Arsenic fate in the Brahmaputra river basin aquifers: Controls of geogenic processes, provenance and water-rock interactions

Applied Geochemistry 107 (2019) 171–186

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Arsenic fate in the Brahmaputra river basin aquifers: Controls of geogenic processes, provenance and water-rock interactions

T

Swati Vermaa,c, Abhijit Mukherjeea,b,∗, Chandan Mahantad, Runti Choudhuryd, Rakesh P. Badonie, Gopal Joshif a

Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur, W.B, 721302, India School of Environmental Science and Engineering, Indian Institute of Technology (IIT), Kharagpur, W.B, 721302, India c CSIR-National Geophysical Research Institute (NGRI), Uppal Road, Hyderabad, 500007, Telangana, India d Department of Civil Engineering, Indian Institute of Technology (IIT), Guwahati, Assam, 781039, India e School of Computer Science & Engineering, Xavier University Bhubaneswar, Odisha, 752050, India f Thermo Fisher Scientific India Pvt. Ltd, Mumbai, 400076, India b

A R T I C LE I N FO

A B S T R A C T

Editorial handling by Huaming Guo

The influence of aquifer sediments provenance and geochemical processes on groundwater solute chemistry, including the fate of groundwater arsenic (As) in aquifers for three distinct tectono-morphic zones (northwestern, northern, and southern [NW, N, and S] in the basin of Himalayan Mega Rivers, i.e., Brahmaputra river basin (BRB), and adjoining the Himalayas and Indo-Burmese ranges, have been delineated. The presence of fluvial re-worked quartz, feldspar, and mica along with orogeny-sourced ferromagnesian minerals contained aquifers sediments in the NW and N regions are hypothesized to be derived from the Himalayan system. The Saquifers mineralogy differs from northern aquifers by a higher proportion of Fe/Mg aluminosilicates, phyllosilicates/clay minerals, derived from the Indo-Burmese ranges. The dissolved As distribution is highly variable among the different alluvial aquifers. The S-region is highly enriched in groundwater As (bdl to 5.53 μM or 415 μg/L, mean 1.77 μM) compared to NW and N regions (bdl to 1.8 μM or 134 μg/L, mean 0.28 μM [60% Ascontaminated samples]; bdl to 2.45 μM or 184 μg/L, mean 0.68 μM [65% As-contaminated samples] respectively). More than 92% of groundwater samples in the S-region are enriched with As (> 50 μg/L), which draws a distinct difference from the NW and N regions of BRB aquifers. Reaction-path models suggest intense chemical weathering of S-aquifer matrix due to the higher equilibrium (more stable) of secondary mineral phases with the solution (groundwater), as compared to NW and N regions. The present study proposes a model for the geological control and effect of weathering/water–sediment reaction on As-fate and mobilization mechanism in the groundwater system of the barely studied Brahmaputra river basin aquifers.

Keywords: Groundwater Arsenic Hydrogeochemistry Brahmaputra river basin Weathering Provenance *bdl= below detection limit

1. Introduction Groundwater is a crucial resource, especially for rural populations and increasingly for urban inhabitants, yet the elevated concentration of dissolved arsenic (As) in groundwater, greater than the World Health Organization (WHO) guideline value for drinking water of 10 μg per liter (μg/L) observed in many countries, and particularly in the South and Southeast Asia. Over the past few decades, the use of As-contaminated groundwater (> 10 μg/L in drinking water) is a primary or rising crisis in floodplains of the South and Southeast Asia. Approximately, 100 million of the rural population affected by unsafe As-enriched untreated groundwater in the South and Southeast Asia including India, Nepal, Bangladesh, Myanmar, Pakistan, Vietnam, ∗

Cambodia, and China (Smedley and Kinniburgh, 2002; Bhattacharya et al., 2007; Nriagu et al., 2007; Mukherjee et al., 2009; Fendorf et al., 2010, and Jia et al., 2017, 2018). Most of the highly As-enriched regions are marked by the Himalayan foreland basin filled with sediments of the Quaternary age (BGS & DPHE, 2001; Guillot et al., 2015). In these regions, the elevated concentration of dissolved As has a geogenic source, where As-enriched minerals might be derived from chemical weathering of the Himalayan rocks (Acharyya et al., 2000; Guillot and Charlet, 2007; Mukherjee et al., 2014). For the, last three decades several authors (Nickson et al., 2000; Ahmed et al., 2004; Ravenscroft et al., 2009; Acharyya et al., 2000, and Mukherjee et al., 2008, 2009) have documented different processes and controls of As-enrichment in groundwater of the Bengal

Corresponding author. Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur, W.B, 721302, India. E-mail addresses: [email protected], [email protected] (A. Mukherjee).

https://doi.org/10.1016/j.apgeochem.2019.06.004 Received 13 November 2018; Received in revised form 5 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0883-2927/ © 2019 Elsevier Ltd. All rights reserved.

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fluvial aquifers (i.e., Bangladesh and West Bengal). In general, it was observed, that to a large extent, the distribution of As-enriched aquifers is controlled by the geologic framework and geomorphic evolution of the basins (Acharyya et al., 2000; Mukherjee et al., 2012). Groundwater arsenic in shallow aquifers of the Brahmaputra river basin (BRB), upstream of the Bengal basin, mostly located in Assam, India is recently known. Till now, some studies (Chetia et al., 2011; Bhattacharya et al., 2011; Mahanta et al., 2015; Verma et al., 2015, 2016; Verma, 2017; Das et al., 2018) have documented the extent of As in the groundwater of BRB. However, the detailed hydrogeological and hydrogeochemical processes that have resulted to the evolution of the groundwater solute chemistry and influence the As fate, is not well known, pending some interim information published in our previous works (Verma et al., 2015, 2016; Choudhury et al., 2017; Verma and Mukherjee, 2018). Thus, the primary objective of this study to elucidate the water-sediments interactions, decipher the sediment provenances, weathering patterns and solute cycling that culminates to the scarcely studied pervasive, severe and groundwater As enrichment in this, 4th largest fluvial system of the world. The present manuscript includes documentation of geochemical and geological controls on As enrichment and liberation in aquifers of three distinct tectono-morphic regions of the BRB (Fig. 1a). Also, this article seeks to evaluate the general geochemical pathways and simulate chemical weathering of aquifer sediments by using the numerical thermodynamic reaction-path models and examine the effect of sediment-water interaction on As-fate in the groundwater of BRB. Hence, we have selected three different tectono-morphic alluvial terrains situated along two distinct orogenic belts, i.e., the Eastern Himalaya and Indo-Burmese Range to examine geological controls on solid-phase and dissolved As, host minerals-phase of As, and investigate the mechanism for As liberation from aquifer sediment to groundwater.

sedimentary, meta-igneous rocks and calc-alkaline plutons of the TransHimalaya Plutonic Belt (TPB) (Supplementary Fig. S1a). Geologically, the eastern syntaxis zone is crucial because it is highly eroded and sediment supply region as described before. The TPB belts divided into two zones, specifically, the Lohit Plutonic belt composed of granodiorite, diorite, tonalite, granite (Sharma et al., 1991; Kumar, 1997; Singh and France-Lanord, 2002; Garzanti et al., 2004) and the Tidding Suture Zone (Fig. 1a). Two major eastern tributaries like Lohit and Dibang flow over the Mishmi Hills and transport huge eroded sediments from calc-alkaline plutonic and tholeiitic metavolcanic rocks. (Sharma et al., 1991; and Kumar, 1997). Afterward, the Siang river enters in Assam alluvial flood plain, as the Brahmaputra braided river channel. Next, it flows as the Jamuna River at the Indo-Bangladesh border near Dhubri. The Subansiri, the Jia Bhareli, the Manas, the Tipki and the Puthimari are major tributaries from the northern side, draining from the eastern Himalayan rocks and merged into the Brahmaputra main river channel. The eastern Himalayan rock formation composed highgrade metamorphic rocks (gneisses and schist) and granitic rocks of the higher Himalayas. Medium-grade phyllite, schist, quartzite, granitic gneiss, and carbonate are dominant rock types in the lesser Himalayas (Dutta et al., 1983). Furthermore, the Siang River drains over Abor volcanic (basaltic rocks formation) associated with Gondwana rocks of the lesser Himalayas (Jain and Thakur, 1978; Kumar 1997; and Yin et al., 2010). The tributaries from the south (e.g., the Kopili, the Dhansiri, the Dihing and the Burhi) are draining the western side of the Indo-Burmese ophiolitic belt. This area consists the ophiolites of Cretaceous and Oligocene age, contains ultramafic and mafic rocks sequence (Acharyya, 2007), and volcanic-arc sediments, and volcanic dykes in shale (Kumar, 1997; Colin et al., 1999). 3. Methodology

