Naturally occurring arsenic in groundwater from the alluvial and deltaic aquifers of Bangladesh is one of the greatest natural disasters in the world. Since 1980s, groundwater in Bangladesh has been extensively exploited for drinking and irrigation purpose. About 6 million “tube wells” tap variably As-contaminated groundwater throughout the country.

This paper explains the spatial and vertical variations of arsenic occurrence in a small drainage basin in central Bangladesh. Concentration of arsenic in 26-analysed groundwater samples from the study area range from <0.001 to 0.24 mg/L. Concentrations distribution reveal variations of arsenic occurrence both spatially and vertically. Finer-grained sediments with high organic matter in areas of sluggish groundwater movement appear to be enriched in arsenic. On the other hand, coarser sediments with less organic matter and active groundwater flushing produce lower arsenic concentrations. Arsenic in groundwater is found only in Holocene alluvial aquifers. Depth profiles of arsenic concentrations show that arsenic occurrence generally higher at intermediate depth (15 to 175 m), and lower at shallow (<15 m) and deeper (>175 m) depth intervals.

Hydro-chemical relationships observed in the study area supports the reductive dissolution of hydrous ferric oxides (HFO) hypothesis for the release of arsenic in groundwater. For example, arsenic concentration shows a good relationship with dissolved iron under reducing condition. Negative correlation of arsenic with dissolved oxygen, nitrate and sulfate, but positive correlation with bicarbonate also implies the important role of microbial-mediated biogeochemical processes. Under aerobic conditions Fe (III) and Mn(IV) oxides remove and concentrate As from water, but subsequent development of anaerobic conditions can reverse this process, leading to the production of groundwater with high levels of As. This has an important implication for removal of arsenic from the groundwater. For example, under progressively more reducing conditions, arsenic can be removed by means of co-precipitation as iron-sulfides.

Variations of arsenic occurrence in the groundwater reveal the complex interplay of local hydrogeology, sedimentology, and geo-microbiologic processes, which have been commonly observed in deltaic regions throughout the world.