Research papersImpact of differential surface water mixing on seasonal arsenic mobilization in shallow aquifers of Nadia district; western Bengal Basin, India
Introduction
High As concentrations (>10 µg/L; permissible guideline value recommended by WHO) are documented in the groundwater of the shallow grey sand aquifers (‘GSA’) (at depths < 70 m bgl) of the Bengal Basin (Dowling et al., 2002, Harvey et al., 2002, Fendorf et al., 2010, Biswas et al., 2012, Chatterjee et al., 2013, Biswas et al., 2014). Frequent patterns of seasonal variations in As concentrations are documented mainly in the shallow GSA groundwaters of the Bengal Basin (Cheng et al., 2005, Savarimuthu et al., 2006, Planer-Friedrich et al., 2012, Biswas et al., 2014, Majumder et al., 2016, Majumder et al., 2017). In contrast, the brown sand aquifers (‘BSA’) (at depths 35–70 m bgl) provide nearly As-free groundwater (<10 µg/L As) (McArthur et al., 2008, Ghosal et al., 2015). Arsenic is commonly found in As(V) sorbed on the surfaces of Fe(III)-oxy-hydroxide phases in the fine sand grains and silty/clay-rich sediments of the Bengal Basin, which is characterized by the high organic contents (Bhattacharya et al., 1997, Nickson et al., 1998, McArthur et al., 2001, Dowling et al., 2002, McArthur et al., 2004). These sediments were part of palaeo-Ganges river flood-delta plain sedimentation events and were strongly influenced by sea-level fluctuations during the Quaternary periods (Acharyya et al., 2000). The principle mechanisms involved in As mobilization from these sediments include dissimilatory reductive dissolution of As(V) coated Fe(III)-oxy-hydroxides, liberating As(III) and Fe(II) in the presence of microbes that facilitate consumption of bioavailable dissolved organic carbon (DOC) in the groundwater for growth and metabolism. DOC in groundwater actively participates during As mobilization process as energy source for microbes and can originate from multiple sources, such as aquifer intercalated organic-rich clay-peat lenses (McArthur et al., 2001, McArthur et al., 2004, Sengupta et al., 2008, Datta et al., 2011) and the natural surface water bodies (ponds, rivers, lakes), that are rich in fresh, labile organic matter (Harvey et al., 2002, Neumann et al., 2010, Lawson et al., 2013).
Seasonal variation in As concentrations in the shallow groundwater of the Bengal Basin is documented highly heterogeneous due to the involvement of multiple factors, including variable rainfall, surface water recharge, mixing, groundwater withdrawal, aquifer redox state, and local lithology (Cheng et al., 2005, Savarimuthu et al., 2006, Planer-Friedrich et al., 2012, Biswas et al., 2014, Majumder et al., 2016, Majumder et al., 2017). However, the subject remains poorly addressed due to the lack of comprehensive knowledge on the interconnected roles of multiple factors driving As distribution over time in the shallow groundwaters of the Bengal Basin. Therefore, an advanced understanding of this topic is needed to identify the potential causes of excessive As in the shallow groundwater and its seasonal fluctuations to better manage the groundwater resources.