2. Study area 3.1. Aquifer sediments/matrix 2.1. Geology of the Brahmaputra river basin (BRB) To examine the aquifer sediment characterization (mineral and bulk chemical composition) and solid phase As and other trace metal distribution in the study area, we have collected sediment samples by “hand-flapper” or sludge drilling method (van Geen et al., 2003) from three different provenances of BRB. The depth of each borehole varies in different locations, i.e., NW-BH = 27 m, N-BH = 24 m, SBH1 = 37 m, S-BH2 = 32 m); mainly depends on the thickness of alluvium in the Brahmaputra floodplain. Collected subsurface sediments were preserved in clean plastic zipper bags and stored on ice at ∼4 °C until further laboratory analysis. All the required analysis of aquifer sediments has been done within two weeks after the samples collection. All collected samples were oven-dried and appropriately crushed with the help of agate mortar and pestle for physical, morphological and geochemical analysis. The particles size analysis by Malvern Mastersizer 2000E/2000MU (Hydro, 2000MU) was performed to determine the grain size distribution of the borehole sediments.

This work was carried out in the Brahmaputra alluvial flood plain, the Indian state of Assam which consists of three different tectonomorphic provenances; classified according to location and geology/ tectonic of the study area. The study area is constrained by two distinct orogenic belts; the north-western (NW) and the northern (N) regions along the eastern Himalayas and the southern part (S) situated close to Naga-thrust belt (the Indo-Burmese range) (Fig. 1b). Tectonically, the study area is defined as the eastern continuity of the Indo-GangeticBrahmaputra foreland basin (Supplementary Fig. S1b). The GangaBrahmaputra river system is the largest river system around the world (Milliman and Meade, 1983; Milliman and Syvitski, 1992; and Singh and France-Lanord, 2002), in term of sediment discharge. It delivers a tremendous amount of sediments to the Bengal basin, where the world's largest delta and marine fan system has been formed (Goodbred et al., 2014). In the study area, the Brahmaputra main river channel receives many tributaries from different directions along its course. These small and large tributaries erode/carry enormous amount of sediments from various litho-formation and deposit as potential aquifer sediments in the Brahmaputra alluvial basin (Supplementary Fig. S1a). The Brahmaputra River originates at an elevation of around 5200 m of Kailash Mountain (in Tibet), where it is known as Tsangpo. The Tsangpo receives three major tributaries, i.e., the Nyang Qu, the Lhasa He (Zangbo), and the Do lung along the ophiolitic belt of Indus-Tsangpo (IT) suture zone. These tributaries are draining or eroding igneous rocks of the Trans-Himalayan batholith and associated Palaeozoic to Eocene sedimentary rock (Goodbred et al., 2014). Subsequently, the Tsangpo flows around the eastern syntaxis (Namche Barwa peaks) and enters in Arunachal Pradesh (India) as Siang or Dihang; where it flows over the highly-metamorphosed rock formation, consisting deformed meta-

3.2. Sediment mineralogy and chemistry The bulk mineral phase in aquifer sediments was measured by thinsection slides under the optical microscope and X-ray powder diffraction (XRD) (PANalytical high-resolution instrument). The aquifer minerals quantification was performed with Cu Kα radiation (30 kV, 20 mA) and positive-sensitive detector, stepped at 2 s/0.02°, from an angular range of 10–70° 2θ. XRD peaks made the identification of minerals in aquifer sediments by PANalytical Xpert High score software with JCPDS-ICDD database (JCPDS-International Centre for Diffraction Data (ICDD), 2000) using calculated powder patterns, and Rietveld whole pattern fitting options. Identification of elemental composition and morphological characterization of minerals grains were also carried out by scanning electron microscopy (SEM) coupled with an energy 172

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Fig. 1. Maps showing a) different geological units of north-east India, b) geomorphological feature of three different tectonomorphic provenances in the study area; classified the north-western (NW) and northern (N) regions along the eastern Himalayas and the southern part (S) situated close to Naga-thrust belt (Indo-Burmese range) (modified from Verma et al., 2016), and shows locations of groundwater and sediment sampling (borehole locations) in different provenance of BRB.

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shallow aquifers system, i.e., < 4–62 m below ground level (bgl) from different locations, 107 from NW, 40 from N, and 52 from S region of the Brahmaputra river basin. The required field, e.g. pH, EH, temperature (T °C), dissolved oxygen (DO), electrical conductivity (EC), and total dissolved solid (TDS) were measured by a multiparameter probe (Hanna 9282). The groundwater sampling started after stabilization of all field parameters by the standard hydrogeochemical procedure (Wood, 1981). Sample filtration was done through 0.45-μm filters. Groundwater samples were preserved by 6N HNO3 (pH∼2) for cations (major and trace metal). Samples for anions analysis were preserved by CHCl3 and unpreserved for HCO3 (Böttcher et al., 1990). The manual titration method with 0.1 H2SO4 was used to obtain the HCO3 concentration by the US Geological Survey alkalinity calculator (http://or.water.usgs.gov/alk). Anions were measured using ion chromatography (Dionex ICS 2100, Thermo Fisher Scientific). The concentrations of major cation and trace metal were measured by inductively coupled plasma with optical emission spectrophotometer (ICP-OES, Thermo Fisher ICAP 6000) with accuracy better than 3%. For analytical calculations, below detection level (bdl) were replaced by dl×0.55 (Sanford et al., 1993).

dispersive X-ray (EDX) at 30 kV, using JSM-6490 Instruments. X-ray fluorescence (XRF) was used to measure bulk chemical composition and elemental oxides data through sediments glass bead prepared by lithium tetraborate (1:3 ratio) for the sample fusion process. The Same procedure was followed to obtain calibration lines using glass beads. The sediment samples from three different sites were analysed for total organic carbon (TOC), using OI Analytical (Aurora 1030) TOC analyser instrument with detection limit 0.1% of sediment. 3.3. Total metal extraction The total metal extraction technique has performed to measure the total concentration of trace metals in aquifer sediments from the different depths of the study area. In the following method, 0.3 g sediment samples diluted with HNO3 and HF (3:1) in 50 ml digestion vial and digested for 1 h at 120 °C and 45 bar pressures. After digestion, the sample was filtered through 0.22 μm Nylon 66 filter paper and subjected to subsequent analysis. ICP-MS (Thermo Scientific) instrument was used to quantify the extracted concentrations of As and other trace elements. The instrument calibration was done by using a series of As calibration standards of known concentration ranging from 10, 50, 100, 500, and 1000 μg kg−1. The highly pure single-element stock solution of As (1000 mg kg−1) was used for making calibration standards.

3.6. Thermodynamic modeling The mineral saturation state distribution of aqueous species in groundwater samples was calculated by Geochemist's Workbench (GWB 10.0.4) software with V8. R6+ database. The module Act2 was used to construct the activity diagrams (i.e. EH-pH) for Fe, As and stability diagrams for K2O-Na2O-CaO-Al2O3-SiO2-H2O systems by using the general approach of equilibrium dissolution, speciation and precipitation reactions.

3.4. Sequential extraction In this study, the gradual chemical leaching of solid phase As, Fe, and Mn from aquifer sediment samples was measured based on sequential extraction method (Wenzel et al., 2001; Keon et al., 2001; Swartz et al., 2004; and Haque et al., 2008). Specifically, the sequential extraction method has performed to examine availability, mobility, and chemical association of solid phase As with aquifer sediments by five different phases/fractions. The first fraction (F1) targets weakly adsorbed elements (As, Fe, and Mn) or poorly bound with the outer sphere of minerals surface, for this phase, 0.05 M (NH4)2SO4 used as reagent at pH 8. Subsequently, the residual sediment from a first fraction (F1) was reacted with 0.05 M NH4H2PO4, at pH 4–5 for removal of strongly adsorbed Fe, Mn, and As and specifically bound with the inner sphere of minerals surface. After that, the residual sediment from a second fraction (F2) was reacted with 0.2 M NH4-oxalate buffer, at pH 3.25 used as reagent (F3) for removal or dissolution of non-labile As coprecipitated or associated with amorphous phase, i.e. amorphous hydrous ferric/ manganese oxyhydroxide minerals like ferrihydrite. Afterward, fourth fraction (F4), the residual from a third fraction (F3) was reacted with 0.2 M NH4-oxalate followed by 0.1M ascorbic acid, at pH 3.25, targets As-associated with well crystalline-phase of Fe/Mn(Al) oxyhydroxide e.g., hematite, magnetite, and goethite. The fifth fraction (F5), first targets, As associated with amorphous sulphides or residual minerals such as orpiment, where mixture of nitric acid (16 N HNO3) and hydrogen peroxide (30% H2O2) used as regents, and finally As associated with aluminosilicate minerals, and residual such as quartz was extracted by mixture of nitric acid and hydrofluoric acid. After digestion, the sample was filtered through 0.22 μm Nylon 66 filter paper and subjected to subsequent analysis. All the extracted products (As, Fe and Mn) from sequential leaching were analysed by ICP-MS (Thermo Scientific) instrument. The instrument calibration was done by using a series of As calibration standards (10, 50, 100, 500, and 1000 μg kg−1) from highly pure single-element stock solution of As 1000 mg kg−1.

