Mobilization of High Arsenic in the Shallow Groundwater of Kalaroa, South-Western Bangladesh

Most of the shallow aquifers (<50 m) of the Southwestern Bangladesh have been producing highly arsenic (As)-contaminated groundwater. This paper aims to provide an insight into the possible causes of high As occurrence along with the geochemical processes controlling the mobilization of As into the groundwater of Kalaroa. After performing a set of investigations including laboratory analysis, statistical analysis, and speciation calculation, it was found that the shallow aquifer (<50 m) has been badly affected with high As (i.e., 231 µg/L) relative to the deep one (>130 m) that still remains safe from As contamination (<1 µg/L) although the latter one produces Na–Cl−-type water. High EC (3950 µS/cm) found in deep groundwater also supports the salinity problem encountered there. Intermediate aquifer (50–78 m), on the other hand, mostly produces Ca–Mg–HCO3−–Cl−-type (mean As 75.1 µg/L)-contaminated water. Concentrations of groundwater parameters of both shallow and intermediate aquifer were, however, observed to be mostly affected by seasonal variation such as getting concentrated in dry period (mean As, 118.10 µg/L, shallow; 70.7 µg/L, intermediate) due to the water shortages and diluted in wet period (mean As: 96.03 µg/L, shallow; 56.35 µg/L intermediate) due to the recharge from the infiltration of surface runoff. In contrast to this, parameters in deep groundwater (>130 m) were reported as stable (mean As: wet period, 0.43 µg/L; and dry period, 0.34 µg/L), since it get less influenced by the discharge- and recharge-related seasonal variations. Speciation calculations with PHREEQC show that Siderite and Vivianite have reached the supersaturated state indicating the possibility of controlling Fe solubility by these two minerals. Low NO3−, SO42−, PO43−, and high Fe concentrations observed in the shallow groundwater reflect the strong reducing environment of the shallow aquifer. The redox buffering could have controlled by the Fe(OH)3/Fe2+ redox pair. Laboratory leaching test confirmed that HCO3− ion plays a vital role in releasing of As along with Fe and Al from As-affected sediment. SEM and XRD analysis indicated the presence of Fe-Al oxyhydroxide mineral in the shallow aquifer sediment. Strong correlations were observed between As and Fe (R2 = 0.57 and 0.7 for wet and dry seasons, respectively), HCO3 (R2 = 0.65 and 0.74 for wet and dry seasons, respectively), Al (R2 = 0.62 and 0.72 for wet and dry seasons, respectively), and a moderate correlation with DOC (R2 = 0.39 and 0.48 for wet and dry seasons, respectively) in aquifer groundwater (<80-m depth). Based on our findings, it can be concluded that mobilization of As is due to the organic carbon instigated dissolution of Fe–Al oxyhydroxide mineral in shallow aquifer. Villagers who used to utilize this contaminated groundwater for drinking were identified to be affected with different skin complications subsequently.