Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision DocumentIn the biomethylation process, arsenic in the sediment is hydrolysed to arseneous acidand is further reduced by bacteria to As 3+ O (OH) that form arsenite in solid state. Thus, the firstrole of bacteria is to increase the ratio of As (III)/As (V) in the sediment. This As (III) readilygoes into aqueous solution to increase arsenic toxicity of water. The next step of change broughtabout by bacteria is biomethylation of As 3+ O (OH), resulting in the formation of CH 3As 5+ O(OH) 2or methyl arsenic acid and (CH 3) 2As 5+ O(OH), i.e. dimethyl arsenic acid or cacodylicacid, which is an extremely toxic compound. In each step, these substances are soluble in waterto increase toxicity. In the fourth step, cacodylic acid is again biomethylated to form (CH 3) 3As 3+or trimethylarsine by aerobic bacteria under oxidizing condition, whereas, anaerobic bacteriaunder reducing condition convert cacodylic acid to (CH 3) 2HAs 3+ or dimethyl arsine. Both aresoluble in water and are toxic. The biomethylation process increases the proportion oforgano-arsenic, which is readily absorbed by plants and animals through soil and water. Thus, thearsenic content of soil and water is reduced. However, the unabsorbed part of theorgano-arsenic being toxic pollutes the soil and water. Hence, the practice of drawing arseniccontaminated groundwater from tube wells for irrigation purposes may ultimately lead topoisoning of surface soil and surface water, which are normally arsenic-free even in arseniferousregions of West Bengal.The Brahmaputra alluvial basin is bounded by lower Himalayan Mountains in the northand northeast. High intensity of rainfall in the catchments and plain areas has contributed to highsediment loads, which have developed the valley into a long stretch of recent and old alluvium.The alluvium near the river is sandier and periodic fluviatile action keeps the alluvium stratified.However, its influence has been gradually obliterated by climate as one move from recent floodplains to old flood plains and then upland. As a result, there is deposition of coarse sand andcoarser river-borne materials along with plant cells and other organic materials cells which maycontain considerable amount of arsenic and other toxic elements. During the course of time,arsenic elements get released in the reducing environment by the process of biomethylation andget shelter within silty and clayey sediments. Some studies (Bhattacharya et al., 1997; Nicksonet al., 1998 and others) also put forward the hypothesis that the burial of sediments, rich inorganic matter, led to strongly reducing conditions in groundwater aquifer, which is facilitated byhigh water table, fine grained surface layers and widely practiced wetland paddy cultivation, aswell as microbial oxidation of sedimentary organic matter, depleting, thereby, the dissolvedoxygen in groundwater. Arsenic is released when arsenic rich iron oxyhydroxides, which areefficient arsenic scavengers, are reduced in anoxic groundwater. Such reduction is driven byconcentrations of sedimentary organic matter.Many experts agreed that the source of such high arsenic, anomaly in groundwateris geological rather than from pesticides or other artificial sources. It is postulated thatarsenic bearing sulphide minerals, the commonest of which in nature is arsenopyrite (FeAsS)and/or its alternation products, had been transported in the geologic past possibly fromthose occurring along the foothills of the Himalayas and deposited with the alluvium inNIH & CGWB 51
Sources and Causes of Groundwater Arsenic Contamination in Ganga-Brahmaputra Plainsthe Ganges-Brahmaputra basin. These extraneous arsenic minerals buried under therecent alluvium are considered to be responsible for contamination. However, arsenopyriteand its alternation products are less toxic and normally insoluble in water. Over andabove this, high arsenic anomaly has suddenly appeared in recent times, as no report ofarsenic contamination of groundwater can be traced earlier than the late seventies. In thepresent condition of emergence of greater area with arsenic pollution, the relation betweenchemistry of arsenic and high arsenic anomaly in groundwater is an interesting subject of study.3.7 Observations, Analysis and AppraisalArsenic contaminations in groundwater have affected highly fertile and thicklypopulated Bengal Delta, Middle Ganga Plain, parts of Brahmaputra Plain and some alluvial areasof North-Eastern Regions. It is by and large influenced by common geomorphological settingand sedimentation pattern associated with Pleistocene-Holocene sea level changes. Shallowlevel arsenic contaminated aquifers of Holocene age are extensive over low-lying Bengal Deltaand discontinuously along narrow entrenched channels in the Middle Ganga Plain. The oxidisedPleistocene sediments are generally free from arsenic. The Late Pleistocene and associatedoxidized sediments constituted the interfluve areas of organic rich Holocene flood plain anddeltaic sediments in the Bengal Delta and Middle Ganga Plain and the Barind. The presence ofLate Pleistocene palaeosol aquitard apparently protects the underlying sediments from thedownward movement of arsenic bearing and organic rich groundwater which might causearsenic contamination. The arsenic contaminated aquifers in the Terai belt, Nepal, are closelycomparable in nature and age to that of the Bengal Delta. The sediments in the Middle GangaPlain and in the Bengal Delta are mainly derived from the Himalaya with minor contribution fromthe Peninsular India. In the Dongargarh Proterozoic rift-zone, in Chhattisgarh, arsenicconcentration was enriched in acid magmatic rocks. In some of its local areas, arsenic and ironwere enriched further in weathered rocks and soils, where Arsenic is adsorbed onhydrated-iron-oxide (HFO), which occurs as coating on clay minerals, biotite, and chlorite. In theBhagirathi-Ganga Delta, Middle Ganga Plain and other arsenic-affected alluvial basins, Arsenicbearing pyrite or any other arsenic minerals are virtually absent in their aquifer sediments.Arsenic generally occurs as adsorbed on HFO, which preferentially enriched fine grained, fluvialand deltaic sediments and occur as coatings on quartz, clay, ferromagnesian clastic grains andorganic matter.Among few hypotheses proposed to explain the possible mechanism of arsenic groundwater contamination, most scientists have settled down to two hypotheses: (i) oxidation ofarsenopyrite or arsenic rich pyrite in soil strata, and (ii) reductive dissolution of arsenic fromsoils.The oxidation hypothesis considers that due to heavy withdrawal of groundwater,aquifer was aerated and oxygen entering into the aquifer oxidised the pyrites rich in arsenic andacid released in the oxidation process leached the arsenic in soluble form in groundwater. While52NIH & CGWB
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ACKNOWLEDGEMENTThe Government of In
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nearly 33 villages in 4 districts i
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(iii) Whether ex-situ removal techn
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6.4 Experiences during Arsenic Remo
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List of FiguresFigure no. Figure ca
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List of TablesTable no. Table Capti
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Mitigation and Remedy of Groundwate
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5.8 SummaryMitigation and Remedy of
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Mitigation and Remedy of Groundwater Arsenic Menace in India

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