Ground Water Arsenic Pollution in West Bengal and Bangladesh: Role of Quaternary Stratigraphy and Sedimetation

Abstract

Arsenic pollution is pervasive at shallow depth in low-land organic rich Holocene (~10,000 year BP) alluvium in southern parts of the Bengal Delta. The upland terraces, mainly made up of Pleistocene (~10,000yr to ~1.8 my BP) sediments and capped by an oxidised zone, are free of arsenic problem [1,2]. Arsenic is sourced mainly from the Himalaya. Damodar fan delta, although entirely drained through Peninsular India, is also marginally affected. No arsenic mineral is present in contaminated alluvial aquifers; instead it occurs adsorbed on Hydrated Ferric Oxide (HFO), which generally coats sediment grains. Arsenic is released to groundwater mainly by bio-mediated reductive dissolution of HFO with corresponding oxidation of organic matter. Arsenic concentration in alluvium is broadly uniform, thus release process rather than source is important factor in arsenic pollution. Dug-well because of their oxygenated nature is useful low cost source of arsenic-safe water even in contaminated areas. This is particularly relevant in areas located away from surface water sources. Arch & Anthropol Open Acc Copyright © Acharyya SK 283 How to cite this article: Acharyya S. Ground Water Arsenic Pollution in West Bengal and Bangladesh: Role of Quaternary Stratigraphy and Sedimetation. Arch & Anthropol Open Acc. 2(5). AAOA.000550. 2018.DOI: 10.31031/AAOA.2018.02.000550 Volume 2 Issue 5 The Holocene sediments beneath YDP in West Bengal and Bangladesh have been tentatively classified into three broad stratigraphic units (Figure 3) based on limited subsurface data [1,6]. These alluvial units are not layer cake in geometry. Thus at places late Pleistocene sediments can occur at shallow depth and incised Holocene channels may occupy deeper levels. The near surface Unit 3 consists of mud, silt, fine sand and locally present peat beds; Unit 2 is dominantly composed of fine, often dirty sand with clay intercalations; whereas, the Unit 1 is coarser, cleaner and sandy. Most arseniferous tube wells generally tap aquifers in Unit 2. C14 (radio carbon) ages from organic matter in the basal Unit 1 range from ~28,300 to ~12,300yr BP. The basal sand and gravel bearing Unit 1 was deposited in entrenched paleochannels during Late Pleistocene and earliest Holocene under low-stand setting [1,6]. Sea-level gradually decreased and reached their lowest level of ~135m during the Late Pleistocene around 18,000yr BP, when the Pleistocene and Late Tertiary sediments located over the present Ganga delta and shelf areas were exposed to erosion and oxidation. Parts of Pleistocene alluvium covering the present delta region remained as incised upland terraces, which were dissected by the proto-Bhagirathi-Ganga-Brahmaputra river system. The Pleistocene terraces, capped by oxidized sediments and hard brown-bluish gray impervious clay paleosol formed interfluve uplands. These were dissected by Holocene sedimentfilled paleochannels and partly buried under younger deltaic Holocene sediments. The hard clayey aquitard corresponding to the Last Glacial Maximum Paleosol (LGMP) strongly influenced groundwater flow and controlled location of arsenic pollution to shallow aquifer in the Bengal Delta [7,8]. The basal sand and gravel unit was not uniformly developed but was mainly restricted to paleochannel floors (Figure 2 &3). Figure 1: Arsenic contaminated areas in the Indian subcontinent. 1-Ganga-Brahmaputra alluvial plain; 2Landforms and depositional environments in parts of Bangal Basin. C: Abbreviated localities; A-Arah; B: Balia; C: Chhapra; G: Ghetugachhi; K: Kolkata, Rn: Raninagar The rising sea level at the initial stages would have flooded the partly sediment covered entrenched valley courses of protoBhagirathi and the Ganga-Brahmaputra rivers, and converted their lower and adjacent parts to fluvial marshes, lagoons and estuaries. Swamps were formed ahead of the rising sea level. High tides in the Bay of Bengal advanced the tidal flat and mangrove growth to invade the delta mouth, which would clog with mud and silt rich in organic content. Thus, clay and silt enriched in organic matter and interbedded with lenticular sand bodies from numerous transient tributary channels dominated the middle unit. Sea-level again began to rise rapidly between 7000 to 5500yr BP, reaching higher than the present level and southern parts of the Ganga-Bhagirathi delta were invaded further by tidal mangrove and encroached by the Bay of Bengal. These sediments occur at 7m to 2m beneath msl. There was widespread development of marine and fresh water peat layers around Kolkata and about 60 km further north during 7000 to 2000yr BP. Arsenic Toxicity in Parts of Damudar Fan-Delta The Damodar river was flowing east to meet the Bhagirathi River during the middle of 18th century, but it has since rotated its course over its fan shifting its mouth 128km to the south. The Ajoy river located further north, still flows east to meet the Bhagirathi river, which was the regional slope also followed by the older Arch & Anthropol Open Acc Copyright © Acharyya SK 284 How to cite this article: Acharyya S. Ground Water Arsenic Pollution in West Bengal and Bangladesh: Role of Quaternary Stratigraphy and Sedimetation. Arch & Anthropol Open Acc. 2(5). AAOA.000550. 