3.7. Reaction-path modeling of water-sediment interaction The reaction-path simulation was performed to examine the sediment-water interaction and predicted the continuous dissolution/precipitation of minerals to developed equilibrium among reactants (minerals) and solution (groundwater) in the shallow aquifer system of BRB. In this study, we have simulated chemical weathering of aquifer sediments of BRB, using the numerical thermodynamic reaction-path model REACT18 (GWB 10.0.4) under atmospheric conditions at 25 °C (Bethke, 2007). The hydrogen ion activity (H+) was measured by pH values and redox state given by EH (Bethke, 2007) to describe sequential changes of bulk composition of secondary minerals and groundwater evolution process (Plummer, 1984; Parkhurst and Appelo, 1999; and Boschetti and Toscani, 2008). Reaction models were generated assuming that groundwater flow and chemistry are at steady state. Groundwater chemistry and aquifer mineralogy are considered to represent initial and final end-members along the flow paths. This simulation allows for the forward predictions of changes in fluid chemistry, mineral composition and mass along the reaction path. In this process minerals, may become saturated and precipitated or continue to dissolve to developed equilibrium in the aquifer system. In these equilibrium models, mineral assemblage of aquifer sediments from different regions, comprise Kfs, Pl, Di, En, Ms, Bt, Mag, Hem, Am, Ilt, Kln, Chl, Qz (Supplementary Fig. S2 a,b,c) is titrated into 1 kg of aqueous fluid (groundwater) to predict sediment-water interaction, which is the dominant process of groundwater evolution. The present study shows the range of possible conditions under which groundwater chemical changes, i.e., continues reactions processes (e.g. dissolution/ precipitation of minerals) are likely to have been developed by varying water-sediment ratio in the aquifer system of the BRB. However, due to lack of quantitative information on physical sediment properties, hydraulic properties and residence time, several ranges of sediment-water ratios were taken in order to visualize scenarios. Therefore, geochemical modeling is suitable for envisaging the

3.5. Aqueous geochemistry Groundwater sampling was performed by following the procedure of Mukherjee and Fryar (2008). This study includes the data-set of 199 groundwater samples, which were sampled from public water supply wells between 2013 and 2014. All groundwater samples collected from 174

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Table 1 The descriptive statistical results of trace-metal concentrations in aquifer sediment samples from the study regions (BRB).

NW-BH n = 11

N-BH n = 9

S-BH1 n = 9

S-BH2 n = 14

Min Max mean Median Std. D Min Max mean Median Std. D Min Max mean Median Std. D Min Max Mean Median Std. D

Depth (m)

Cr mg/kg

Mn mg/kg

Fe mg/kg

Co mg/kg

Ni mg/kg

Cu mg/kg

Zn mg/kg

As mg/kg

Cd mg/kg

Pb mg/kg

1.52 25.91 11.68 15.24 8.00 3.05 24.38 14.23 13.71 7.39 3.05 36.57 18.29 19.81 9.91 1.52 32.00 16.76 15.24 9.28

16.90 28.90 21.98 18.24 4.46 40.26 86.24 55.46 48.87 17.12 24.11 67.30 42.75 48.89 12.33 38.11 71.24 59.04 60.42 10.02

8.65 44.9 18.38 11.31 12.22 29.32 52.01 37.57 39.52 8.71 22.66 48.26 30.84 28.90 7.39 20.50 33.30 26.34 24.64 4.15

1482.5 1628.46 1544.21 1590.90 53.39 1260.53 1780.50 1453.44 1377.10 205.38 1366.22 1620.50 1436.72 1371.92 90.86 1465 1646.08 1566.48 1573.86 67.13

0.46 1.12 0.87 0.97 0.23 0.39 1.07 0.73 1.07 0.26 0.36 1.08 0.71 0.48 0.28 0.68 1.20 1.03 0.98 0.14

2.48 4.61 3.28 4.11 0.68 2.68 5.70 3.60 3.50 0.90 2.44 5.18 3.19 2.64 0.86 2.48 5.96 4.31 4.19 1.23

2.13 8.48 5.37 2.12 2.24 3.05 10.49 6.13 10.48 3.10 2.04 8.60 5.03 3.11 2.52 2.24 8.50 5.63 5.30 1.73

2.24 11.42 9.35 2.24 2.54 4.10 12.09 9.12 11.10 2.98 4.07 13.08 7.61 5.61 3.55 8.44 11.27 10.36 11.18 0.80

9.26 12.58 11.04 10.24 1.48 12.34 17.40 15.08 14.87 1.82 10.05 18.90 15.45 10.90 2.77 10.42 14.48 12.40 12.47 1.11

0.04 0.06 0.05 0.05 0.01 0.05 0.08 0.06 0.07 0.09 0.04 0.07 0.06 0.07 0.01 0.04 0.06 0.05 0.04 0.01

0.58 0.88 0.71 0.80 0.09 0.42 1.12 0.73 0.61 0.24 0.30 0.96 0.58 0.9 0.18 0.54 1.18 0.84 0.89 0.19

k-feldspar (Kfs), biotite (Bt), muscovite (Ms), amphibole (Am), and quartz (Qz). Moreover, some grains of pyroxene (Px) can be observed on the basis of pleochroism and second-order interference colours in sediments samples of NW and N aquifer. The sub-angular or subrounded shape of sediment grains, indicate immaturity of minerals grain due to the insignificant distance between sediment provenance and depositional sites. The intergranular grains are filled up by fine rock fragments and brownish or opaque hydrated iron hydroxides. In Nregion, Kfs are highly altered; Opx grains present which showing pleochroism (pink–brown to pink–green) and second order interference colours. Opx and Qz are highly fractured and in some slides evidence of fluid percolation occurred which indicates the highly-weathered condition. Biotite are highly weathered which probably led to the formation of some of chlorite. Identification of S-aquifer mineral composition is slightly difficult under the microscope because the aquifers lithology of S-region dominated by clay fraction with subordinate amount of sand (medium and fine). Sediment samples are highly enriched with clay content, i.e., kaolinite (Kln), illite (Ilt), although some grain of Bt, Kfs, Pl, and Opx can be recognized. Feldspars show cloudy appearance under polarized light indicating their partial alteration to secondary minerals like sericite and kaolinite. Some green patches also occurred in slide those indicate chloritization (Supplementary Fig. S2 a, b, c). The detailed study of aquifer mineral compositions done by XRD is depicted in Supplementary Table S1. The occurrences of Pl, Kfs, Bt, Ms, Am, Chl, Px, and Qz are confirmed by peaks or pattern of XRD in aquifers sediments of NW and N regions (Supplementary Fig. S2 a, b). Furthermore, many iron (Fe) bearing minerals like hematite (Hem), wuesite (Wue), goethite (Gt), siderite (Sd) and magnetite (Mag) along with some small peaks of As content minerals, i.e., arsenolite (Ars), cobaltite (Cob), and arsenopyrite (Apy) present in various depths of aquifer sediments. The mineral compositions of S-aquifers are slightly different from the other two studied terrains (NW and N parts). Samples collected from S-aquifers are characterized by dominance of clay minerals like kaolinite (Kln), illite (Ilt), montmorillonite (Mnt) etc. along with Kfs, Pl, Am, Ms, Bt, Qz, and Px (Supplementary Figure S2 c). It also confirmed the minor presence of olivine (Ol) in the sediment sample of various depth in the southern (S-BH1, BH2). XRD data shows multiple peaks of As-bearing minerals but less abundance of Fe content minerals (Supplementary Table S1). High abundance of phyllosilicate minerals (mica, chlorite, and clay minerals) might be provide available reactive surfaces or inner sites for As adsorption or precipitation (Dixit and Hering, 2003; Wenzel et al., 2001; Seddique et al., 2008), potentially via Fe-oxyhydroxide molecular bonding on the edges minerals

plausible reaction between aquifers mineral composition and groundwater, at an assumed local thermodynamic equilibrium. In this study, the reaction-path simulation was performed to examine the hypothesis that the aquifer sediments of S-region are more chemically altered compared to NW and N aquifers of BRB, and the sediment-water interaction (chemical weathering) represents a major controlling factor of groundwater evolution in the Brahmaputra basin.

