Genesis of arsenic contamination of groundwater in alluvial Gangetic aquifer in India S.K. Acharyya & B.A. Shah Department of Geological Sciences, Jadavpur University, Kolkata, India ABSTRACT: Arsenic pollution in groundwater mainly affects major parts of the Ganga- Brahmaputra delta in West Bengal and Bangladesh, as well as, parts of narrow entrenched lower- middle sections of the Gangetic floodplains. Arsenic adsorbed on hydrated ferric oxides (HFO) was preferentially entrapped in organic rich deltaic Holocene sediments and less frequently in its floodplains. Severe reducing condition that developed later mobilized arsenic to groundwater mainly in the deltaic domain. The sediment cover on the Pleistocene uplands in the Bengal Basin and the interfluve Ganga plain are free of arsenic problem. Arsenic is mobilized to groundwater by bio-mediated reductive dissolution of HFO. Strong reducing nature of groundwater in the Bengal Basin and parts of affected flood plains is shown by high concentration of iron (р9–36 mg/L), which is generally low (Ͻ1mg/L) in the Ganga alluvial plain upstream of the Bengal Basin indi- cating that groundwater is not adequately reducing in nature to mobilize arsenic. 1 INTRODUCTION Arsenic contamination of groundwater affects extensive low-lying deltaic areas in the Bengal Basin, located mainly to the east of Bhagirathi river in West Bengal (India) and Bangladesh, and parts of narrow entrenched middle Ganga floodplain in parts of Bihar, Jharkhand and eastern Uttar Pradesh (UP; Fig. 1). The upper permissible limit of arsenic in potable water is set at 50␮g/L in 17 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Figure 1. Map showing Quaternary basins in northern parts of the Indian subcontinent. 1. Ganga Alluvial Plain. 2. Bengal Basin. Abbreviated localities: A – Allahabad, BX – Bauxar, C – Chhapra, B – Balia, P – Patna, BG – Bhagalpur. Copyright © 2005 Taylor & Francis Group plc, London, UK India, whereas, recommended limit is 10␮g/L by WHO. Arsenic concentrations in tube-well water exceed 50␮g/L limit manifold in parts of the affected area and several cases of arsenical diseases have been recorded, particularly from the pandemically affected parts of the Bengal Basin. Arsenic is also recorded to have entered into food chain from areas irrigated with arsenic laced groundwa- ter (Sanyal & Naser 2002). There is wide variability in level of arsenic concentrations in space and time, with high values restricted to some ‘hot-spots’, whereas, arsenic safe zones are also located within broadly affected areas. Regular monitoring of arsenic level in tube-wells is thus a necessity within arsenic affected and potentially risk areas. Arsenic contamination also affects isolated patchy areas, some of which are located on arsenic enriched acid magmatic rocks. In the affected areas in northern parts of the Proterozoic Dongargarh rift zone, Chhattisgarh, tube-well water is contaminated but dug-wells are generally arsenic free except in Kaurikasa area, where arsenic enriched regolith and soil are exposed. In this area even some dug-wells are arsenic polluted (Acharyya 2002). 2 ARSENIC AFFECTED FLOOD-DELTA PLAINS IN WEST BENGAL Pandemic arsenic contamination in the Bengal Basin is essentially confined to the low-lying Ganga-Brahmaputra flood-delta plain located downstream of the Rajmahal Hills. The low-lying flood-delta basin of the Ganga, Bhagirathi, Jamuna and old Bhramhaputra rivers is entrenched and incised on the Pleistocene terraces during the low stand setting of the terminal Pleistocene period. The Pleistocene upland (and hilly belt of older rocks) flank the western margin of the Bengal 18 Figure 2. Landforms and decompositional environments in the Bengal Basin. Legend: 1. Hills of older rocks. 2. Laterite/Ferrisol Pleistocene upland. 3. Older Alluvial Plain. 4–7. Younger Alluvial Plain. 4. Recent Flood and Deltaic Plain. 5. Interdistributary Swamp. 6. Tidal Swamp. 7. Trippera Surface. 