2018.DOI: 10.31031/AAOA.2018.02.000550 Volume 2 Issue 5 proto-Damodar and other tributary rivers of the western upland. The Bhagirathi River system continued to remain to the east of the proto-Damodar and Damodar fan (Figure 2). Although arsenic affected areas are mainly located to the east of the Bhagirathi river, a few areas west of the Bhagirathi River are also arsenic affected (Figure 2) [7]. Many affected areas are located over the Damodar fan-delta, where maximum As concentrations of 85-90μg/L have been recorded. Figure 2: Geomorphologic and Quaternary-morpho-stratigraphic map west of Bhairathi river. 1-Laterite plain (Pleistocene), 2Older Alluvial Plain (OAP) Pleistocene-Holocene), 3-Younger Delta Plain YDP (Holocene), 3A Damodar fan-delta; 3B-Bhagirathi delta plain; 4-Recent Bhagirathi plain Source and Release of Arsenic No specific sources of arsenic could be identified for the affected alluvial areas e.g., Bhagirathi-Ganga delta, Damodar fan-delta. Roy Chowdhury et al. [9] and Chakraborti et al. [10] postulated that the opaque particles and pyrite in the aquifer sediments were the source of arsenic. Nickson et al. [4] on the other hand, suggested that arsenic is released by reduction process and sourced from sulphide deposits such as the copper belt in Jharkhand (erstwhile Bihar) and Gondwana coal basins of Damodar valley. The potential sources suggested by Nickson et al. [4], are located far south and lower in level from the arsenic affected Ganga tributary system. Acharyya et al. [1] listed sulphide occurrences, some of which are arsenic bearing, from the Himalayan foothills. As the Himalayan range is subjected to high erosion and intense rainfall during the Holocene, these minor occurrences were transported As-bearing products to the Ganga-Brahmaputra basin. Furthermore, Arsenic concentration in biotite-chlorite bearing silt fraction, from the piedmont areas feeding the Ganga and the Brahmaputra Rivers, are also generally found to be higher than normal. Acharyya et al. [1] and Acharyya & Shah [7] also reported virtual absence of pyrite or any arsenic minerals in the contaminated aquifer. The Damodar River system located mainly between the Chhotanagpur and Hazaribagh plateaus exclusively belongs to Peninsular Indian. Gondwana coal basins, minor arsenic mineralizations in mica-belt in Hazaribagh plateau and minor sulphide occurrences in Chhotanagpur plateau might have sourced mild arsenic Arch & Anthropol Open Acc Copyright © Acharyya SK 285 How to cite this article: Acharyya S. Ground Water Arsenic Pollution in West Bengal and Bangladesh: Role of Quaternary Stratigraphy and Sedimetation. Arch & Anthropol Open Acc. 2(5). AAOA.000550. 2018.DOI: 10.31031/AAOA.2018.02.000550 Volume 2 Issue 5 contaminations affecting the Damodar fan-delta [7]. Mineralogical studies on sediment cores and tube well sludge from arsenic affected and unaffected surrounding areas in the Bhagirathi – Ganga delta and Damodar fan-delta corroborate that arsenic rich pyrite or any other arsenic minerals are rare or absent. However, rare presence of biogenic pyrite is recorded in reducing environment often in association of degraded plant remains [8]. Biogenic pyrite is noted to grow along clastic grain boundaries and as cement like overgrowth on magnetite. As-bearing nature of biogenic pyrite indicates co-precipitation and sorbing of arsenic in pyrite. These biogenic pyrites therefore have locked arsenic and thus acted as sinks and not sources of arsenic in groundwater. Figure 3: Profile of Holocene sediments across Ganges delta in West Bengal. 1-3 are broad morpho-stratigraphic units. The oxidation of pyrite and arsenopyrite under oxic condition would release arsenic and produce acid and thus increase concentration of Fe and SO4 in groundwater. The release of arsenic to groundwater in the Bengal Delta was earlier inferred to be due to oxidation process [9,10]. This is unlikely as pyrite or any other As-mineral are absent or rare in alluvial aquifer sediments. Instead, release of arsenic to groundwater in alluvial aquifer is inferred to be caused by reductive dissolution of iron oxyhydroxide mediated by biota [1,3,4,11]. Biomediated reductive dissolution of hydrated iron oxide (HFO) that occurs mainly as adsorbed coatings on sediment grains and corresponding oxidation of sedimentary organic matter is regarded as the main mechanism. Ferrous ion, released by the IRB from sediment coatings of HFO or other Fe-bearing mineral phases possibly reacted with abundantly present bicarbonate in groundwater to precipitate siderite concretions, which grew around sediment grains and/or centers of IRB colony [12]. Biogenic objects a few microns in length and less than a micron width oc