4. Results 4.1. Sediment geochemistry and mineralogy The descriptive statistical results of trace-elements concentrations in the sediment samples from various depths are summarized in Table 1. The Mn concentrations show wide variation among three studied regions, varying from 8.65 to 45 mg kg−1 (ppm) in NW and 29–51 mg kg−1 in N region while it shows relatively low concentrations in S region ranges from 20.5 to 48.26 mg kg−1. Iron (Fe) does not show a considerable variation in aquifer sediments of BRB; the mean values are almost similar (1527 mg kg−1 in NW; 1491 mg kg−1 in N and 1436–1566 mg kg−1 in S). The average Co, Ni, Cu, Zn, Cd, Pb concentrations are not much varying among aquifer sediments of different provenances, but Cr shows high values in N and S regions. The geochemical composition of sediment samples of four representative boreholes (NW-BH, N-BH, and S-BH 1, 2) from different studied regions of BRB is given in Supplementary Table S2. The aquifer sediments show wide-ranging from coarse sand to clay in texture and grey to dark grey in colour. Major oxide data shows that SiO2 concentrations range between 52 and 78 wt% in all boreholes from different locations. High concentration of SiO2 associated with sand and low concentration in clay samples. The Fe2O3 gives high concentrations in the aquifer sediments of S-region range between 8 and 12 wt% (Supplementary Table S2). Subsequently, Al2O3 varies from 10 to 18 wt % among different borehole sediments, the TiO2 range from 0.4 to 1.3 wt% and CaO from 0.42 to 3.8 wt%. The K2O contents are quite high from 2.6 to 6 wt%; however, MgO, MnO, Na2O show small percentage in all the aquifer sediments. However, the southern borehole (S-BH1, BH2) are mainly composed of clayey sediments, enriched with less mobile or immobile elements such as Al2O3, Fe2O3, MgO and TiO2 compared to sand fraction from NW and N boreholes. The present dataset suggested that dark grey clays are highly enriched with immobile components (Supplementary Table S2). The petrographic analysis shows the dominance of plagioclase (Pl), 175

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Fig. 2. SEM analysis shows the morphological characterization of minerals grains of aquifer sediments from various depths of different borehole (NW-BH, N-BH, SBH1 and S-BH2) in study area; where Ab (albite), Kfs (k-feldspar), Bt (biotite), Pl (plagioclase), Mnz (monazite), Rt (rutile), Am (amphibole), Px (pyroxene), Cpx (clino-pyroxene), Opx (ortho-pyroxene), Chm (chamosite), ilmenite (Ilm), titanite (Ttn).

and it ranges from 0.95 to 4.98 mg kg−1 in all studied regions of the BRB. However, fine sediments contain more TOC as compared to coarser sediments (Fig. 9). The statistical summary of total solid phase As in fine-grained and coarse-grained sediments of different studied regions of the BRB are presented in Table 1. The distribution of solid phase total As concentration varies from 9.2 to 18.9 mg kg−1 in the aquifer sediments among all different studied regions in the BRB. Although, these aquifer sediments are not unusually enriched with As compared with average values found in the Ganges and downstream Bengal Basin (McArthur et al., 2004; Nickson et al., 1998, 2000; Chowdhury et al., 1999; and Bhattacharya et al., 2002). The highest solid-phase As concentration (18.9 mg kg−1) found in the aquifer sediments of S (S-BH1) aquifer at a depth of 14 m, where the sediments are mostly contained grey/dark grey clay with high TOC content (3.6 mg kg−1) (Fig. 9). Also, maximum solid-phase As concentrations are associated with high ferromagnesian and As-bearing (cobaltite) in sediment samples of S-BH1 (Supplementary Table S1). Furthermore, total solid phase As concentrations do not show a considerable variation in NW (9.26–12.58 mg kg−1) and N (12.3–17.5 mg kg−1) aquifer sediments. As discussed earlier, NW and N aquifers predominantly composed of coarser fraction (fine, medium, coarse sand, and silt) and associated relatively low As content in compare to S-aquifers. Fig. 9 shows the correlation between total solid phase As, grain size and total organic carbon (TOC), according to depth variation in each boreholes of different studied regions. In aquifer sediments of S-regions total solid phase As shows positive correlation

interlayer hydroxyl sheet structures (Dzombak and Morel, 1990). The SEM/EDX results suggest some interesting information about chemical weathering and alteration of minerals in subsurface sediment samples. Samples of NW-aquifers show presence of heavy minerals, i.e., monazite (Mnz), rutile (Rt), and highly altered grain contains Am, and Bt (Fig. 2), might be derived from medium to highly metamorphosed gneisses and schists rocks of the Eastern Himalayas. Northern-aquifer sediments show profoundly altered ferromagnesium silicates (Am, Bt) at depth of 10 m and alteration to ilmenite (14 m), indicating that sediments associated with igneous and metamorphic rocks (the Eastern Syntaxis and Himalayas). Sediment samples of S-region, show chemical and mineralogical alteration of Kfs, Pl, Fe/Mg aluminosilicates like pyroxene (Px) and Am, and the presence of heavy minerals i.e. titanite (Ttn), ilmenite (Ilm) at various depth (Fig. 2), representing weathering of mafic igneous rock and associated pegmatites of Naga Hills.

4.2. Arsenic distribution in aquifer sediment The stratigraphically characterization of aquifer sediments is shown in Fig. 9, where the sediment colour varies from greyish-brown to dark grey. According to particle size analysis data, the aquifers sediments of NW and N dominated by large particles (∼0.45 mm), predominantly composed of coarser fraction (fine, medium, coarse sand, and silt). The particle size of S-aquifer sediments generally less than 0.1 mm, which comprise fine sediments i.e., clay, silt and very fine sand. However, the proportion of TOC significantly varies between fine and large particles, 176

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Fig. 3. Box plot shows the statistical summary of groundwater solutes in all studied regions.

NW-BH and N-BH, respectively. In the S-region, aquifer sediments samples from S-BH1 and S-BH2, the concentrations of As derived from crystalline (hydrous) Fe (Mn, Al) oxides varies from 0.9 to 2.6 and 0.8–1.8 mg kg−1 in S-BH1 and S-BH2. Undoubtedly, most of the As is extracted in fraction 4 (F4) which targeting crystalline (hydrous) Fe (Mn, Al) oxides, i.e., goethite, hematite, magnetite, and birnessite, higher in NW and N sediment than S-aquifer. Fraction 5 (F5): The total extractable solid-phase As in the residual phase (silicates and clay minerals) range from 0.4 to 1.8 mg kg−1 in the sediment samples from NW-BH. In N-region, As concentration varies from 0.6 to 1.9 mg kg−1 in sediment samples of N-BH. In the S-region, aquifer sediments samples from S-BH1 and S-BH2, the concentrations of As derived from the residual phase varies from 0.8 to 1.4 and 0.5–0.7 mg kg−1. The concentration of structural-As which derived from residual phase (F5) is comparatively low, especially in S-aquifer sediments (Fig. 9). Thus, the general observation is that most of the extracted As associated with crystalline phase of Fe (Mn, Al) oxides in NW and N aquifers, but in S-aquifer As must be linked to an amorphous phase of Fe-Mn oxides (Supplementary Table S3).

with TOC, and related to the grain size of aquifer sediments. Subsequently, the fine particles (S-aquifers) mostly grey/dark grey clay exhibit higher As concentration compared to coarse sediments (NW and N regions).

