Following Priyanka Pulla’s excellent article in The Hindu about the alarming arsenic levels in the groundwater under West Bengal and Bangladesh, it’s worth elaborating on the geology part of the story.
The Holocene age began about 12,000 years ago and continues today. The Pleistocene age began about 2.5 million years ago and ended when the Holocene began. Now, why are arsenic levels much higher in the shallower Holocene age aquifers than in the deeper Pleistocene age aquifers?
The ultimate source of arsenic in the groundwater are the high Himalayan rocks and the Indo-Burman ranges. Minerals like biotite, magnetite, ilmenite, olivine, pyroxene and amphiboles contain arsenic. When they get weathered in the catchment area and in the deposits in alluvial plains, they release arsenic.
This arsenic is absorbed by secondary minerals, such as iron hydroxides like goethite. Under oxidising conditions, the arsenic is immobile and remains sequestered in the iron hydroxides. However, when these sediments encounter organic-rich reducing conditions, the bacterial reduction of iron releases arsenic into groundwater.
During the Pleistocene age, the high Himalayas were glaciated. Important source of arsenic, like the Indus ophiolite belt and the high-grade gneisses, were covered in ice and couldn’t release sediments. As a result, a lesser amount of arsenic made its way onto the alluvial plains.
Since the climate then was also cooler and drier, less organic material accumulated in sediments of the alluvial plains. The conditions were oxidising and so the arsenic remained sequestered in iron hydroxide minerals.
Next, the sea level was also much lower, so almost the entire continental shelf was dry land. The Ganga and the Brahmaputra rivers met the sea much more to the south than they do today. Similarly, reducing environments like the delta-front marshes, ponds and estuaries were also located much further to the south.
Then, sedimentary conditions changed about 12,000-15,000 years ago. The glaciers melted and exposed arsenic-bearing rocks in the high Himalayas, and more arsenic made its way on to the alluvial plains. The sea level also rose, and water flooded the continental shelf.
As a result, the reducing environments of the Pleistocene delta-front were drowned, and shorelines shifted northwards. The climate was warmer, encouraging the growth of vegetation. Reducing delta-front environments like swamps, coastal marshes and lakes developed on former alluvial plains.
This map shows the position of shorelines between 7,000 and 4,000 years ago, along with the location of wells with high levels of arsenic. The study that this map is from focuses on Bangladesh, but similar conditions in West Bengal as well. (It was published in October 2007.) You’ll notice the clustering of wells with higher amounts of arsenic along the older shorelines. Organics-rich delta-marshes and swamps developed here. The bacterial reduction of iron hydroxides released arsenic into the groundwater.
The distribution of arsenic is also correlated with topography. The map below shows high arsenic levels in groundwater coinciding with topographic lows (i.e. depressions in the land). Such low-lying areas accumulate more fine sediments and organic material. Similar processes apply to the West Bengal portion of the land as well.
So, a change in climate and sea levels from the Pleistocene to the Holocene exerted strong control on the distribution of arsenic in the alluvial plains of Bangladesh and West Bengal.