STRATIGRAPHIC AND GEOMORPHIC CONTROLS ON ARSENIC DISTRIBUTION IN THE BENGAL BASIN, BANGLADESH
A positive correlation of arsenic with iron and bicarbonate, and negative correlation with sulfate, nitrate and dissolved oxygen, suggest that arsenic is released to the groundwater in a reducing environment. It is inferred from mineralogical and geochemical studies that these reducing conditions are developed due to biogeochemical reactions triggered by bacteria in organic-rich alluvial sediments. We suggest that distinct geomorphic and stratigraphic settings control these biogeochemical reactions.
Arsenic is found in groundwater from only Holocene alluvial aquifers; groundwater tapped from older Plio-Pleistocene sediments is free from arsenic. Arsenic concentrations vary within alluvial sediments in different geomorphic regions. Highly contaminated areas include the Ganges delta plains in the southwest, the Meghna flood plains in the east and southeast, and the active delta plains in the south. Arsenic contamination is relatively lower in the Brahmaputra flood plains in the northern region. The Himalayan piedmont plain in the northwest (the Teesta Fan) is found to be arsenic free. Finer sediments with high organic matter in areas of sluggish groundwater movement appear to be the arsenic hot spots. On the other hand, coarser sediments with less organic matter and active groundwater flushing yield arsenic-safe aquifers. Depth profiles of arsenic concentrations show that arsenic is generally higher at intermediate depths (15-175 m) and lower at shallow (<15 m) and deeper (>175 m) levels.
This study is limited by non-availability of consistent data from all the geomorphic provinces. Further systematic studies would assist in mitigating the problem by providing key information in locating arsenic-safe aquifers.