Stable isotope and biogeochemical study of arsenic contamination in shallow groundwater at seasonal time intervals from West Bengal (Nadia district)

Abstract

Seasonal monitoring of groundwater is essential to ascertain safe water for the population and ecosystem for drinking, irrigation, domestic purposes and to keep track of groundwater quality with natural (climate change adaptation) and anthropogenic manipulations (excessive withdrawal). It helps to arrive at essential decisions regarding groundwater management policies and adopt contaminant mitigation strategies. The seasonal trend of groundwater As is highlighted to identify the significant concerns and minimize the As risk. The present study provides a comprehensive idea about the seasonal mobilization process of 'Arsenic’ (As) in the shallow groundwater (<60 m bgl) of the Western Bengal Basin. Arsenic is a siderochalcophile metalloid that is carcinogenic. Arsenic contamination in shallow groundwater of the Bengal Basin has been an alarming issue since 1984, affecting millions of population's health and intensified with growing water demand. Arsenic concentration in shallow groundwater is found in excess contents; >10 μg/L (permissible limit assigned by WHO) however varies at seasonal time intervals over a year due to multiple factors, i.e., climate, recharge, biogeochemistry, and subsurface lithology. The present study aims to capture the probable redox drivers (i.e., the effect of variable rainfall and excessive groundwater withdrawal, recharge influences, the biogeochemical cycle of redox-sensitive elements) responsible for As abundances in the shallow aquifer at seasonal time intervals over multiple years. Our study has been conducted at Nadia district (Chakdaha, Haringhata block) in West Bengal (India), which is one of the 'As-hot spots' with >15% of the population reportedly being affected by arsenicosis. We have monitored shallow groundwater samples originating from reducing grey sand aquifers/GSA (<70 m bgl) and a few groundwaters from deep aquifers for multiple years (2016-2019) covering dry seasonal time intervals (i.e., postmonsoon: when the groundwater level remains at near maxima after the monsoonal recharge and pre-monsoon: when the groundwater level attains minima due to excessive withdrawal practice), when the probabilities of exposure to As toxicity is maximum via excessive groundwater utility due to minimal rainfall. In addition, we have monitored the composition of deeper water from several ponds (collected from just above the pond sediment-water interface) in proximity to the monitoring bore-wells, river water, rainwater at multiple dry seasonal time intervals to estimate the possible infiltration process into the shallow aquifers. Excessive groundwater withdrawal and drop in the water level (up to ~5 m bgl) in the shallow regional aquifer during dry periods; promotes surface water mixing with the aquifer. Our results showed a strong relationship between rainfall amounts and regional groundwater level in shallow aquifers at a monthly time scale, i.e., heavy rainfall events coinciding with an increment in groundwater level. We have used stable isotopes (δ18O, δ2H) as conservative tracers to deduce source and seasonal recharge contribution to the shallow aquifer. In the mixing model, the mass conservative approach using δ18O, D-excess tracers have shown 4- 14% mixing of deep pond water with the shallow aquifer water during dry winter and premonsoon time; however, there are exceptions for the pre-monsoon time of particular years, when regional shower contributed to the aquifers. Limited observations of pre-monsoonal groundwater for the stable carbon isotope ratios in the dissolved organics (δ13C-OC) showed a total ~34% ± 0.7% (applying mass conservative approach) mixing of pond-derived young labile organic carbon into the shallow groundwater. Our observation is consistent with earlier reports on the 14C-DOC signature from the region capturing traces of pond-derived labile organic carbon in the shallow groundwater. Such mixing causes the generation of anoxia and promotes As mobilization. In the majority of the shallow groundwater samples of the present study, an increasing total As contents (+ΔAs) and developing reducing conditions during dry pre-monsoon periods compared to the post-monsoon periods is evident, supporting our interpretation. However, the magnitude of such seasonal deviations in groundwater As varies over multiple years under different seasonal hydrological conditions. The few deep groundwater samples from GSA (>70 m bgl) show a predominance of historical rainwater with minimum mixing influence from the overlying shallow groundwater. However, the water samples from brown sand aquifer/BSA (35-70 m bgl) show a probable effect of evaporation connected with variable climate at a historical time or mixing of heavy pore water from the sediment intercalated clay-peat lenses. Arsenic mobilization in groundwater is linked to the process of dissimilatory reductive dissolution of As-coated Fe(III)-oxy-hydroxides and/or reduction of adsorbed arsenate [As(V)] from the aquifer sediment coupled to the microbial metabolism of available DOC in an anoxic condition. Based on multiple redox-sensitive geochemical tracers (i.e., total As, Fe, SO42-, NO3-, Mn, Cl-, DOC content, oxidation-reduction potential) in the seasonal shallow groundwater samples, our observations have documented contrasting variable seasonal trends in total As contents and development of redox conditions between post-monsoon and premonsoon time intervals over multiple years. Such seasonal patterns in As in shallow groundwater are probably linked to the dependency of redox controlled biogeochemical processes acting in the shallow aquifer on excess DOC supplied by the differential surface water recharge (rainwater, deep pond water) during dry pre-monsoon periods, triggering anoxic conditions for high As mobilization. Alternatively, we have proposed a possible influence of organic-rich pore water mixing from the shallow aquifer intercalated clay-peat lenses of variable thickness in the adjoining groundwater chemistry and total As contents; based on limited observations. The mixing of pore water intensifies with excessive withdrawal from ‘squeezed’ clay-peat lenses and often triggers regional land subsidence. The satellite-based observation of an overall ~12.5 mm land subsidence over the monitoring period covering 2017-2020 with a maximum rate of ~3.3mm/yr is noted in and around the study region, supporting our interpretation. Based on multiple geochemical tracers (i.e., total As, Fe, SO42-, dissolved inorganic carbon content [DIC]) and stable isotopes [δ13C-DIC, δ34S-SO42-], described in the present study, we have highlighted the role of carbon and sulfur biogeochemical cycles on seasonal As mobilization process in GSA groundwater. Our observation documents the microbiallymediated reduction of As-coated Fe(III)-OOH coupled to methanogenesis (by ‘carbonate reduction’) process triggers high As mobilization in the shallow and a few deep groundwater samples (Gr-1) with limited bacterial sulfate reduction (BSR) state (due to low initial SO42- contents). Such a process is more evident during dry pre-monsoon time. However, few shallow groundwater samples (Gr-2) with abundant SO42- contents show evidence of active BSR state and lower As mobilization, with however no conclusive inter-seasonal pattern being recorded for SO42- reduction. Rayleigh fractionation model with fractionation of δ34SSO42- isotopes by BSR process simulates the intensity of BSR at seasonal time intervals in the groundwater (Gr-2), which is found independent of the corresponding total As contents. The multi-proxy-based approach documented in the present study can serve as a template to study and compare the inter-seasonal pattern of groundwater total As contents from other regions of India and Bangladesh, providing information for decision-making on groundwater As mitigation and adoption strategies.