4.3. Sequential extraction of solid phase arsenic The extracted concentration of As, Fe, and Mn from different mineral phases described in Supplementary Table S3, and the section below summarized separate pools or five different fractions of solid phase As to understand the factors controlling As mobilization in the Brahmaputra alluvial aquifers. Fraction 1 (F1): In aquifer sediments of NW-region, the concentration of physio-sorbed (outer sphere surface complexed or F1) As varies from 0.3 to 0.6 mg kg−1 and percentage varies 8.1 to 14.3. Likewise, the As concentration range from 0.2 to 0.6 mg kg−1 and portion varies 3.9%–9.5% in aquifer sediment samples N-BH. In the aquifer sediments of S-BH1 and S-BH2, the physio-sorbed As concentration range from 0.4 to 0.9 mg kg−1 and 0.5 to 0.7 respectively; percentage varies between 6.5 to 11.1 and 10.1 to 12.1. Fraction 2 (F2): The concentration of chemisorbed (inner sphere surface complexed) As ranges from 0.3 to 0.7 mg kg−1 and 0.28 to 0.72 in NW-BH and N-BH, respectively (Supplementary Table S3). In the Sregion, aquifer sediments samples from S-BH1 and S-BH2, the chemisorbed As concentration ranges from 0.4 to 0.9 and 0.5–0.7 mg kg−1, respectively. The results of chemical extraction show that S-aquifers contain high sorbed phase As (F1 + F2) concentration compared to NW and N aquifers sediments. The “sorbed” As also referred to as reactive or ions exchangeable, previously mentioned that S-aquifer contains an enormous amount of clay minerals which might be an available reactive site for As-sorption. Fraction 3 (F3): The concentration of As associated with amorphous phase or poorly crystalline-amorphous hydrous Fe (Mn, Al) oxide, i.e., ferrihydrite or Fe(OH)3 ranges from 0.3 to 0.7 mg kg-1 in NW-BH. Also, the As concentration in N-region varies from 0.4 to 0.9 mg kg-1. In the S-region, aquifer sediments samples from S-BH1 and S-BH2, the concentrations of As derived from amorphous or poorly crystallized Fe(Mn, Al) oxide are relatively higher in S-aquifer than NW and N samples (Supplementary Table S3), varies from 1.5 to 3.6 mg kg−1 and 1.9 to 2.1 in S-BH1 and S-BH2. In this fraction (F3) As might be derived from dissolution of biotite, chlorite, ferrihydrite and some carbonate minerals (wollastonite and dolomite) in S-aquifers. Fraction 4 (F4): The extracted solid -phase As in fraction 4 (F4) varies from 0.8 to 1.7 and 1.1–2.7 mg kg−1 in the sediment samples of

4.4. Hydrogeochemistry and groundwater arsenic The groundwater solutes chemistry of three different tectonomorphic regions (NW, N, and S) is provided in Supplementary Table S4. Our previous study (Verma et al., 2016) has documented detail groundwater solute chemistry of the BRB aquifers. In brief, groundwater is dominated by Ca2+ and Mg2+ cation in NW and N regions, whereas Na+ cation extensively present in S-groundwater. In all studied regions HCO3− is the dominant anion, but the highest concentration occurs in S-aquifer (Fig. 3). However, the level of Cl− and SO42- are generally low. The BRB groundwater is circum-neutral to slightly alkaline with pH in the range of 6.5–8.5. The redox-sensitive parameters, i.e., EH and dissolved oxygen (DO) are low (> 1 mg/L, mean values 0.4–0.7 mg/L for all studied regions), indicating reducing condition in groundwater of BRB (Supplementary Table S4). Solutes (e.g., Fe and Mn) indicative of reducing condition (e.g., Fe and Mn) are exhibit high concentration in BRB (mean: Fe 268 μM, 480 μM; Mn 20 μM, 28 μM in NW and N region, respectively) than the S-region (mean: Fe 140 μM; Mn 3.37 μM) (Fig. 3). In the BRB, groundwater samples have As concentrations rising to 5.53 μM or more than around 50 times the WHO drinking water guideline value (≥0.13 μM or 10 μg/L) (Supplementary Table S4). Groundwater samples from two distinct tectonic-setting, i.e., the Eastern Himalayas (NW and N) and the Naga-thrust belt/Indo-Burmese 177

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minnesotaite, phengite (mica) and quartz are present in dissolved form, as the amount of reacted sediments increase albite is introduced in system (Fig. 4, N-BH). Reaction-path simulations between representative sediment samples from S-region (S-BH # 28 m) and mean groundwater solution gives mineral assemblages, i.e., diaspore (aluminium oxide/hydroxide), dolomite, muscovite, saponite-Mg, nontronite-Mg (smectite group), pyrolusite, mesolite (zeolite) and quartz in groundwater system and hematite expected to precipitate. With increment of the water-sediment ratio, minerals e.g., diaspore, saponite-Mg, and nontronite-Mg (smectite) are getting precipitated and muscovite, celadonite, saponite-Ca, calcite, nontronite-Ca, and pyrolusite are expected to be thermodynamically stable in the S-aquifer system (Fig. 4, S-BH). Observations from the thermodynamic reaction-path simulations point toward the weathering of high-temperature mafic minerals and intense chemical alteration of S-aquifer matrix by higher equilibrium of secondary mineral phases [ i.e., saponite, and nontronite (smectite group), kaolinite, daphonite (chlorite group) and celadonite (altered biotite)], compared to NW and N regions.

range (S) show striking variance in the groundwater arsenic concentrations. The N and NW regions are relatively low in dissolved arsenic than the S regions, which might be caused by the differences in the geology, tectonic evolution, and weathering pattern in the studied regions. More than 68% of samples are contaminated with dissolved As in groundwater of all different tectono-morphic regions in the BRB. However, highest As enrichment occurs in S-aquifers (As ∼ 5.53 μM) and almost 92% collected groundwater sample from S-site are enriched with dissolved As (Supplementary Table S4). Fig. S3 shows that reduced As (III) is being dominant species over oxidized As (V) form in all collected groundwater samples. 4.5. Insights from geochemical modeling The thermodynamic stability of minerals phase in groundwater is measured by saturation index (SI) calculation, which gives under-saturated, equilibrium, and saturated minerals species. Groundwater of NW and N sides are mostly under-saturated with carbonate mineral species; however, groundwater of S-region slightly under-saturated to equilibrium (Supplementary Table S5). Thus, dissolution of carbonate minerals controls high concertation of HCO3− in groundwater of the BRB. Calculated SI values show that Mn-phases should be present in dissolved form due to thermodynamic instability in the groundwater system. However, Mn could be precipitated as carbonate phase (rhodochrosite) or in combination with other carbonate phases, i.e., FeCO3 as expected in the reducing groundwater system. The activity diagram (Supplementary Fig. S3) shows that NW samples mostly plot in the stability field of Fe2+; but, N and S groundwater samples plot toward Fe3+ (hematite) stability zone within pH 6.5–8.5 ranges, gives iron precipitation as Fe (III) phase from groundwater. The As-phases are thermodynamically unstable, or groundwater is under-saturated with As-minerals, indicating that As should be present in the dissolved form after mobilization and precipitation of As-phases from groundwater is not thermodynamically favourable. Supplementary Fig. S3 illustrates that As mobilization leading with As(III) species than As(V) in groundwater of the BRB. Stability diagrams (Supplementary Fig. S3) show NW groundwater is in equilibrium with kaolinite, indication weathering of Kfs, Pl, and Ms. However, N and S samples mostly plot in transition between kaolinite and smectite stability fields, suggesting solutes in groundwater might be resulting from chemical leaching of Kfs, Pl, Bt, and some ferromagnesium silicate minerals, i.e., Px, Am, and Ol. Thermodynamic equilibrium relationship between groundwater chemical evolution and secondary mineral formation has mentioned in Fig. 4. The results of reaction-path modeling suggest continuing reactions process (i.e., dissolution/precipitation of minerals) to developed equilibrium among reactants and groundwater solution in aquifers of BRB (Fig. 4). In reaction-path simulation, present mineral assemblage (Supplementary Table S1) from the different depth of NW-BH, N-BH, and S-BH used as reactant into the groundwater (solution) of the various studied region of the BRB. The reaction-path simulation predicted the precipitation of siderite, kaolinite, saponite, dolomite, daphnite, and dissolution of muscovite, calcite, rhodochrosite, k-feldspar, pyrolusite, and quartz (Fig. 4, NWBH). However, the water-sediment ratio also a controlling factor of precipitation/dissolution of secondary minerals (Fig. 4). When, sediment (1 gm.) from 4.5 m of NW-BH, titrated into 1 kg of mean groundwater composition (water-sediment ratio 1000:1) siderite is getting precipitated. Subsequently, as sediment amount increase (water: sediments, i.e., 100:1, 10:1, 1:1) it leads the precipitation of saponite-Mg, saponite-Ca, kaolinite and paragonite, muscovite, dolomite, albite, calcite, daphnite, and saponite-Na. In the same way, aquifer mineral composition from 14 m of N-BH react with mean groundwater composition of N-region, where kaolinite, siderite, daphnite, (chamosite) and dolomite are expected to be precipitate; however, rhodochrosite, muscovite, calcite, saponite-Ca,

