Identification of causes of arsenic contamination in graoundwater of Bangladsh

Abstract

Arsenic contamination of groundwater in shallow aquifers has become a scvere water supply and hcalth problem in many parts of Bangladesh. A national survey has idcntified 1.44 million tubewell contaminated with arsenic exceeding the Bangladesh drinking water standard of 0.05 mg/L and 38,420 cases of arsenicosis patient. In this study, groundwater, river water, suspended sediment of the river water, riverbed sediment, and floodplain sediment of three rivers (Jamuna, Padma and Meghna), and borehole sediment of alluvial plains of five arsenic affected districts of Bangladesh were collected and studied in an effort to identify sources of arsenic and mechanisms of its mobilization in the subsurface. Arsenic content in river water, suspended sediment, riverbed sediment, and floodplain sediment of three rivers - Jamuna, Padma and Meghna was analyzed. Arsenic was found in suspended sediment at concentrations comparable to average arsenic content in soils in Bangladesh, but very little arsenic was found in the river water samples. The sediments were fractionated into sand, silt and clay fractions and the arsenic content in each of the fractions was determined. Highest concentration of arsenic was found in clay fraction of the sediment, followed by silt, with lowest concentration in sand. The arsenic content of fine-grained materials like clay is higher because they have higher surface area as well as adsorption sites per unit volume/weight. The beels and haors receive higher quantity of Arsenic as the clayey materials are deposited in these stagnant water bodies. It has been found that the districts with beel and haors and low-lying areas are severely affected by Arsenic contamination of groundwater. An estimated 11,000 metric tons of arsenic is transported through Bangladesh each year from upstream with 2.4 billion tons sediment; a large fraction of this arsenic however is discharged in the Bay of Bengal with the sediment. The suspended sediment and river bed sediment may be an important source of arsenic in Bangladesh. Arsenic concentration in the floodplain sediments of three rivers - Jamuna, Padma and Meghna as well in the borehole sediments of alluvial plains of Sylhet, Chandpur, Munshiganj, Kushtia and Nawabganj were analyzed. Arsenic concentrations of these sediments have been found to be comparable to average arsenic contents in soils of Bangladesh. Arsenic present in different chemical forms in the sediment samples from floodplain and alluvial plain were analyzed by sequential extraction method. Relatively small amount of Arsenic was found to be present as ionically bound and strongly adsorbed forms. Comparatively a larger amount of Arsenic was found to be associated with amorphous iron oxyhydroxides. However, there are variations in the chemical forms of Arsenic associated with sediment, depending on the geological history of the sediment. The Arsenic present in the sediments as ionieally bound (IB), strongly adsorbed (SA), and associated with amorphous iron oxydydroxides (Ala) accounts for significant quantities of total arsenic, which could be mobilized under reducing condition. Arsenic and other relevant water quality parameters of groundwater samples were determined. Arsenic concentration was found to be positively correlated with ammonia, alkalinity and hardness of groundwater. This relationships support the process of Arsenic mobilization with the microbial breakdown of organic mailers in the sub-soil environment. Arsenic was also found to be inversely correlated with oxidation-reduction potential (ORP) of groundwater. The phase stability diagrams show that the eH-pH values of groundwater samples in the arsenic affected areas lie in the iron and arsenic soluble ficlds of the diagram. It indicates that arsenic and iron in the groundwater is likely to remain in soluble arsenite and ferrous form. The presence of soluble iron and arsenic (1II) in fresh groundwater supports the findings. The presence of very low sulfate concentration found in the groundwater is also consistent with a reducing environment, and negates Arsenic mobilization as a result of oxidation of Arsenic bearing minerals. Arsenic release from sediment was studied in the laboratory by simulating anaerobic reducing environment prevailing in aquifers in arsenic affected areas by introducing organic carbon in the form of glucose and molasses in closed reactors. Significant quantities of Arsenic were released in aqueous phase from sediment under anaerobic reducing condition at low Eh while no arsenic release was observed in control batch where enough oxygen was allowed to enter into the reactor to maintain aerobic condition. Arsenic was released from the sediments only when Eh values were lowered to less than zero. The study demonstrated that bacteriamediated reducing state was a requirement for the release of arsenic from sediments.