The South-West Monsoon (SWM) rainfall (June through September) which accounts for 82 % of the annual rainfall over India, significantly recharges the shallow aquifers of the Bengal Basin (Mukherjee et al., 2007b). The time interval between October and May is designated as the dry period when the freshwater abundance due to the interplay of the meteorological parameters is less prominent, and the demand is high for the groundwater. The dry period is further divided into three periods: post-monsoon (October-December), winter (January- February), and pre-monsoon (March-May). After the SWM, the groundwater level remains high in the post-monsoon period due to the monsoon recharge effect. However, during the pre-monsoon period, the groundwater level recedes maximum due to excessive abstraction practices for irrigation and drinking purposes. Pre-monsoon showers (‘Norwester’ or locally known as ‘Kalbaisakhi’) and rainfall due to cyclonic activities account for 18 % of the annual precipitation over India (Mukherjee et al., 2007a, Mukherjee et al., 2007b). Such rain provides occasional hydrological input to the shallow aquifers during dry periods by percolation through unsaturated soil and sedimentary desiccation features like mud cracks. In dry periods, groundwater provides the only potential water resource (∼50–60 % from shallow aquifers), essentially meeting the agricultural demand (Sikdar et al., 2018). Excessive groundwater abstraction during dry periods causes a sharp drop in the local groundwater level of the shallow aquifers (Harvey et al., 2002, Neumann et al., 2010, Lawson et al., 2013, Biswas et al., 2014, Mukherjee et al., 2018), which generates local depression zones in the groundwater levels (Sikdar et al., 2018). The drawdown in the groundwater level of the shallow aquifer is often compensated by the a) infiltration of organic-rich surface water from the base of ponds, lakes, and rivers (Harvey et al., 2002, Neumann et al., 2010, Lawson et al., 2013, Mukherjee et al., 2018) or b) mixing of organic-rich pore-water from the adjoining intercalated clay-pockets. Mixing of such organic-rich water into the shallow aquifer is likely to trigger anoxic conditions and higher As mobilization by driving the anaerobic microbial activity. There has been evidence of land subsidence in the adjoining areas of the Bengal Basin due to the compaction of sub-surface fine-grained sedimentary strata by excessive pumping (Sahu and Sikdar, 2011, Planer-Friedrich et al., 2012, Mihajlov et al., 2020, Mozumder et al., 2020, Pathak et al., 2022a). Seasonal changes in the storage volume of the shallow aquifers occur due to the cycles of monsoonal recharge and dry time groundwater abstraction (Planer-Friedrich et al., 2012). The process generates an expansion and squeezing effect in the aquifer intercalated clay-peat layers, which may perturb the sub-surface aqueous chemistry by expulsion and mixing of the organic-rich pore-water. Approximately ∼20 % mixing of organic-rich pore water derived from the squeezing of aquifer intercalated clay pockets caused high As release via excess supply of DOC to the adjoining groundwater (Mihajlov et al., 2020).
A variable temporal pattern of dissolved total As concentration in the shallow groundwater has been explained based on the seasonal recharge cycles. Multiple studies, registered higher dissolved total As concentrations in shallow groundwater during the wet periods compared to the dry periods in an annual cycle (Cheng et al., 2005, Savarimuthu et al., 2006, Planer-Friedrich et al., 2012, Biswas et al., 2014, Majumder et al., 2016, Majumder et al., 2017). Such an observation is commonly attributed to the monsoonal recharge bringing anoxia either through the inflow of DOC or saturation of sediment pore spaces; conducive for high As release. In contrast, Government agencies like Central Ground Water Board (CGWB) and other studies (Cheng et al., 2005, Biswas et al., 2014) reported elevated dissolved total As concentrations in the shallow groundwater during dry periods compared to the wet period. The possible dilution effect in the shallow groundwater during the wet time due to the mixing of rainwater and surface runoff as input can explain such temporal distribution of dissolved total As concentrations. In addition, the introduction of organic-rich anoxic water from the bottom of the natural surface water bodies into the shallow aquifer during the dry period facilitated by excess groundwater withdrawal can lead to high As mobilization.
The majority of the past studies reporting heterogeneous As distribution in the shallow groundwater over time were conducted between seasonal time intervals over only an annual cycle. Therefore, repeated seasonal observations on the same set of bore-wells over multiple years are required to understand better the seasonal As mobilization process in the shallow groundwater under variable hydrological conditions in yearly cycles. A comprehensive effort to monitor the seasonal As trends in the shallow groundwater (mid-screen depth < 70 m bgl) over multiple years (2016–2019) was carried out in this study to address the interconnected roles of multiple factors responsible for As release. In this study, the groundwater sampling was done from one of the As hot spot regions of the Bengal Basin, situated in the Nadia district, West Bengal, India. We focused our observations between the post-monsoon and pre-monsoon periods (coinciding with the period of maximum abstraction of shallow groundwater). Our current objectivity focused on a quantitative understanding of the source of recharge water contributing to labile organic matter and responsible for high As mobilization in the shallow groundwater at the seasonal time intervals by inducing anaerobic microbial activity. For this purpose, we measured the abundance of conservative solute (dissolved Cl-) and stable isotopic ratios (δ18O, δ2H) in the seasonal shallow groundwater samples along with δ13C-DOC signatures in the groundwater. For a better understanding of the active redox-biogeochemical processes responsible for the seasonal release of As in the shallow aquifer, we simultaneously measured multiple dissolved redox-sensitive solutes and redox-parameter (i.e., dissolved total As, Fe, Mn, SO42-, NO3–, DOC concentration, Oxidation Reduction Potential ORP value) with emphasis on their seasonal patterns.