5. Discussion 5.1. Weathering pattern and processes Geochemical weathering of aquifer sediments is primary controlling factor to introduce major ions (K+, Na+, Ca2+, Mg2+, SO42- and Cl−) and trace metal (Fe, Mn, Cr, Cu, Zn, As, etc.) in groundwater by breakdown of aquifer mineral compositions (Grant, 1964; Tardy, 1971; Nesbitt and Young, 1989; and Drever, 1997). The nature of sediment provenance, the degree of weathering, fluvial processes, and climatic condition are the major controlling factor for chemical alteration/ weathering procedure (McLennan et al., 1993; Mukherjee et al., 2009). During chemical weathering of aquifer sediments, Ca2+, Na+, and K+ released at the early stage of chemical weathering, consider as the mobile element; subsequently, (Al3+, Si, Ti4+, Fe3+/2+) released by weathering/alteration of secondary or residual minerals (Grant, 1964; Nesbitt and Young, 1989). Weathering intensity mostly measured by the degree of depletion in the mobile elements relative to immobile elements (Nesbitt and Young, 1982). Previously mentioned (section 4.1), plagioclase and k-feldspar are major mineral phases in aquifer sediments of the BRB; therefore, chemical index of alteration (CIA) and chemical index of weathering (CIW) used as the parameters to measured chemical weathering intensity via alteration of k-feldspar and plagioclase to clay minerals (kaolinite, smectite, illite, and chlorite). Nesbitt and Young (1984), suggested an equation through applying elemental oxides values, CIA = [Al2O3/(Al2O3+CaO*+Na2O + K2O)] × 100

(1.1)

Where CaO* represents the amount of CaO integrated into the silicate phases. Accordingly, intense chemical weathering or alteration is reflected by high CIA, i.e., 70–100; whereas lower CIA values (i.e., 50 or less) signify the least alteration of aquifer sediments in the BRB. The present weathering analysis indicates that the aquifer sediments show a considerable variation in CIA, and CIW values in different alluvial regions of the Brahmaputra basin. The NW-aquifers sediments have CIA ranging from 55 to 66 and N aquifers sediments have CIA values 45–56 (Supplementary Table S2) indicates that these sediments are poorly weathered due to immaturity and rapid transport along the course of the river (i.e., distance from sediments provenance to depocentres). Singh et al. (2005) calculated CIA values of bedload and suspended sediments of the Brahmaputra mainstream and its tributaries; reported that sediments carried via northern and eastern Himalayan tributaries (Himalayan drainage) show low CIA values (less clay contents). However, sediments carried by southern tributaries give high CIA values (65–85); similarly, the present study also shows high CIA values (i.e., 65–75) of southern aquifer sediments, indicating more 178

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Fig. 4. Plots of the reaction-path simulations with various range of water: rock (1000:1, 100:1, 10:1 and 1:1) to calculate groundwater-sediment interaction from selected sediment samples from depths (NW-BH # 3 m; N-BH # 14 m, and S-BH # 28 m) and mean groundwater composition, where (+) dissolution; (−) precipitation.

Fig. 5. Plots of A–CN–K with CIA values on the vertical axis (after Nesbitt and Young, 1984, 1989) diagram to calculated weathering intensity and trend in aquifer sediments of various alluvial-regions in the BRB.

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and As-enrichment in the foreland basin aquifers, e.g., the Brahmaputra basin, and suggested that the ophiolites, arc-related rocks lithologies are the source of As in groundwater of the BRB. Brookfield (1998) and Yin et al. (2006) have proposed the paleohydrological system of the Himalayan orogenic belt and suggested that major Himalayan rivers possibly flowed directly from the south Tibetan plateau to paleo-foreland basin (i.e., Siwaliks) during the Miocene. This paleo-drainage system of the Himalayan orogenic belt eroded As-enriched arc derived magmatic rock (basic to ultrabasic composition rocks) and deposited the weathered As-rich sediments in the paleoforeland basin (Guillot and Charlet, 2007; Verma et al., 2016). Therefore, during the Miocene, an enormous amount of As-laden sediments might be transported from ophiolites of Indus-Tsangpo (IT) suture zone by the paleo-Himalayan river system to the Indo-Gangetic-Brahmaputra paleo-foreland basin (Siwaliks). These paleo-foreland sediments act as secondary provenance in the NW and N regions; furthermore, the eastern Himalayan syntaxis contains the high amount of mafic rock, which might also include a mafic minerals provenance especially in Naquifers of BRB. Many studies have documented a correlation between sediments geochemical composition and tectonic setting of foreland sedimentary basin (Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986, 1988; Bhuiyan et al., 2011; Guillot et al., 2015; and Verma et al., 2016). Bivariate plots of elemental oxides (Fe2O3+MgO) vs. TiO2 and Al2O3/ SiO2 provide tectonic-sedimentological classification to categorized different tectonic settings, e.g., passive margin, active continental margin, continental island arc, and oceanic island arc (Bhatia, 1983). Most of the NW and N aquifer samples are falling close to the active continental margin field, and some scatter in the vicinity of the continental island arc margin (Fig. 7). Accordingly, aquifer sediments of NW and N (which contain abundance of ferromagnesian minerals, i.e., Am, Hem, Man, Bt, Opx, Cpx) could have been derived from felsic continental margin and some mafic igneous rocks i.e. gabbro, andesites/diorites and dacites/granodiorites (contain up to 1500 mg kg−1 solid-phase As [Henke, 2009]) of the Himalayan orogenic belt (TransHimalayan plutonic belt). Following observations suggest, the NW and N aquifers sediment possibly derived from As-enriched source rock, after deposition, they became altered, weathered, mixed and re-deposited, further became a part of the main aquifers. On the other hand, S-aquifer samples scatter in the oceanic island arc field, however few plots across the boundaries between the continental and oceanic island arc (Fig. 7). This observation is supported by the marked presence of pyroxene (Opx and Cpx), olivine, titanite, Chamosite, magnetite, and ilmenite which might have been derived from arc-related lithologies and Naga-ophiolite belt, and comprise significant proportion of As-enriched mafic and ultra-mafic rocks (Hattori et al., 2005; Guillot and Charlet, 2007; Verma et al., 2016). The southern tributaries are draining the As-enriched mafic and ultra-mafic rocks (ophiolites) of the Naga Hills, which eroded/carried As-enriched weathered sediments from primary provenance and deposited in the Sregion of the BRB (Verma et al., 2016).

