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Geochemical investigation of arsenic discharge components in the Bengal Basin groundwater Carolyn B. Dowling, Robert J. Poreda, Asish R. Basu, and Scott L. Subsides

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Sampling Sixty-eight groundwater tests Bangladesh West Bengal (India) 3 He/3 H groundwater ages Major components by Ion Chromatography Trace components by Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) Sediments from penetrate center Trace components by ICP-MS Compare adsorbed versus disintegrated follow component focuses

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Groundwater Depth Profile of Dissolved As Figure 2. Profundity profile of broke up As. The groundwater arsenic fixations over the WHO standard (0.01ppm; 0.13 μ M) are moved in the shallow wells (<60m). The more elevated amounts of As in the more profound wells ( > 60m) are doubtlessly from the wells screening both profound, without as and shallow, As-rich groundwater.

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Dissolved As versus Fe, CH 4 , and NH 4 Figure 3 . Disintegrated As versus Fe, CH 4 , and NH 4 . Broken down arsenic levels from every one of the wells in the Bengal Basin are plotted against iron, methane, and alkali. There are just frail to humble connections amongst's arsenic and iron, methane, and smelling salts.

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Dissolved As versus CH 4 , Fe, and NH 4 in Faridpur and Laxmipur Figure 4 . Broken down As versus CH 4 , Fe, and NH 4 in Faridpur and Laxmipur. Disintegrated arsenic levels from wells situated in Faridpur and Laxmipur are plotted against methane, iron, and smelling salts. There are sensible connection (r 2 = 0.8-0.9) amongst arsenic and methane, iron, and alkali. The As-CH 4 and As-NH 4 , relationships demonstrate that there are dynamic organisms in the anoxic groundwater. The As, CH 4 , and NH 4 affiliations joined with the Fe information bolster bacterial decrease of FeOOH as the principle discharge instrument of As into the groundwater. The specimens encountering low levels of microbial movement (in light of CH 4 levels) are prohibited from the As-Fe connection.

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Watershed Hydrology Figure 5. Schematic cross segment of groundwater stream in the Bengal Basin along the NW-SE (A – A') line. All wells are anticipated to the An A' line in Figure 1. There are numerous city wells (up to 150 m) that contain tritium and have a 3 He/3 H groundwater age. Shallow wells at Laxmipur, Faridpur, and Kustia are tritium dead with high biogenic methane. More profound wells in the upper aquifer, with no tritium and direct measures of radiogenic 4 He, are over 100 years of age. In the lower aquifer, the groundwater has no tritium, raised methane and helium focuses, and home times of more noteworthy than 1000 years.

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Laxmipur Groundwater and Sediment Oxalate Extraction versus Depth Figure 6 . Groundwater and dregs oxalate extraction information plotted against profundity at Laxmipur, Bangladesh. Select broke up and adsorbed information from the water and residue oxalate extraction examinations (arsenic and iron) are plotted against profundity at Laxmipur. The groupings of broke down As and Fe are 5 times not exactly the adsorbed mass portion. Adsorbed As and Fe show a comparable pattern. Grain estimate assumes a critical part in controlling the groupings of follow metals broke up in the groundwater and adsorbed onto dregs. The drop in the adsorbed metals seen at 39 m compares to a sandier interim in the penetrate center.

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Sediment As-Fe As/Fe Ratios with Depth Figure 8. Adsorbed As-Fe molar proportions plotted against profundity. There is a general diminishing in the proportions with profundity, coming about because of more groundwater flushing the lower silt and expelling a portion of the As from the framework. Figure 7. Adsorbed Fe versus adsorbed As in the residue divisions. There is a general connection (r2 = 0.72) between adsorbed Fe and As demonstrating that Fe and As originate from a similar source. It additionally brings up the significance of grain size on follow metals on the grounds that the mass silt has the most reduced centralization of As and Fe while the fine-grained portion has the largest amounts.

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Groundwater Age Dating

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Research Questions? Why do we think about Arsenic in groundwater? Is it an issue in the Bengal Basin? Which wells are defiled by Arsenic? Where are the wells found? What are their profundities? Does As associate with different components? What are the wellsprings of As? Dregs? Modern contamination? Horticultural contamination? Why is it an issue in the Bengal Basin?

