Abstract
In order to better understand the mobilization of arsenic to form arsenic polluted groundwater, geochemical and mineralogical investigations have been carried out on the continuous core samples (up to 90.1 m depth) at transitional area of Holocene and Pleistocene deposits at Sonargaon of Narayanganj district, Bangladesh. Lithologial characteristics and geochemical data demonstrated that elevated levels of total arsenic (max. 53.4 mg/kg) are found in organic-rich fine-grained silty-very fine sand rather than those of coarser sandy sediments. Arsenic does not show significant correlation with amount of clay fractions. Weakly positive correlation between As and total iron is observed in the upper aquifer (< 28.6 m) of Holocene and lower aquifer (38.5 - 90.1 m) of Pleistocene sediments, while the samples from the middle aquitard (28.6 - 38.5 m) of Pleistocene sediments do not give any correlation. As shows a good positive correlation with Al2O3 throughout the sediment column. Relationship between As and TOC in the middle aquitard sediments shows a very good positive correlation, on contrast, no distinctive relationship can be found in the samples from the upper and lower sand units. Sequential extraction of arsenic exhibits that As is mainly fixed in (1) organic and (2) sulfide and silicate phases in all of the samples. As concentration of the sediments is in accordance with concentrations of mica, Al2O3, Fe2O3, but not well with particle size, indicating that the biotite is a primary source of As in the studied area. Vertical flow of groundwater along well pipes and/or those surroundings accelerates the chemical weathering of this mineral to release As into the aquifer and persistently exists near the bottom of upper aquifer, because the groundwater above the thick confining layer is probably stagnant and has not been experienced sufficient flushing until present.