intense chemical weathering or alteration of aquifer mineral composition in the S-region than NW and N parts (Fig. 5). The molar proportions of elemental oxides values are plotted in ACN-K triangular diagrams (Fig. 5), where A = Al2O3, CN= CaO*+Na2O, K= K2O (Fedo et al., 1995; Nesbitt and Young, 1984, 1989; and Nesbitt et al., 1996) to understand chemical weathering trend of aquifer sediments of the BRB. The bulk chemical composition of NW-aquifer sediments plots close to the feldspar joining line or A-K boundary in A-CN-K diagram (Fig. 5), signifying that feldspar is most dominant Al-silicate minerals where Kfs is somewhat high than Pl. Likewise, samples of N-aquifer fall in between feldspar joining line; therefore, Pl and Kfs proportion are almost the same, suggesting the relatively less chemical alteration. However, S-aquifer sediments plot at the higher side of the diagram close to illite (Ilt) field, indicating the preponderance of clay minerals, due to the breakdown of the feldspar fraction and loss of mobile elements, i.e., CaO and Na2O suggest intense chemical weathering. Furthermore, high rainfall and greater residence time of sediments in the basin because of lower relief make the condition more favourable for intense chemical weathering. 5.2. Provenance of aquifer sediments/matrix The aquifers sediment characteristic (mineralogical and elemental oxides composition) are applied to estimate sediments provenance, tectonic settings in their formation, and the redistribution or chemical changes in sediments during and after deposition (sedimentary processes) from the source region (the Eastern Himalayas and Naga Hills) to the deposition site (the Brahmaputra alluvial plain) (McLennan et al., 1993; Nesbitt et al., 1996). Previous studies (Roddaz et al., 2006; Bhuiyan et al., 2011; Guillot et al., 2015) suggested that sediment chemical composition also indicates chemical weathering intensity, physical process e.g., erosion, and hydraulic sorting, rate of sediment supply, and element mobility during sediments transportation from source to sink. Fralick and Kronberg (1997) and Lupker et al. (2012) suggested a binary diagram of major oxides against SiO2 to measure the hydraulic behaviour of minerals and mobility of their elements. Bivariate plots of Al2O3, Fe2O3, and TiO2 vs. SiO2 give much strong negative correlations (r2 = 0.7, 0.9, and 0.7, respectively) in the case of S-aquifer sediments than NW and N regions (Fig. 6). Previously mentioned, S-aquifer sediments are clay, however, NW and N aquifer mostly sand (fine, medium and coarse). Subsequently, CaO and MnO are not showing any trend, but SiO2 vs. Na2O gives positive correlation in NW and N regions of the Brahmaputra basin. The concentrations of Na2O, CaO and MnO decrease in S-aquifers (fine fraction); suggests that these elements are more sensitive or mobile during chemical weathering (Fig. 6a). In contrast, sediments of southern aquifers are highly enriched with Al2O3, Fe2O3, and TiO2, indicating that during chemical weathering these elements are highly immobile and due to sorting effects concentrated in fine clay fraction. Fig. 6b shows the behaviour of three immobile elements (Al2O3-Fe2O3-TiO2) in the aquifer sediments of three different locations. The binary plots of Al2O3 vs. Fe2O3 and Al2O3 vs TiO2 demonstrate elements provide a good correlation with each other in sediments of S-BH-1, 2 (southern region), because of similar hydraulic sorting; which indicates that sediments derived from same provenances or similar rock types. However, in the case of NW-BH and N-BH sediments, they are not showing any significant correlation, and weathering trend suggest that these less chemically altered aquifer sediments might be derived from different provenance and lithotypes.

5.4. Arsenic fate in BRB 5.4.1. Water-sediment interaction and Arsenic-liberation Sediment-water interaction is possibly the primary mechanism of groundwater evolution and quality. It directly depends on the cycling of mineral phases (Stumm and Morgan, 1996, and Drever, 1997). As discussed in section 4.1, aquifer sediments of the BRB (NW, N and mostly S- region) were found to be highly enriched with Pl, Kfs, Bt, Am, Cpx, Opx minerals with accessory mafic minerals and Kln, Chl and smectite (Sme), were identified as a significant alteration product. The clay minerals abundance in the aquifer sediments of the BRB is mainly controlled by degree and rate of weathering processes. These weathering processes established through thermodynamic stability

5.3. Influence of morpho-tectonics on aquifer composition and asenrichment Several studies (e.g. Saunders et al., 2005; Stanger, 2005; Guillot and Charlet, 2007; Mukherjee et al., 2014; Verma et al., 2016; Verma, 2017) have proposed a hypothetical link between Himalayan orogeny 180

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Fig. 6. a) Binary diagram of major oxides versus SiO2 of all aquifer sediments to demonstrate elemental mobility; b) Diagram shows bivariate relationship between two immobile elemental phases to measure hydraulic sorting effect during weathering processes in aquifer sediments of different studied regions in the BRB.

Ahmed et al., 2004; Chakraborty et al., 2007; Seddique et al., 2008; Xie et al., 2008) high abundance of phyllosilicate minerals (mica, chlorite, and clay minerals) and Fe-oxides/oxyhydroxides-coated feldspar, illite, chlorite and residual magnetite might provide available reactive surfaces or inner sites for As adsorption or precipitation, potentially via FeMn and Al oxyhydroxide or hydroxide. These mineral phases are dominant components in the fine sediments fraction, which adsorb a significant amount of As and act as carriers of As in the sediments. The adsorbed As onto fine sediments (clay minerals) might be mobilized as a pH-dependent adsorption/desorption (Anawar et al., 2003). Also, the present data-set has shown that particle sizes of aquifer sediments might have a controlling factor for As distribution and mobilization in groundwater, as strong association of solid phase As with fine sediments fraction, i.e., clay, and silt. In the present study, arsenic shows the positive correlation with Al2O3, and Fe2O3 in all sediment samples from various regions (NW-BH, N-BH, and S-BH) but gives more specific or good relationship in aquifer sediments of S-region (Fig. 8). Smedley and Kinniburgh (2002) suggested that due to chemical weathering of sedimentary rock, As might be concentrated or accumulating on the surface of immobile phases, i.e., Al2O3 and Fe2O3 or secondary iron in fine clay sediments, which can act both as sources and sinks for As, depending on geochemical conditions. This relationship might be explained that As association

relationship between the collected groundwater samples and the aluminium-silicate minerals viz. feldspar, muscovite, and clay minerals (Fig. 4). The aquifer mineralogical composition of the study area has shown the dominance of feldspar along with phyllosilicates minerals (i.e., illite, chlorite, and biotite), where feldspar is highly reactive minerals and releases most of Ca into the groundwater during chemical weathering (Drever, 1988). Fig. 4 shows that alteration or dissolution of k-feldspar added sufficient K+ in solution, which leads to precipitation of kaolinite and appearance of muscovite in the groundwater system (eq. (1.2)). The concentration of dissolved Ca2+, Mg2+, and HCO3− in groundwater steadily increase until calcite and dolomite precipitates, and chlorite begins to precipitate (eq. (1.3)). 3Al2Si2O5(OH)4 (kaolinite) + 2K+ (aq) = 2KAl3Si3O10(OH)2 muscovite + 3H2O (aq) +2H+ (aq) (1.2) 5Mg2+(aq) + 3SiO2 (aq) + 2Al3+ (aq) = Mg5Al2Si3O10(OH)8 (chlorite) + 16H+

(aq)

+

12H2O (1.3)

In the present study, the sediment-water interaction is suggested through reaction-path models, indicating the formation of high secondary minerals mostly clay minerals, i.e., saponite, and nontronite (smectite group), kaolinite, daphonite (chlorite group) and celadonite (altered biotite). According to previous studies (Anawar et al., 2003;

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more easily released into the groundwater (Nickson et al., 2000; Islam et al., 2004) than silicate-bound As (Fig. 9). 4FeOOH(sorbed As) + 14CH2O + +8HCO3−(aq) + 6 H2O + As (aq) (release)

7H2CO3 = 4Fe2+(aq) (1.6)

Therefore, chemical weathering intensity or high abundance of immobile elements might be the influencing factors for As-association with aquifer sediments of the different studied regions (NW, N and S) in the BRB.

5.4.2. Arsenic phase and mobilization The discussion so far has suggested that grain sizes and mineralogy of aquifer sediments might have controlling factor for As distribution and mobilization in groundwater, as the highest As enrichment occurs in S-aquifers (As ∼ 5.5 μM), where the aquifer composition is dominated by phyllosilicates (i.e., illite, chlorite, and biotite) and clay minerals compared to NW and N regions. Therefore, clay minerals, are the major hosts of arsenic, which contributed both adsorbed and structural arsenic to the arsenic budgets. Non-parametric Spearman's rho (ρ) correlation has suggested that various hydrogeochemical parameters are controlling factor for As release into groundwater of the study area (Supplementary Table S6). Detailed observation of the relationship between As and redox-sensitive species and solutes in the BRB is presented in Verma et al. (2016). In this study, the concentration of dissolved As shows negative to very weak correlation with NO3−, SO42- and EH, which propose that groundwater As-enrichment is associated with redox-dependent mobilization in the study area (Supplementary Table S6). The sequential extraction analyses demonstrate that most of the extractable solid phase-As is associated with well-crystallized, Fe(Mn, Al)-oxides/hydroxides in NW and N aquifers sediments (Fig. 9). A significant correlation of As with redox sensitive elements (Fe, Mn) have been observed in the groundwater of the NW and N regions. It is broadly accepted that reductive dissolution of As-bearing crystalline and poorly crystalline Fe/Mn oxides/hydroxides minerals phase coupled to the organic matter oxidation is the most significate mechanism of As enrichment in the alluvial aquifers of the Himalayan river basin (Bhattacharya et al., 1997; Nickson et al., 2000; Dowling et al., 2002; Harvey et al., 2002; Swartz et al., 2004; Charlet and David, 2006; Polizzotto et al., 2008; Mukherjee et al., 2008; Verma et al., 2016). Therefore, this is the most reasonable explanation for As-enrichment in groundwater of the NW and N regions in the BRB, where As released by reduction of As-containing crystalline Fe-Mn oxides minerals phase. In the groundwater of S-region, concentrations of As and HCO3− are very dominant but do not demonstrate any mutual relationship (Supplementary Table S6). Oxidation of organic matter might generate high HCO3− during FeOOH reduction and released As into groundwater (eq. (1.6)). Arsenic shows a positive correlation with pH, suggesting that pH-dependent sorption/desorption reactions may also play a significant role to mobilize As (Anawar et al., 2003). However, a substantial portion of total solid phase-As is extracted from “sorbed” phase (F1 + F2) and amorphous Fe(Mn, Al)-oxide/oxyhydroxides (F3) in S-aquifer sediments (Fig. 9). The present study suggested that high abundance of phyllosilicate minerals (mica, chlorite, and clay minerals) and Fe-oxides/oxyhydroxides-coated feldspar, illite, chlorite and residual magnetite might contribute adsorbed As, as a co-precipitated in Fe-Mn and Al oxyhydroxide or hydroxide and structural As in residual clay and silicates minerals. Moreover, variation in pH values can be a potential controlling factor to As mobilization from sorbed or labile phase due to surface adsorption and complexation reactions on reactive sites of mineral surface, especially clay minerals (Goldberg, 2002; Dixit and Hering, 2003; Wenzel et al., 2001; and Hamon et al., 2004). Previously mentioned, S-aquifers contain a significant amount of labile As (surface exchangeable or reactive As) compared to NW and N aquifer sediments.

