Geochemical controls and future perspective of arsenic mobilization for sustainable groundwater management: A study from Northeast India

• Sulphide oxidation enhances As mobilization in the drier months.
• Phosphate fertilizers play a minor and localized role in As enrichment.
• As in soil samples is majorly associated with fraction III and IV.
• As and Fe showed negative correlation due to siderite (FeCO3) deposition.

Groundwater and soil sampling was conducted in Jorhat district of Assam, India during monsoon (June 2013) and post-monsoon (January 2014) seasons to observe the geochemistry of groundwater arsenic. The hydrochemistry of the region is dominated by silicate and carbonate weathering. Redox potentials (ORP) were found to be mostly reducing and indicated the presence of a mixed aquifer system. Pyrite oxidation appeared to be one of the pathways of As mobilization during the drier periods, especially pre-monsoon. Close correlation between As and PO43− in both seasons implied that both contaminants may share similar adsorption sites and may thus be co-released. Iron (hydr)oxides appear to be the common source of As and PO43−, especially during the post-monsoon season, when conditions are more reducing. Isolated incidences of PO43− fertilizer usage in the tea growing areas could also account for the positive correlation between As and PO43− in the Jorhat district. Results of the sequential extraction showed that Fe (hydr)oxides yielded a high amount of As, indicating the role played by reductive hydrolytic processes. However Fe may behave non-conservatively in the groundwater, leading to its poor correlation with As. Speciation modeling using Visual MINTEQA2 v 3.1 revealed that the groundwater of the region is under-saturated with respect to As and a number of other mineral phases. This knowledge combined with the fact that the high As levels were detected in patches reveal that the As contamination in the region is in its initial stage and will increase in the near future. Reduction in groundwater abstraction for irrigation can prevent the contamination of deeper aquifers due to drawdown. Surface water use in the Jorhat district can be enhanced for sustainable management of groundwater resources.