Role of Quaternary stratigraphy on arsenic-contaminated groundwater from parts of Barak Valley, Assam, North–East India

Groundwater arsenic survey in Cachar and Karimganj districts of Barak Valley, Assam shows that people in these two districts are drinking arsenic-contaminated (max. 350 μg/l) groundwater. 66% of tubewells in these two districts have arsenic concentration above the WHO guideline value of 10 μg/l and 26% tubewells have arsenic above 50 μg/l, the Indian standards for arsenic in drinking water. 90% of installed tubewells in these two districts are shallow depth (14–40 m). Shallow tubewells were installed in Holocene Newer Alluvium aquifers are characterised by grey to black coloured fine grained organic rich argillaceous sediments and are mostly arsenic contamination in groundwater. Plio-Pleistocene Older Alluvium aquifers composed of shale, ferruginous sandstone, mottle clay, pebble and boulder beds, which at higher location or with thin cover of Newer Alluvium sediments are safe in arsenic contamination in groundwater. 91% of tubewell water samples show significantly higher concentrations of iron beyond its permissible limit of 1 mg/l. The iron content in these two districts varies from 0.5 to as much as 48 mg/l. Most of the arsenic contaminated villages of Cachar and Karimganj districts are located in entrenched channels and flood plains of Newer Alluvium sediments in Barak-Surma-Langai Rivers system. However, deeper tubewells (>60 m) in Plio-Pleistocene Older Alluvium aquifers would be a better option for arsenic-safe groundwater. The arsenic in groundwater is getting released from associated Holocene sediments which were likely deposited from the surrounding Tertiary Barail hill range.