Fig. 7. Diagram discriminate different tectonic setting, e.g., passive margin, active continental margin, continental island arc and oceanic island arc based on concentration of elemental oxides (Fe2O3+MgO) vs. TiO2 and Al2O3/SiO2 (after Bhatia, 1983)

with fine-grained aquifer sediments and precipitation on the surface of secondary iron and residual phase (Al2O3 phase) in fine clay sediments may be influenced by chemical weathering processes in the study area. Chakraborty et al. (2007) and Seddique et al. (2008) have discussed adsorption behaviour of micas (muscovite and biotite) for As and proposed that biotite have more effective adsorption site for As than muscovite. The present study shows that As positively correlate with K2O, and dominance of mica (Bt) in the aquifer mineral composition of BRB, suggesting that As might be released from the chemical weathering and dissolution of As-laden mica (Bt and Ms) in the groundwater system of the Brahmaputra basin. Eq. (1.4) shows that Fe2+ release into the groundwater due to chemical weathering of biotite. Subsequently, Fe2+ react with O2 (local oxidant) to form FeOOH can act as a potential sorbent for As adsorption and precipitation (Dzombak and Morel, 1990). 2KMgFe2AlSi3O10(OH)2 (biotite) + 14CO2+ 15H2O (aq) = Al2Si2O5(OH)4 + 4Fe2+ (aq) + 2K+ (aq) +14HCO3− (aq) +4H4SiO4 (1.4)

4Fe2 +(aq) +

1 O2 (aq) + H2 O(aq) = 2FeOOH + 2H+ (aq) 2

(1.5)

However, due to Fe(III) reductive dissolution, this adsorb As on reactive surface sites of Fe oxyhydroxides/oxides (i.e. ferrihydrite) is 182

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Fig. 8. Diagram shows correlation between Al2O3, Fe2O3, K2O and TOC contents and total As concentration in NW-BH, N-BH, S-BH1 and S-BH2.

6. Conclusion

Groundwater As shows negative correlation with redox-sensitive parameters, e.g., Fe [ρ = −0.30], Mn [ρ = −0.35] (Supplementary Table S6) suggested that As and Fe may not be simultaneously released by reductive dissolution of Fe-Mn oxide and hydroxides (Islam et al., 2004). Additionally, poor correlation between dissolved As and Fe may rise due to resorption of released As by reduction of Fe/Mn-OOH onto residual or partially reduced Fe/Mn oxides-hydroxides (McArthur et al., 2004). Therefore, sorption reaction on reactive surface sites of ferric oxides-hydroxides (i.e., ferrihydrite) minerals might be an important mechanism of As liberation in the groundwater system of S-region. The mineralogical study confirmed that aquifer sediments of the BRB dominated by phyllosilicate minerals (mica, chlorite, and clay minerals) and Fe-oxides/oxyhydroxides-coated feldspar, illite, chlorite and residual mineral phase, which might contribute adsorbed As, as a coprecipitated in Fe-Mn and Al oxyhydroxide or hydroxide and structural As in residual mineral phases (clay) and silicates minerals. However, the contribution of structural As is insignificant, because it required more energy to release into groundwater system than adsorbed and precipitated arsenic. Thus, the present study suggests the chemical weathering of aluminium-silicate minerals produced secondary iron (Fe oxides-hydroxides) and residual phase of metal oxides, favourable for the accumulation of solid-phase arsenic on the surface of poorly crystalline or amorphous phase (Fe/Mn-OOH) and crystalline Fe/Mn oxides minerals. Subsequently, the reductive dissolution of minerals, i.e., Fe/Mn oxides/ hydroxides due to microbially mediated redox reaction with high organic carbon (TOC) might be the dominant mechanism for As enrichment in groundwater of the Brahmaputra river basin aquifers. Also, the competitive adsorption/desorption reactions play a significant role in As mobilization from water-aquifer matrix interaction in aquifer sediments of the BRB.

The present study of water-rock interaction has been executed in three Himalayan orogeny-controlled sites of the rarely studied Brahmaputra river basin (BRB), the 4th largest fluvial system of the world. The study concludes that the hydrochemistry and trace element concentrations are mainly dependent on the tectono-morphic influence on sediment provenance and their interaction with groundwater along flow-path in the high-altitude geomorphic terrains. This is reflected in the sediment mineralogy and solute phase distributions in downstream aquifers. The major mineral compositions of the BRB aquifer sediments indicate the presence of feldspar (Kfs and Pl), mica (Bt and Ms), and orogeny-sourced ferromagnesian minerals, (Am, Hem, Man, Bt, Opx, Cpx) along with some heavy minerals, i.e., monazite (Mnz), and rutile (Rt) in NW and N aquifer sediments. Accordingly, sediments on the northern side of the Brahmaputra river are hypothesized to have been derived from granitic (felsic) terrains of the Eastern Himalayan (Higher, Lesser, Siwaliks) rocks and igneous and metamorphic rocks of the Eastern Syntaxis. The mineral compositions of S-aquifers differ from the northern aquifers by the presence of the higher proportion of Fe/Mg aluminosilicates, phyllosilicates or clay minerals, and heavy minerals, i.e., titanite (Ttn) and ilmenite (Ilm), derived from weathering of mafic and ultramafic rocks, calc.-alkaline rocks, and gabbroic complex (ophiolite) of the Naga-ophiolitic belt. Therefore, it is evident that the aquifer sediments in the northern and southern sides of the BRB alluvial aquifers are derived from upstream orogenic provenances that have evolved through differential tectonic history. Furthermore, sedimentwater interaction along flow path, as envisaged through reaction-path models suggests intense chemical weathering of S-aquifer matrix, reflected by the higher equilibrium of secondary mineral phases, i.e., saponite, and nontronite (smectite group), kaolinite, daphonite (chlorite group) and celadonite (high altered biotite), as compared to NW and N regions. The groundwater of S-region aquifers is more 183

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Fig. 9. Diagram show total concertation of solid-phase As and sequentially extractable As, and relationship with grain size and total organic carbon (TOC) in BRB sediments for a) NW-BH, b) N-BH, and c) S-BH1.

severely As enriched (dissolved As up to 0.42 mg/L [5.53 μM]), in comparison to NW and N regions. The present study proposes that chemical weathering of As-enriched sediment derived from arc-sourced mafic minerals (e.g., olivine, pyroxenes, chamosite, and titanite) facilitates leaching of more soluble, physio-sorbed solid phase As from the

surface of poorly crystalline or amorphous mineral phases into the groundwater system of BRB. Therefore, high variation in groundwater As concentrations among various tectono-morphic regions of the BRB may be explained in term of tectonic set-up and change in lithofacies, where As subsequently mobilized through the differential, provenance184

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influenced sediment-water interaction and biogeochemical redox processes into groundwater of the BRB.

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