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Some Answers Arsenic sullying is a main problem Source is common Bulk dregs supplies As to the groundwater Microbial intervened lessening of iron oxy-hydroxides a.k.a. the microbial breakdown of FeOOH

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Time Line World Health Organization (WHO) Until 1970s, populace utilized dirtied waterways Drilled 2 million groundwater wells Most wells are defiled with arsenic (As) Levels are more prominent than WHO greatest contaminant level (MCL) of 0.01 ppm or 0.13 M Symptoms of Arsenic harming grow gradually 30-60% of the populace is influenced

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Arsenic Geochemistry Species As(V), Arsenate, AsO 4 3-As(III), Arsenite, As 2 O 4 2-30-60X lethal and 5-10X versatile As firmly adsorbs onto press oxy-hydroxides (FeOOH) As-loaded FeOOH are saved in estuaries and wetlands

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Existing Theories of As Release Oxidation of pyrite (Rarely utilized any longer) Requires oxic water Competitive trade with phosphorus Phosphate (PO 4 3-) Dissolved As and P trade for each other Dissolution of iron oxy-hydroxides (FeOOH) FeOOH emphatically adsorb As Correlation amongst Fe and As Anaerobic microorganisms

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Adsorption of chose anions on hydrous ferric oxide as a component of pH Figure 5-9. Drever, The Geochemistry of Natural Waters 3 rd Edition

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Background ^^^^^^^ ^^^^^^^^ Himalayas Brahmaputra Bangladesh and West Bengal State, India Quaternary stores Ganges-Brahmaputra Himalayas Sea level changes and stream relocation Complex stratigraphy of coarse and fine-grained residue. Ganges India Bangladesh Bay of Bengal (Modified from http://www.geoexplorer.co.uk)

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Sampling Where is the Arsenic found? Groundwater science Is the Arsenic originating from the residue? Silt science What is the watershed hydrology? Groundwater stream

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Sampling Sixty-eight groundwater tests Bangladesh West Bengal (India) Sediment Drill center River

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Groundwater Depth Profile Is As an issue? More than 60% of tests above 0.13 M Where are the wells? All through the nation What are the profundities? Most elevated amounts of As at shallow profundities (< 60 m)

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Does As associate with others? Press (Fe) Previous studies interface As and Fe Weak connection amongst's As and Fe (r 2 =0.37) Methane (CH 4 ) & Ammonia (NH 4 ) Microbial movement Weak to unobtrusive relationship (r 2 : 0.39-0.55)

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Correlations with Arsenic Faridpur and Laxmipur As-rich territories Faridpur Laxmipur Strong connections with CH 4 , Fe, & NH 4 ( r 2 : 0.8-0.9)

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Are microorganisms included? As-CH 4 and As-NH 4 connections As microorganisms are oxidizing natural matter, they are separating FeOOH Microbes changing over As(V) to As(III) Microbes Shewanella alga BrY MIT-13 Geospirillum barnesii SES-3

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Groundwater Age Dating 3 H/3 He Age Dating Technique Tritium ( 3 H) is framed Above ground atomic testing Cosmogenic responses ( 14 N + n = 3 H + 12 C) Component of water particle ( 3 H 2 O) 3 H rots to 3 He t 1/2 =12.4 yrs Groundwater home time: t=(1/l )ln{1+( 3 He*/3 H)}

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Groundwater Age Dating Variations in ground-water speeds ~0.4 m/yr ~3 m/yr Complicated stratigraphy Complex dispersion of As

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Watershed Hydrology

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What is the wellspring of As? Silt impact groundwater Mineralogy Grain measure Adsorption/desorption Dissolved As and Fe have comparable examples Adsorbed As and Fe have similar examples Bulk equipped for providing As to groundwater

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Sediment As-Fe Modest relationship at any profundity r 2 =0.7 Sources of As and Fe in all strong stages might be the same Microbial disintegration of FeOOH Grain estimate assumes a critical part

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As/Fe Ratios with Depth As-Fe proportions diminish with profundity More groundwater has moved through the more profound dregs Removed As from more profound aquifer framework

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Overview of As Release Rain Vadose Zone (unsaturated) Phreatic Zone (immersed) Aerobic creatures devour O 2 Anaerobic organisms decrease FeOOH Releases Fe and As Dissolved As levels Biological action Adsorption responses Vadose Zone Recharge Phreatic Zone Oxygen Present No Oxygen Present Microbial Activity FeOOH As Adsorption

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Summary As in groundwater 30-60% populace is influenced 60% of the specimens above WHO MCL (0.13 M) Depth under 60 m Anoxic groundwater more noteworthy than 60 yrs Complicated appropriation of As in groundwater

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Summary Source of As The As-loaded residue Released from the residue through microorganisms Bulk silt are fit for providing the greater part of the arsenic to the groundwater

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Present The Bad News Groundwater will have high arsenic levels for a drawn out stretch of time. The Good News The drinking supply wells can be penetrated to further profundities.

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Future Research? Widespread Problem?? Fast gathering of silt from Himalayas Yangtze River Irrawaddy River Mekong River Sea level changes and waterway movement Mekong Delta, Vietnam Dong The Nguyen Feb. 2004 AAAS travel proposition Future NSF proposition Ganges Brahmaputra (Modified from http://www.central.k12.ca.us)