Section snippets
Study area
The field area (∼48 km2 in Haringhata and Chakdaha blocks, Nadia district, West Bengal) (Fig. 1a) selected in the present study has received significant attention due to excess As concentrations in the shallow groundwaters inspected for over > 30 years. It falls under the Gangetic alluvial plain and belongs to the Sonar Bangla Aquifer (Mukherjee et al., 2007a) (elevation: 0–20 m above MSL) in proximity to the Hooghly River (also known as Hugli River).
Groundwater modeling over the study region
Materials and methods
Here we monitored bore-well waters tapping the shallow GSA (mid-screen depths < 70 m bgl) and BSA (mid-screen depths 35–70 m bgl). The groundwater sampling from shallow GSA included n = 11 bore-wells each during Nov-16, Apr-17, Feb-18, and May-18; n = 35 bore-wells each during Dec-18, and Apr-19 time periods. However, the groundwater sampling from BSA included n = 6 bore-wells each during Nov-16 and Apr-17; n = 1 bore-well each during Dec-18 and Apr-19 period (Fig. 1a). We provided a
Vertical distribution pattern of As and As mobilizing solutes in groundwater:
The vertical distribution pattern of dissolved total As concentrations in the integrated seasonal groundwater samples (mid-screen depths: 4–70 m bgl) over multiple years was compared with the adjoining lithological characteristics of the aquifers. The groundwater samples in the present study from mid-screen depths <70 m bgl mainly originated from the shallow GSA, and a few groundwater samples from mid-screen depths of 39.6–51.8 m bgl represented BSA. Dissolved total As concentrations in the
Identification of the possible mineral phases in the shallow GSA groundwater from the calculated saturation indices values
In the geochemical modeling, the Saturation Index value or SI > 0 implies a supersaturation condition, which indicates precipitation of mineral phases from the solution is thermodynamically favorable. However, precipitation of mineral phases can be inhibited by slow rates of reactions. Whereas SI < 0 indicates an undersaturation condition, which suggests dissolution of mineral phases into the solution without further precipitation (Drever, 1997). The calculated SI values of the shallow
Conclusion
This study highlights the interconnected roles of multiple factors driving seasonal variation in As release in the shallow aquifer between the post-monsoon and pre-monsoon periods over 2016–2019. We analyzed groundwater samples from the shallow reducing grey sand aquifers (GSA) and a few samples from less reducing brown sand aquifers (BSA) that showed distinct seasonal variation patterns in the dissolved total As concentrations over multiple years. Unlike previous findings, our current
CRediT authorship contribution statement
Pousali Pathak: Conceptualization, Methodology, Formal analysis, Data curation, Investigation, Visualization, Validation, Writing – original draft. Prosenjit Ghosh: Conceptualization, Funding acquisition, Writing – review & editing. Abhijit Mukherjee: Resources, Writing – review & editing. Utsab Ghosal: Resources, Writing – review & editing. Mao-Chang Liang: Formal analysis, Writing – review & editing. Pradip K. Sikdar: Resources, Writing – review & editing. Ritika Kaushal: Formal analysis,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The work is supported by the Indian Institute of Science Ph.D. fellowship. We express our gratitude to Mrs. Ranita Banerjee, Mrs. Noor Muzakkira (Centre for Earth Science, IISc, Bangalore, India), and Dr. Sumanta Bagchi (Centre for Ecological Sciences, IISc, Bangalore, India) for helping in the analytical measurements of dissolved total Arsenic (As) and dissolved organic carbon (DOC) concentration in the water samples needful for this study. We are thankful to Mr. Kathiravan Merran (Centre for
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2022, Applied GeochemistryCitation Excerpt :However, they may not be the predominant drivers in the current scenario since circum-neutral pH ranges were recorded in the groundwater samples. Additionally, the calculated Saturation Index values in the groundwater samples using PHREEQC Interactive 3.3.7–11094 software and the Wateq4f database (Pathak et al., 2022b) showed mild equilibrium to slight supersaturation concerning the possible carbonate mineral phases. The mean SI values included Aragonite +0.28, Calcite +0.43, Dolomite +0.93, Siderite +0.38 in shallow GSA groundwater samples, and Aragonite +0.47, Calcite +0.62, Dolomite +1.30, Siderite +0.03 in deep GSA groundwater samples.
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