GROUNDWATER ARSENIC DISTRIBUTION ALONG AN EAST-WEST TRANSECT, CENTRAL BANGLADESH

Groundwater in alluvial aquifers in Bangladesh and many other deltaic areas around the world are affected by elevated dissolved arsenic. Geochemical analyses of groundwater aquifers in the central Bangladesh provided interesting results on groundwater Arsenic distribution along an approximately East-West transect through the central Bangladesh.

The groundwater arsenic content has been examined along transects close to an effluent stream in Narayanganj, east-central Bangladesh. Arsenic concentrations vary significantly along groundwater flow path based on their geomorphic features and local geology. In case of a transect away from the stream, the arsenic level ranges from 0 to 0.124 mg/L whereas for another transect that runs parallel to the stream ranges from 0 to 0.197 mg/L. For the latter, one sampling point found to be arsenic hotspot (0.197 mg/L) compared to adjacent areas at that depth (25 ft) that are safe in terms arsenic contamination (~0 mg/L). It is not obvious yet whether the irrigation well next to that sampling location had any influence on this high value of Arsenic content.

ICP-OES data from pore fluid samples collected along the east-west transect, from Chandpur to Chapai Nawabganj districts suggest considerable variation of groundwater arsenic concentration within uppermost aquifers, and also in upper confining units in a basin-wide scale. The sampling plan was to find the influence, if any, on sedimentation of three major rivers (the Ganges, Brahmaputra, and Meghna) on groundwater Arsenic concentrations. The upper aquifer, which starts at 8 feet 7 inches, was safe whereas the widely exploited aquifer by the locals in Kustia (west) was found unsafe (0.231 mg/L) at a depth of 120 feet. The arsenic content was within acceptable limits in Chapai Nawabganj, Natore (contains oxidized host sediments), Shirajganj, and Narayangaj (east) (in As safe areas). Strikingly, the arsenic concentrations are found to be higher than the Bangladesh allowable limit (>0.05 mg/L) in pore fluids of upper confining units in Kustia (west) and Chandpur (east) districts at 4 feet and 5 feet respectively.

Ongoing work on composition of host sediments of groundwater may reveal additional information on Arsenic distribution pattern in Bangladesh.