Some groundwater samples of Bengal Basin contain 10 microgram per liter or more As. I am interested to know those minerals which release arsenic to the groundwater.
The arsenic concentrations in the soils are found to be higher than in the aquifer sediments, with concentrations as high as 40ug/g at the surface in Bangladesh The provenance of groundwater As of Bengal basin may be hypothesized to be sourced to the Himalayan orogenic belt, where the contaminant might have originated by deep - seated tectono - magnetism and subsequently introduced to the superficial system by exhumation. Later sedimentary processes transported the As - laden sediments from the orogenic belt to the peripheral foreland basin of of Bengal where As is released from solid phase to the circulating groundwater.For more details consult https://link.spinger.com
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 oxidizing conditions, the arsenic is immobile and remains sequestered in the iron hydroxides. Also, due to the activities of microbes, closely associated to the groundwater sources, the release of arsenic from the above said minerals happens frequently. The microbial oxidation of those minerals resulted in releasing Arsenic bound to those minerals and get dissolved in groundwater and finally contaminate the surface soil after continuous extraction.
Dear Sir (Mr. Arnab Majumder),
With due respects, I am pleased to express my thanks to you for your immediate response. I do agree with your opinions. My next query is that why the arsenic concentration in groundwater near the Himalayan orogenic belt is uneven. I will highly appreciate if you can add some cross reference please.
With best regards,
Sudip Saha
Dear Sir (Professor Dr. Sylvester Peter Antai),
I would like to convey my respects to you. I am pleased to express my gratitude to you for your immediate response.
Thanking you Sir,
Sincerely yours,
Sudip Saha
Dear Sudip,
To my knowledge, the rock mineral compositions are different than the soil underground, closely associated to the groundwater aquifers and hence the dissolution of parent rock minerals at the lower Himalayan belt differs according to its groundwater extraction. The chances of underlying rock layers creates confined aquifers and drilling through those layer might create some oxygenic channel via which the redox change might happen to the arsenic bound minerals. further extraction of water resulted in dissolution of arsenic in the aqueous phase. But the bed rock formation under the soil Horizon of A to Horizon B differs at different foothills of Himalaya. This resulted different rate of mineral dissolution and thus different arsenic concentration at different places.
For some extra distributional information Please go through this-
Bengal arsenic, an archive of Himalaya orogeny and paleohydrology
Arsenic is a Chalcophile element. It is spread in surface environments at low concentrations, and its concentration in the upper continental crust is 1.5 ppm. Arsenic is distributed in different minerals. Around 60% of arsenic minerals in nature are estimated to be Arsenates, 20% as Sulfates and sulfide salts . and the remaining 20% are distributed in Arsenides, Arsenites, Oxides, alloys and polymorphs of elemental arsenic. Arsenic is found in low concentrations in most igneous and metamorphic rocks. In sedimentary rocks, the concentration of arsenic varies considerably. Arsenic is concentrated in clays and fine-grained sediments, especially those containing sulfides that are rich in organic matter, secondary iron oxides and phosphates. Arsenic enters the aquatic environment easily as arsenic is highly mobile in weathering environments. Its concentration in natural waters varies depending on the availability of arsenic sources in the region and the geochemical conditions of the aquatic environment.
Classical college Textbook examples are Realgar, Orpiment, Arsenopyrite
( Chemistry 2nd Paper, Sanjit Kumar Guha, Lecture Publications, Dhaka, Bangladesh)
Please go through the following RG links.
Article A Review of Groundwater Arsenic in the Bengal Basin, Banglad...
Article Nature of arsenic pollutants in groundwater of Bengal basin-...
Not all of us are from Bangladesh, so do not expect many answers or contributions to your question. The bulk of the answers you could get by simply going on Google, please.
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