GEOCHEMICAL AND HYDROLOGIC MODELING OF ARSENIC ENRICHMENT IN THE COASTAL AQUIFERS OF THE GANGES DELTA

Arsenic contamination in the shallow groundwater of Bengal Basin is essentially confined to the early-mid Holocene deltaic sediments. Holocene sea level rise and high sea level stand created sluggish hydrologic flushing and reducing conditions, contributing to enrichment of arsenic in the groundwater. By contrast, arsenic levels in deeper sediments deposited during Pleistocene glaciation are much lower due to faster hydrologic flushing in response to low sea level stand. Hydrologic simulations show that initial arsenic loads in aquifers may be completely flushed out in a few thousand of years during low sea level stands. Geochemical modeling indicates significant desorption of As(V) from oxides in response to increasing pH (> 8.5) or ionic competition during saltwater intrusion. Ferric oxyhydroxides may dissolve and release sorbed arsenic under moderate reducing environments.

Unlike areas in central Bangladesh, groundwater arsenic concentrations in the areas of study do not show significant correlation with Fe or Mn, suggesting that its mobilization in coastal areas may not be controlled by bacterial iron or manganese reduction alone. Arsenic rich groundwater tends to be found in areas where high concentration of Fe and Mn minerals with their absorbed and coprecipitated As have accumulated in the sediments. This accumulation occurs as a result of natural sedimentation processes at the saltwater intrusion interface in which high As groundwaters are concentrated. Elevated groundwater arsenic concentrations (up to 553 ppb) are currently only found in low salinity shallow aquifers within the study area, however saltwater intrusion and competition processes may lead to arsenic desorption from the deeper aquifier sediments. These results indicate that our monitoring and modeling efforts in the coastal regions of the Ganges Delta could prove to be useful in predicting areas susceptable to deteriorating water quality resulting from increasing salinity and arsenic content.