Relic surface water (clay-pore water) input triggers arsenic release into the shallow groundwater of Bengal aquifers

Pathak, P and Ghosh, P and Banerjee, S and Chatterjee, RS and Muzakkira, N and Sikdar, PK and Ghosal, U and Liang, MC and Meeran, K (2022) Relic surface water (clay-pore water) input triggers arsenic release into the shallow groundwater of Bengal aquifers. In: Journal of Earth System Science, 131 (2).

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Abstract

The geogenic source of arsenic (As) in groundwater is an indisputable fact. Arsenic contamination in the shallow groundwater of West Bengal is an alarming health issue with progressive growth of regional population and demand for water. The As contamination in shallow groundwater is facilitated by microbial activities, which depend on the inflow of dissolved organic carbon (DOC), providing a temporary reducing condition conducive to the release of As from aquifer sediments. Here, we present a documentation over several years of seasonal observations on dissolved total As contents and stable isotope ratios in shallow groundwater, ‘deep pond water’ (collected from the deepest part of the pond just above the sediment–water interface), river water and rainwater from the Chakdaha region, Haringhata block, Nadia district, West Bengal. The current study shows, for the first time, a large similarity in the isotopic composition of seasonal deep pond water with the shallow aquifer-intercalated clay sediment-trapped pore water (data adopted from the literature on a Bangladesh region). We highlight the possible processes wherein seasonal eutrophication during dry time followed by monsoon time flooding and sedimentation allows the burial of organic-rich clayey sediments with trapped DOC-rich pore water, which serve as a source of clay-pocket pore water. The isotopic composition of clay-pore water formed at a historical time scale resembles the deep-bottom water composition of surface water bodies and provides an ideal composition responsible for triggering the seasonal release of As into the shallow groundwater. The processes such as excess withdrawal of groundwater during dry periods and consequent squeezing of aquifer sediment-intercalated clay-lenses can expel its organic-rich contents to trigger anaerobic microbial activities and As release into the adjoining water. Using D-excess and δ18O as conservative tracers, we have shown that ~13–14% mixing (exclusion) of clay-pore water to the adjoining As-uncontaminated (≤10 µg/L) shallow regional groundwater is sufficient to cause high As (>10 µg/L) mobilisation. [Figure not available: see fulltext.] Research highlights: Shallow groundwater samples (<60 m bgl) collected at seasonal time intervals (from Nadia) show increasing dissolved total arsenic (As) contents during dry pre-monsoon periods, featuring mixing of aquifer-intercalated clay-lens-trapped pore water with the adjoining groundwater.Excessive groundwater withdrawal during dry periods generates compaction of the intercalated clay-lenses placed within the shallow aquifer and actuates the process of clay-pore water expulsion which causes land subsidence.An estimation of ~13–14% mixing (exclusion) of aquifer-intercalated clay-pore water (high As, organic-rich) to the adjoining As-uncontaminated (≤10 µg/L) regional shallow groundwater is sufficient to trigger high As mobilisation.Aquifer-intercalated clay-pore water is equivalent to ‘relic surface water’, making the presence of palaeo-water of quaternary time scales.

Item Type:	Journal Article
Publication:	Journal of Earth System Science
Publisher:	Springer
Additional Information:	The copyright for this article belongs to the Springer.
Keywords:	aquifer; arsenic; clay; dissolved organic carbon; eutrophication; isotopic ratio; pond; porewater; rainwater; river water, Chakdaha; India; West Bengal
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences Division of Mechanical Sciences > Divecha Centre for Climate Change
Date Deposited:	22 Jun 2022 05:31
Last Modified:	22 Jun 2022 05:31
URI:	https://eprints.iisc.ac.in/id/eprint/73615

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