These grains are the repositories of our precious drinking water.
Unfortunately some layers of these rock grains contain various forms of arsenic. It is still more unfortunate that in some tracts like West Bengal and Bangladesh this arsenic is released to mingle with the water content. Water from these when tapped for drinking cause arsenic toxicity amongst humans, animals and even flora including crops.
Vast tracts of west Bengal are in the grip of arsenic toxicity. Not only humans, arsenic has not spared even the animals and the crops. Whosoever consumes water from arsenic afflicted tube wells becomes a victim. Unfortunately the polluter of ground water is no other but the nature. It has been established that the water from the tube wells in the depth range of 25-50m has the maximum arsenic content, with values exceeding 500 parts per billion (ppb). The maximum permissible safe limit of arsenic in drinking water is 50 ppb. In order to overcome the problem deeper tube wells were drilled to a depth of 120-140m. Alas, over a period of time it was found that in the affected area water from about 30-40% of these tube wells had an arsenic content of more than 50 ppb.
Arsenic poisoning is one of the worst natural environmental hazards being faced by the people of West Bengal and Bangladesh. In order to save the population the technologists have come-forth with various filters, which are mostly based on surface techniques. The problem is so widespread that people are forced to use costly filters at the community and or domestic level.
In 2004 Debkumar Bhattacharyya, P.K. Mukherjee, A.K. Ray and S. Sengupta of Geological Survey Of India (GSI), Kolkata carried out a survey and came out with an interesting report published in Current Science. They have recommended use of clay-candle domestic water filter. Their contention is based on the fact that water pumped for drinking is exposed to atmospheric oxygen. This causes oxidation of soluble ferrous iron oxide to Fe(III) oxy-hydroxide and arsenic species to As(V). This species of arsenic is less soluble and gets quickly adsorbed on to amorphous Fe(III) hydroxide and turns in to suspended particle in water filtered by the clay-candle domestic filter. In their study carried out in Nadia and 24 Parganas districts they found that in 62% of water samples studied by them more than 60% of arsenic is removed making water safe for drinking.
More recently Taraknath Pal and Pradip Kumar Mukherjee of GSI, Kolkata have come out with quite an innovative solution of filtering arsenic which eliminates the need of an external filter. Their study published in January 2008 issue of Current Science is based on identification of a particular aquifer in the sub-surface that has arsenic content lower than the permissible limit.
The Nature has taken nearly 1.75 million years to deposit layers of sand and clay brought by the rivers Ganga and Brahmaputra in their flood-plains in west Bengal. Compared to these the sediments deposited by the river Damodar are slightly older. Floods are an annual feature and huge quantities of sand of varying grain sizes and clay are brought by the rivers and deposited. They are deposited in a cyclic manner with the coarsest grains being at the base and the finest clays at the top of the cycle. Hundreds of such cycles are present in the subsurface and each coarser layer is laden with water.
The water laden grey sand layers or the aquifers occur from a depth of 20 to 50m, a depth commonly tapped by the people for drawing water. Government owned community tube-wells tap deeper grey sand layers enriched with organic matter at depths of 60-140m. These grey sand layer aquifers are unfortunately rich in arsenic too, the root cause of widespread toxicity in the region. What perplexed Pal and his colleagues was that aquifers in the flood plains of Damodar River valley are generally free from arsenic. The aquifer sand of Damodar flood plain and its equivalent in Bangladesh is known as orange sand, because of its color.
Arsenic toxicity is a matter of grave concern and the scientists are leaving no stone turned to find a way out. In order to find out the characteristics of the aquifer sand and their arsenic content etc 140 boreholes were drilled between Baruipur in the south (near Sealdah) to Malda in the north in west Bengal.
It was a chance Pal and his co-workers noticed from the borehole data of these wells what others had missed. They observed that a 5-10m thick orange sand horizon occurs in the Ganga and Brahmaputra flood plains at a depth of 40 to 50m sandwiched between two grey sand bearing aquifers. When traced it was established that it is the same horizon that occurs as orange sand in Damodar flood plains. A unique feature of this horizon between Baruipur and Malda is that it has a layer of brown hard clay at the top and a layer of bluish grey hard clay with little pore water at the bottom.
The sand grains of the orange sand are stained with iron-oxy-hydroxide indicating that in the geological past the sand grains brought by the river were exposed to the Sun continuously for hundreds of decades. The exposure caused oxidation which imparted a yellowish-brown color to the grains. In contrast the grey sand is rich in mica and organic matter. Mica flakes shine in the Sunlight and often people refer it as 'silver sand'.
Taking clue from Damodar flood plain, Pal and his co-workers worked on a hunch that the orange sand horizon of Ganga and Brahmaputra flood plains might yield arsenic free water! They chose Ghentugachi and Gotra villages in Nadia district where arsenic toxicity is at its peak. People are dying in these villages because of this menace they report. Four tube-wells were drilled in these villages in 2006 and water was drawn from the orange sand aquifer only. Surprisingly the arsenic content of water from this horizon is much below the permissible limit, compared to grey sand, water from which literally spews arsenic.
Yet another interesting observation is that the arsenic content of the sediments in orange sand is higher but the process of release of arsenic from sediments in to water has not yet started in this horizon they state. This is because the orange sand is protected from developing reducing conditions responsible for the release of arsenic from the sediments to ground water.
Since the orange sand is sealed with clays at the top and bottom the possibilities of intermixing of arsenic rich waters from over or underlying grey sand aquifers are minimized.
The four tube wells drilled in Ghentugachi and Gotra villages were periodically monitored for more than a year and it was found that the arsenic content of water remained within safe limits. The villagers there have already started consuming water from these wells and have benefited.
The orange sand thus could act as a natural filter for arsenic struck state of West Bengal and Bangladesh as well. Since the sand occurs at a much shallow depth tube wells drilled to tap it will remain cost effective and save the cost of surface filtration says Pal.
The study of Pal and his colleagues though in a primitive stage speaks volumes about the future possibilities. Identification of safe aquifers and tapping them is perhaps the best way to keep the spread of arsenic toxicity under control. A word of caution however, is that water quality from such tube wells has to be periodically examined so that there is no chance of arsenic surreptitiously sneaking in to safe aquifers.