Hydrogeo-morphological influences for arsenic release and fate in the central Gangetic Basin, India

Geochemical influences on arsenic (As) and other solutes along with regional groundwater flow path were inferred in parts of the central Gangetic Basin. The median concentration of As in groundwater was higher (0.046 mg/L) in Piedmont and (0.006 mg/L) in younger alluvium while very low (0.002 mg/L) in older alluvium. The median As concentrations in core sediments of Piedmont, older alluvium and younger alluvium were observed as 5.12, 11.2 and 11.6 mg/kg, respectively. Approximately 70% of the samples fell in As(OH)3 or As(III) field and 30% samples fell in HAsO2/4 or field of As(V) in Eh–pH plots. In contrast ∼60% of the samples fell in the FeOOH field and 40% in Fe(II) field in the Eh–pH plots. Cation exchange in Piedmont and older alluvium, while carbonate dissolution in younger alluvium controlling the groundwater major solute chemistry. The predictive outputs of inverse modelling congruence with predicted/ hypothetical groundwater flow directions. It was observed that geomorphology and the groundwater flow path also have substantial influence on solute chemistry and As distribution. Redox controlled adsorption/ desorption of Fe–Mn (oxyhydr)oxide was suggested as the probable mechanisms for As liberation in Piedmont, while multiple processes control the mobilization and presence of As in the younger alluvium. Study also concluded that the shallow aquifers of Piedmont and younger alluvium are contaminated with As while aquifers of older alluvium having low As contamination, could be the safe alternate for the residing people.