Mitigation and Remedy of Groundwater Arsenic Menace in India

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Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentimportance for aquifer development, health, and remediation. Early work (DPHE, 1999) showedthat sea-level change strongly influenced the distribution of arsenic-pollution, apparentlyconfining it to sands deposited after the low-stand of sea-level at the Last Glacial Maximum(LGM; ≈20 ka; Umitsu 1993, Lambeck et al. 2002); wells screened in underlying older sandswere believed in 1999 to be arsenic-free. But of wells in the post-LGM aquifers, only 50% arepolluted by arsenic above the concentration level of 50 µg/L, and a full 75% contained less thanthe local drinking-water limit of 50 µg/L, so sea-level variations alone could not explain thedistribution of arsenic-pollution.Polizzotto et al., (2008) using hydrologic and (bio)geochemical measurements, showedthat on the minimally disturbed Mekong delta of Cambodia, arsenic is released from nearsurface,river-derived sediments and transported, on a centennial timescale, through theunderlying aquifer back to the river. Owing to similarities in geologic deposition, aquifer sourcerock and regional hydrologic gradients their results represented a model for understandingpre-disturbance conditions for other major deltas in Asia. Furthermore, the observation of stronghydrologic influence on arsenic behavior indicated that release and transport of arsenic aresensitive to continuing and impending anthropogenic disturbances. In particular, groundwaterpumping for irrigation, changes in agricultural practices, sediment excavation, levee constructionand upstream dam installations will alter the hydraulic regime and/or arsenic source materialand, by extension, influence groundwater arsenic concentrations and the future of this healthproblem. This model was supported by Harvey (2008) who postulated that the arsenic originallycame from eroded Himalayan sediments, had been washed down into low-lying regions. It iswidely believed that this arsenic dissolves and enters the groundwater under anaerobicconditions. It is, therefore, unsurprising to find that highly contaminated groundwater originatesfrom pond sediments: the steady settling and decomposition of organic material at the bottom oftropical ponds take up all the oxygen that diffuses, or is carried by downward flow, into thesediment. Water passing through pond sediments could also contain organic carbon that, ondecomposition, might help liberate arsenic from deeper sediments, adding to the contamination.But any organic carbon that is already contained in deeper aquifer sediments probablycontributes less to biogeochemical processes because it is not replenished, and what remains istypically of low reactivity.The model proposed by Polizzotto and Harvey was contradicted by Sengupta et al.,(2008) from their analysis of time-series data collected over two years for δ 18 O, δ 2 H, and Ca,Mg, K, and Cl, concentrations for 10 ponds in, and upflow of, an arsenic-polluted region ofsouthern West Bengal. They compared the compositions of As-polluted groundwater from wellswith the compositions of waters in ponds upflow and within the range of influence of the wells;and revealed that conservative tracers (δ 18 O, δ 2 H, K), and other tracers (Ca, Mg) of pondwaterand groundwater were distinct and there were no overlaps between the composition oftwo sources. These indicated that water from ponds was not the source of arsenic in thecontaminated groundwater.NIH & CGWB 47
Sources and Causes of Groundwater Arsenic Contamination in Ganga-Brahmaputra Plains3.5 Transport modeling to understand arsenic movementGeochemical, physicochemical and biological characteristics usually explain chemistryof arsenic with regard to its occurrence, release, dissolution, sorption, mechanism, etc. insoil-water phases at local scale. Groundwater, being dynamic, when it is in a specific hydrogeologic setup, fate assessment of point values of arsenic concentration in terms of distributionand transport in a groundwater domain on a regional scale could provide a good insight tounderstand movement of arsenic in that domain. Numerical groundwater flow and transportmodeling is one of the powerful techniques by which one can determine spatial and temporaldistribution of arsenic mobilization, understanding of transport phenomena, zoning of fresh andcontaminated water, etc. that can help evolve management strategies of an aquifer for differentstress conditions under a framework of hydro-geological setting. Modeling is a framework ofcomputational tool, basically derived from the conceptualization of processes, which areotherwise known, but modeling itself cannot explain the physical processes. Numerous studieshave been attempted by many investigators related to modeling of groundwater arseniccontamination, however, the findings of numerical modeling has remained mere exercises withnumber of 'ifs and buts' owing to lack of proper understanding of chemistry of arsenic in thegeo-environmental and geochemical processes.National Institute of Hydrology (NIH) and CGWB in year 2001 carried out a modelingstudy in the Yamuna sub-basin, Nadia, and North 24-Paraganas districts to quantify spatial andtemporal variation of arsenic contaminated groundwater for different geo-hydrologic settingsand to suggest possible remedial measures for the arrest of spreading. It was reported from thestudy that the distributions of observed arsenic concentration have local high peaks with spreadingin their surroundings implying localized in-situ sources spread over in the vicinity of thesource by the influence of groundwater movement exaggerated by exploitation. And their localizedactivation has no bearing on the influence of sources from other domain or by the mobilizationof arsenic from one pocket to another: those had been propagated by the local disturbances.From another analysis, it was shown (Majumdar et al., 2002 ) that by appropriately locatingpumping wells in the freshwater zone, within the scattered zones of arsenic contaminated water,one can withdraw arsenic free groundwater without influencing movement of arsenic in thecontaminated zones. In other words, it indicated that by adopting appropriate aquifer managementstrategies, fresh groundwater tapping from the scattered zones of fresh water, adjoining tothe zones of arsenic contaminated water is possible without perturbing movement of arsenicfrom the contaminated zones.Michael and Voss (2008) derived a method for reducing the concentration of arsenic inthe Bengal Basin's water supply, from a groundwater modeling study, which could provide thepopulation with safer water for drinking and irrigation. As an alternate to other solutions such asex-situ treatment of arsenic contaminated water by filters, the authors performed a quantitative,large-scale hydro geologic analysis and numerical simulation of the entire Bengal Basin, looking48NIH & CGWB
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ACKNOWLEDGEMENTThe Government of In
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(iii) Whether ex-situ removal techn
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6.4 Experiences during Arsenic Remo
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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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