8. Location of boreholes along section lines. Abbreviations: Bl-Balagarh, B-Barind, C – Calcutta, Dk-Dhaka, Gh – Ghetugachi, Kh – Khulna, M – Madhupur tract, Md – Malda town, L – Lalmai Hills, R – Rajmahal Hills, TC – Tripura-Chittagong Hills. Copyright © 2005 Taylor & Francis Group plc, London, UK 19 Figure 3. Profiles and correlation of Holocene sediments. A: Section AB (location shown in Fig. 2) across Ganga- Bhagirathi delta in West Bengal. B: Section CD (location shown in Fig. 2) across Jamuna flood plain and Ganga delta in Bangladesh. Modified after Umitsu (1993). 1–3 are broad Holocene stratigraphic units referred in the text. Basin in West Bengal (India), whereas, those at Barind and Madhupur areas mark the northern and central parts of the basin in Bangladesh. The Bengal Basin is flanked to the east by the Tertiary hills (Fig. 2). The Pleistocene sediments on incised up-lands are oxidized to iron-stained, heavy mineral deficient sand and brown-orange stiff clay. These were well flushed by groundwater over longer period and are arsenic free. The Holocene subsurface units beneath the arsenic affected younger delta plain of the Bengal Basin are tentatively subdivided into three units (Acharyya et al. 2000) and the classification is also adopted in Bangladesh (Uddin & Abdullah 2003). The ‘basal unit’ of late Pleistocene – early Holocene sequence comprising gravelly sand above the disconformity was deposited as incised channel fills or fluvio-deltaic sand of proto-Ganga-Bhagirathi-Bharahmaputra rivers around 18,000 to 10,000 yr. B.P., when sea level rose rapidly. The basal gravelly sands are generally mica- ceous, heavy mineral rich and coarsening northward from fine to medium and coarse sand. The ‘basal unit’ of the Holocene channel fill sands is generally free of arsenic problem. The Ganga-Brahmaputra delta sedimentation of the ‘middle unit’ was induced over large area beginning around 10,000 yr. B.P., when rapid rise of sea level led to back-flooding and overtop- ping of the low stand entrenched channels and the oxidized late Pleistocene surfaces. Continued high stand, during most of the early-mid Holocene period flooded partly sedimented valley courses and converted their lower and adjacent parts to fluvial marshes, lagoons and estuary. The ‘middle unit’ comprises silt dominated mud and fine sand that commonly contain wood and other plant fragments, shell clasts and marine organic remains. High sediment load from the rapidly eroding Himalayas competed with rapid sea level rise to enforce continued sluggish deltaic sedi- mentation. The lenticular muddy sand bodies generally form numerous transient distributary channels (Acharyya et al. 2000). Most arsenic contaminated tube-wells mainly tap aquifers in the ‘middle unit’, which were very poorly flushed by groundwater due to their deltaic setting. Thus any arsenic released from these sediments following burial accumulated in groundwater (Fig. 3). Copyright © 2005 Taylor & Francis Group plc, London, UK The top mud facies of the ‘upper unit’ presently cap sandy sequences throughout the Bengal Basin. These are deposited during rapid sea level rise since 7000 yr. B.P., when sea level reached higher than present level and the southern parts of the Ganga delta was invaded by tidal mangrove and encroached by the Bay of Bengal. There was extensive development of marine and fresh water peat during 7000 to 2000 yr. B.P., within the clayey sequence of the ‘upper unit’, which were mainly confined to southernmost part of the basin. The ‘upper unit’ sediments are also enriched in arsenic but there is minor development of aquifers that are free of saline water within this sequence. 3 ARSENIC AFFECTED AREAS IN MIDDLE GANGA FLOOD PLAIN Narrow tracts of arsenic affected areas are recorded recently from middle Ganga flood plain from parts of Bihar, Jharkhand and eastern UP, and it is apprehended that arsenic contaminations would affect wide regions of the Ganga alluvial plain (Chakraborti et al. 2003). The affected areas are confined to the Newer Alluvium (Holocene) within narrow entrenched active flood plain. Major parts of the Ganga alluvium interfluve upland plains are unlikely to be affected according to us (Acharyya & Shah 2004a). Sedimentation in these entrenched flood plains was also influenced by sea level fluctuation during the Holocene, causing increased aggradation and forming fluvial swamps (Singh, 2001). The arsenic bearing aquifers are located close to the Ganga and western down faulted side of the Ghaghra river channel-floodplain in parts of Bauxar, Bhojpur and Balia districts, but extensive area exposing or having oxidized Older Alluvium (Pleistocene) at shallow depths to the east of Ghaghra river and along the northern bank of the river Ganga in Chhapra and Baishali districts are unaffected (Fig. 1). Over 80% of the Ganga alluvium plain is represented by vium plain close to its confluence with the Ganga is also free of arsenic contamination. Our study in this area is under progress. 