Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentarsenic emission in the environment. There are several metallurgical plants, cement factories,incineration and chemical industries in eastern and Northeast India which contribute to theemission of arsenic into the environment. However, there is no data available, on the exacttonnage of arsenic entering the environment. A secondary leading industry near greater Kolkata,West Bengal, releases arsenic to the environment. The maximum concentration in soil of thatarea was reported to be 9740 ± 226 mg/kg while the minimum was 17.5 ± 0.52 mg/kg (Chatterjeeand Banerjee, 1999). Leaching of arsenic in groundwater is also expected in the vicinity of areasof landfills containing waste and hazardous waste piles (Boyle and Jonasson, 1973; Tripathi etal., 1997; Pandey et al., 1998). The use of fertilizers and insecticides also causes highconcentration of arsenic in soil compartments. There is a lack of information on theanthropogenic deposition of arsenic, within the extensive alluvial tract of the Ganga-Brahmaputrariver basin. The arsenic-affected areas are the parts of the lower delta plain of the Ganges andfoothills of Brahmaputra and Barak valley. The sources of arsenic are natural or may partly stemfrom anthropogenic activities like intense exploitation of groundwater, application of fertilizers,burning of coal and leaching of metals from coal-ash tailings. However, it has been contemplatedthat the Ganges-Brahmaputra basin has rather been undisturbed by anthropogenic sourcescompared to industrialized countries, where river basins have generally been affected byindustrial activities (Huang et al., 1992).3.3 Occurrences of Arsenic in GroundwaterSeveral studies suggested that the groundwater arsenic contamination is mostly restrictedto the alluvial aquifers of the Ganges delta comprising sediments carried from the sulphide-richmineralized areas of Bihar and elsewhere surrounding the basin of deposition (Bhattacharya etal., 1997; Das et al., 1995). However, recent studies indicated that the vast tract of Indo-Gangeticalluvium extending further to the west and the Brahmaputra alluvium have elevatedconcentrations of arsenic in wells placed in the late Quaternary and Holocene aquifers. Arsenicreleased during the weathering of sulphide minerals is generally adsorbed onto thesurface of iron oxy-hydroxides that precipitated under oxidizing conditions normallyprevailing during the deposition of the Holocene sediments. However, redox processes inthe sediments triggered the reductive dissolution of iron oxides that transferredsubstantial amounts of arsenic in aqueous phases through biogeochemical interactions(Amaya, 2002; Smedley and Kinniburgh, 2002). Arsenic-containing groundwater inGanga-Brahmaputra River basin is hosted by the sediments deposited by the rivers during thelate Quaternary or Holocene age (< 12 thousand years). Lithology of those late Quaternarysediments includes sands, silt and clay. Mineralogical composition of those sediments consists ofquartz, feldspars, illite and kaolinite and the fine-grained over bank facies are rich in organicmatter (Nickson et al., 1998; Ahmed, 1999; Datta and Subramanian, 1998; Sikdar and Banerjee2003). There is a thick layer of newer alluvium containing sand, silt and clay, which spread out bynumerous rivers that originate from the Himalayas both in the north and northeast. Mostenvironmental arsenic problems, recognized so far, are the result of mobilization undernatural conditions.NIH & CGWB 43
Sources and Causes of Groundwater Arsenic Contamination in Ganga-Brahmaputra Plains3.4 Mechanisms of As MobilizationIn most studied areas it was seen that high-arsenic groundwater was not related toareas of high arsenic concentration in the source rock. Two key factors were identified: first,there should be very specific biogeochemical triggers to mobilize arsenic from the solid/sorbed phase to groundwater, and second, the mobilized arsenic should have sufficienttime to accumulate and not be flushed away, that is, it should be retained in the aquifer(Smedley and Kinniburgh, 2002). In other words, arsenic released from the source should bequick, relative to the rate of groundwater flushing. There are number of processes for mobilizationof arsenic in groundwater namely, (i) mineral dissolution, (ii) desorption of arsenic under alkalineand oxidizing conditions, (iii) desorption and dissolution of arsenic under reducing conditions, (iv)reduction of oxide mineral surface area, and (v) reduction in bond strength between arsenic andholt mineral surface (after Smedley and Kinniburgh, 2002).Oxidation of sulphide minerals (pyrite-FeS2) was advocated strongly by manyinvestigators in West Bengal as the cause of groundwater arsenic contamination (Das etal., 1994). According to this hypothesis, arsenic is released from the sulfide minerals(arseno-pyrite) in the shallow aquifer due to oxidation (Mandal et al., 1998). The loweringof water table owing to over exploitation of groundwater for irrigation is the cause ofrelease of arsenic. The process can be explained as: the large-scale withdrawal of groundwatercauses rapid diffusion of oxygen within the pore spaces of sediments and, thereby, increasesdissolved oxygen in the upper part of groundwater. The newly introduced oxygen oxidizes thearseno-pyrite and forms hydrated iron arsenate compound known as pitticite in presence ofwater. This compound being very soft and water-soluble, the light pressure of tube-well waterbreaks the pitticite layer into fine particles and make it readily soluble in water. It then seeps likedrops of tea from the teabag and percolates from the subsoil into the water table. When the tubewellis in operation, it comes out with the extracted water (Safiuddin and Karim, 2001). Suchoxidation processes could explain possible mechanism of As mobilization in some parts of theaquifers, particularly at the shallowest levels but may not be the main cause of groundwaterarsenic contamination in the Ganges-Brahmaputra river basin. A recent research study explainedthat desorption or dissolution of arsenic from iron oxides could be the process onregional distributions of arsenic in water (Smedley, 2004). According to this process, aseries of changes in the water and sediment chemistry as well as in the structure of iron oxidestake place at the onset of reducing conditions in aquifers. Many of these changes are poorlyunderstood on a molecular scale. Broadly, it can be stated that some critical reactions totransform to reducing conditions and subsequent arsenic release are likely to take placeto reduce arsenic from its oxidized (As(V)) form to its reduced (As(III)) form. Under manyconditions, As (III) is less strongly adsorbed to iron oxides than As (V); and reduction in suchcase involves a net release from adsorption sites. Dissolution of the iron oxides themselvesunder reducing conditions is another potentially important process. Under aerobic andacidic to neutral conditions, adsorption of arsenic (As (V)) to iron oxides is normally strong andaqueous concentrations are usually low. However, the sorption is less strong at high pH.44NIH & 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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Mitigation and Remedy of Groundwate
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5.8 SummaryMitigation and Remedy of
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