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

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 delta O-18 as conservative tracers, we have shown that similar to 13-14% mixing (exclusion) of clay-pore water to the adjoining As-uncontaminated (<= 10 mu g/L) shallow regional groundwater is sufficient to cause high As (>10 mu g/L) mobilisation.

Automated summary

This article discusses the geogenic source of arsenic in groundwater and examines the seasonal variations in dissolved arsenic content and stable isotope ratios in groundwater, deep pond water, river water, and rainwater. The study finds a similarity in the isotopic composition between deep pond water and clay-pore water in shallow aquifers, which contributes to the seasonal release of arsenic into groundwater.