4 SOURCE AND RELEASE OF ARSENIC IN GROUNDWATER No specific sources of arsenic could be identified for the Ganga-Brahmaputra river system and potential sources are located both in the Himalayas and Peninsular India. Contrary to claim other- wise, our mineralogical studies indicate that arsenic rich pyrite (Fig. 4) or arsenic minerals are rare or absent in the aquifers from affected areas in West Bengal. However, rare presence of biogenic pyrite is recorded in reducing environment often in association of degraded plant remains (Acharyya 2001, Acharyya & Shah 2004b). These have acted as sinks for and not sources of arsenic. Arsenic contamination is moderate in aquifer sediments from affected and adjacently located arsenic safe sediments from arsenic polluted and safe zones located within overall arsenic affected areas in West Bengal reveal common occurrence of iron-coated quartz and clay (illite) grains, iron- manganese-siderite, magnetite and biotite/chlorite, which are arsenic bearing (Acharyya 2001, Pal et al. 2002). Sludge samples from Balagarh block (Fig. 2), Hoogly district, contain minor fractions of peaty wood fragments (Fig. 5) within which arsenic is locked in authigenic and frambroidal pyrite (Acharyya & Shah 2004b). Aggregates of Fe-Mn-siderite concretions often having biogenic colony like structure and frambroidal pyrite have been found in aquifer sediments from Balagarh and Ghetugachhi area in Hoogly and Nadia districts respectively (Pal et al. 2002, Acharyya & Shah 2004b). Arsenic release by oxidation of pyrite has been disapproved in general, because pyrite is nearly absent in the affected aquifer sediments and sulfate concentrations are very low in affected ground- water. Biomediated reductive dissolution of hydrated ferric oxides (HFO) that occur mainly as coat- ings on sediment grains and corresponding oxidation of sedimentary organic matter is regarded as the main mechanism, which mobilizes arsenic to groundwater from aquifer sediments (Bhattacharya et al. 1997, Nickson et al. 1998, 2000, Kinniburgh & Smedley 2001). Arsenic sorbed 20 Copyright © 2005 Taylor & Francis Group plc, London, UK areas in West Bengal (Ghetugachhi and Baruipur area; Fig. 2). Studies on drill cores of aquifer older Alluvium interfluve upland (cf. Kumar et al. 1996), which is also unaffected. The Son allu- 21 Figure 4. Biogenic pyrite (marked P) in a carbonaceous shale clast. Pyrite growth often follows grain boundary. Arsenic bearing nature of pyrite revealed by SEM-EDX scan, Balagarh-Sripur area, Balagarh. Figure 5. Degraded woody fragment where pyrite is replacing cell structures. in discrete phases of Fe-Mn-oxyhydroxide was preferentially entrapped in argillaceous and organic rich early-mid Holocene deltaic sediments and the Holocene floodplain sediments. Recent studies has established that iron-rich groundwater is produced by the activities of anaerobic heterotropic Fe 3+ reducing bacteria (IRB), which preferentially reduce and dissolve least crystalline discrete Copyright © 2005 Taylor & Francis Group plc, London, UK phases of HFO, with consequent release of its sorbed arsenic and other trace elements to ground- water. Ferrous ion, released by IRB from Fe-bearing mineral phases or HFO sediment coatings pos- sibly reacted with abundantly present bicarbonate in groundwater to precipitate siderite concretions, which grew around sediment grains and/or centers of IRB colonies (Acharyya & Shah 2004b). Reduction of HFO is common and intense in affected aquifer in the Bengal Basin and parts of Ganga