ABSTRACT
ABSTRACT: This study presents the results of the investigation of arsenic (As) enrichment in groundwater of three alluvial aquifers at theBengalDelta Plain (BDP) in Sonargaon inNarayanganj, Chandina in Comilla, and Sirajdikhan in Munshiganj districts in South-central Bangladesh. Water samples were collected from these sites from wells with screens placed at different depths and the hydrogeochemical characteristics and redox status were determined. The highest DOC and HCO−3 concentrations were found at Sirajdikhan site and lower concentrations at Sonargaon and Chandina sites. In contrast, the highest NH+4 concentrationswere found at Chandina site and concentrations at other sites were much lower. The correlation between dissolved As and Fe was high at Sirajdikhan and Sonargaon sites, but not at Chandina site. Also, at Chandina site dissolved Mn concentrations were low, suggesting that Mn(IV) redox buffering step was missing. Speciation modeling indicated a possibility of siderite precipitation at all sites, but precipitation of rhodochrosite only at Sonargaon and Sirajdikhan sites. Calculated log PCO2 were very high (reaching −1.37 at Sirajdikhan site), suggesting production of CO2 in redox processes. The hydrogeochemical trends and modeling results suggest that dissolved As may be de-coupled from dissolved Mn, when Mn(IV) content in solid phase is low (or when released As is re-adsorbed) and from dissolved Fe (when precipitation of Fe(II) minerals controls Fe concentrations). Furthermore, several redox processes may operate simultaneously, depending on kinetic constraints and refractoriness of Fe(III) minerals.