floodplain. This is demonstrated by high concentration of dissolved Fe (р9–36 mg/L) in arsenic contaminated groundwater (Acharyya et al. 1999, Acharyya & Shah 2004a,b). Under stronger reducing condition and in presence of organic carbon, sulfur reducing bacteria (SRB), instead would precipitate pyrite which would co-precipitate arsenic from groundwater. The chemistry of arsenic affected tube-well water is so far based on study of mixed samples from the entire column. An inflatable packer-stradle-pump assembly was used by us (Guha et al. in prep.), to test chemical characteristics of aquifer water from a specific depth. Interpretation of sed- iment and water analysis indicates that iron-reducing condition develop at several levels releasing arsenic from sediments to the groundwater. Although arsenic is present in the sediments through- out the entire depth of boreholes, it is not released under nitrate and sulfate reducing conditions. Clayey lenses in the aquifer create low permeability zones preventing electron acceptors like nitrate and sulfate to reach these levels where iron reducing conditions and release of arsenic prevail. The presence of tritium, high 14 C (ϳ80–112 pMC) and ␦ 18 O values (Ϫ3.5 to Ϫ5.5 ‰) in shallow aquifer groundwater in the Bengal Basin (Shivanna et al. 2000, Agrawal et al. 2000) indicates con- tinuing recharge from local rain, surface and floodwater. Extensive groundwater irrigation has accel- erated flow of groundwater that brought dissolved degraded organic matter in contact with HFO bearing sediments, enhancing reduction process and triggering release of arsenic (Acharyya 2001). Arsenic contamination in groundwater in alluvial aquifer is typically confined to organic rich fluvio- deltaic sediments e.g. Ganga-Brahamaputra delta in the Bengal Basin (Nickson et al. 1998, 2000, Kinniburgh & Smedley 2001), Red and Mekong River deltas in Vietnam (Berg et al. 2001). Major parts of the Ganga alluvial plain is also subjected to equally intensive groundwater irrigation, but bulk of the interfluve upland in the Ganga plain corresponding to Older Alluvium (Pleistocene) (cf. narrow entrenched floodplain in the parts of lower-middle Ganga plain in Bihar, Jharkhand and UP are arsenic contaminated. Arsenic affected local pockets may also occur in northern fan areas as recorded from terai region in Nepal (Chitrakar & Neku 2001, Bhattacharya et al. 2003). 5 CONCLUSIONS Pandemic arsenic pollution in groundwater mainly affects low-lying entrenched flood-delta plains of the Bengal Basin covering parts of West Bengal (India) and Bangladesh. The contaminated aquifers are mainly confined to delta sediments deposited around 10,000–7500 yr. B.P., when sea level rose rapidly establishing high stand setting. Arsenic contamination also affects Holocene entrenched floodplain in parts of lower-middle Gangetic plain in Bihar and eastern UP. Sediment cover on Pleistocene uplands in the Bengal Basin as well as, in interfluve uplands in lower-middle parts of the Gangetic floodplain are oxidized and free of arsenic. Pyrite or any other arsenic bear- ing mineral are nearly absent in aquifer sediments. Arsenic sorbed in phases of iron-oxyhydroxide was preferentially entrapped in organic-rich clayey deltaic sediments and partly in floodplain sed- iments in lower-middle sections of the Ganga plain. Severe reducing conditions developed later, mainly in the delta sediments and partly within entrenched floodplains, releasing arsenic to groundwater by reductive dissolution of iron-oxyhydroxides. Arsenic contaminated groundwater from the affected areas is thus generally enriched in iron (р9–36mg/L). REFERENCES Acharyya, S.K. 2001. Arsenic pollution in groundwater from lower Ganga plains, Bengal Basin. Indian Journal of Geology 73: 1–19. 22 Copyright © 2005 Taylor & Francis Group plc, London, UK Kumar et al. 1996) are unaffected by arsenic contamination. However, Holocene alluvium within Acharyya, S.K. 2002. Arsenic contamination in groundwater affecting major parts of southern West Bengal and part of western Chhattisgarh: Source and mobilization process. Current Science 82: 740–744. 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