Mechanism of arsenic release to groundwater, Bangladesh and West Bengal

Applied Geochemistry 15 (2000) 403±413 Mechanism of arsenic release to groundwater, Bangladesh and West Bengal R.T Nickson a, J.M McArthur a,*, P Ravenscroft b, W.G Burgess a, K.M Ahmed c a Geological Sciences, University College London, Gower St., London, WC1E 6BT, UK Mott MacDonald International Ltd., 122 Gulshan Avenue, Dhaka -1212, Bangladesh c Department of Geology, University of Dhaka, Dhaka -1000, Bangladesh b Received January 1999; accepted 13 August 1999 Editorial handling by R Fuge Abstract In some areas of Bangladesh and West Bengal, concentrations of As in groundwater exceed guide concentrations, set internationally and nationally at 10 to 50 mg lÀ1 and may reach levels in the mg lÀ1 range The As derives from reductive dissolution of Fe oxyhydroxide and release of its sorbed As The Fe oxyhydroxide exists in the aquifer as dispersed phases, such as coatings on sedimentary grains Recalculated to pure FeOOH, As concentrations in this phase reach 517 ppm Reduction of the Fe is driven by microbial metabolism of sedimentary organic matter, which is present in concentrations as high as 6% C Arsenic released by oxidation of pyrite, as water levels are drawn down and air enters the aquifer, contributes negligibly to the problem of As pollution Identi®cation of the mechanism of As release to groundwater helps to provide a framework to guide the placement of new water wells so that they will have acceptable concentrations of As # 2000 Elsevier Science Ltd All rights reserved Introduction Following independence, the governments of Bangladesh, assisted by aid agencies, have provided most of the population with bacteriologically-safe drinking water by providing tubewells that abstract water from subsurface alluvial aquifers This achievement has reduced the incidence of waterborne disease only to replace it with another problem: water from many of the tubewells is contaminated with naturallyoccurring As (Saha and Chakrabarti, 1995; Dhar et al., 1997; Bhattacharaya et al., 1997, 1998a, 1998b; Nickson et al., 1998) Concentrations of As in water * Corresponding author E-mail address: j.mcarthur@ucl.ac.uk (J.M McArthur) from tubewells can reach mg lÀ1 levels (Badal et al., 1996) and frequently exceed both the provisional guideline concentration for drinking water set by the World Health Organisation (10 mg lÀ1 WHO, 1994) and the Bangladesh limit for As in drinking water (50 mg lÀ1; Department of the Environment, Bangladesh, 1991) The problem seems likely to a€ect a signi®cant proportion of the 3±4 million tubewells in Bangladesh (Arsenic Crisis Information Centre; http://bicn.com.acic/, 15/05/99) Whilst the calamity may be alleviated by using water from other sources for public supply (e.g rain or surface water), the attendant storage and bacteriological problems make this dicult The authors believe that by identifying the chemical and geological processes that give rise to As contamination, it might be possible to use that knowledge in a predictive manner to site 0883-2927/00/$ - see front matter # 2000 Elsevier Science Ltd All rights reserved PII: S 8 - ( 9 ) 0 - 404 R.T Nickson et al / Applied Geochemistry 15 (2000) 403±413 Fig Conurbations in Bangladesh that were sampled for this study; the scale does not permit individual wells to be di€erentiated, excepting for irrigation wells outside of town sites Tungipara, in the district of Gopalganj, is 100 km SW of Dhaka (inset) The area within the dotted line marks the border of the Madhupur Tract new tubewells and possibly to remediate existing tubewells, so as to continue the development of a groundwater resource that is bacteriologically safe As a contribution to this end, it is shown here that the As present in Bangladesh groundwater cannot derive from the presently accepted mechanism, whereby water-level drawdown from abstraction allows atmospheric O2 into the aquifer and so allows the oxidation of Asbearing pyrite, with a concomitant release of As to groundwater (Das et al., 1995, 1996; Roy Chowdhury et al., 1998) Such a mechanism is incompatible with the redox chemistry of the waters Arsenic produced this way would be adsorbed to FeOOH, the product of oxidation (Mok and Wai, 1994; Thornton, 1996; references therein), rather than be released to groundwater The As in the groundwater derives from reductive dissolution of As-rich Fe oxyhydroxide that exists as a dispersed phase (e.g as a coating) on sedimentary grains The reduction is driven by microbial degradation of sedimentary organic matter and is the redox process that occurs after microbial oxidation of organic matter has consumed dissolved-O2 and NO3 ness underlie much of Bangladesh (Khan, 1991) Upwards ®ning sequences from braided river deposits to meander deposits and ultimately to ¯oodplain deposits are common (Ghosh and De, 1995) The nature of ¯uvial deposits, however, makes dicult the de®nition of laterally continuous or contiguous sedimentary layers The evolution of the most recent parts of the sedimentary sequence in the Ganges Alluvial Plain have been discussed by Davies (1989, 1994) and Umitsu (1985, 1993) During the last glacial maximum (18 ka BP), the base-level of the rivers was some 100 m lower than in interglacial times During this low-stand of sea-level, the sediments were ¯ushed and oxidised, thereby giving rise to their characteristic red/brown colour The Madhupur Tract (underlying Dhaka city) and the Barind Tract are two areas of Plio-Pleistocene sediment that survived this period of erosion As sea level rose, late Pleistocene-Holocene sediment in®lled the valleys with ¯uvial sands, silts and clays Sedimentological setting During May and June, 1997, groundwaters were sampled from 17 wells in Dhaka City that tap the Plio-Pleistocene Dupi Tila aquifer of the Madhupur Fluvial and deltaic sediments up to 10km in thick- Material and methods PTW PTW HTW HTW 11M Manikganj Prod Well Prod Well Well (Beutha) Well 11 (Dashora) Well 10 (Dashora) Savar Irrigation well No.6 IRR HTW HTW HTW PTW 128 PTW 28 PTW 128 31 91 75 37 23 42 6.89 6.74 6.84 6.25 6.58 6.55 5.95 6.38 5.99 6.14 6.06 6.31 7.00 5.97 6.08 6.04 5.82 5.51 5.85 6.25 6.76 6.64 6.40 pH 68 5.88 116 6.72 99 6.97 6.92 6.92 6.92 58 58 58 170 68 177 144 133 114 170 158 141 149 146 58 60 57 60 59 69 68 67 Narayanganj Palpara 992/2 Palpara 307/3 Palpara 311 Majdair Dewbogs OHT Baruhall Pump 157 138 132 173 165 75 68 63 70 67 PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW PTW top base Type Screen (m) Dhaka Banam Rd 18 Nayanagar Mohakhali DOHS Goran-1 Magdapara Bijoynagar OHT Armanitala Nilkhet W.H Dhanmondi No.8 Pallabi Sec 10 BIBM Mirpur Shamali Ulan Hazaribagh-4 WAPDA colony Tejgaon Location 300 510 480 690 670 550 970 960 960 860 840 250 180 210 210 210 210 340 470 500 460 210 250 180 240 330 540 360 330 30 0 18 20 12 12 12 0 66 10 22 51 82 84 85 21 122 58 116 105 61 148 27 26 82 26.0 25.2 25.3 24.9 24.5 24.2 27.6 25.8 26.6 26.8 27.8 26.3 28.9 28.9 26.2 26.1 26.4 26.0 26.2 25.5 25.2 25.8 26.4 25.3 25.4 24.8 26.3 25.8 25.9 7.5 27.1 21.4 13.8 19.5 10.4 150 151 151 72.8 110 24.6 17.1 21.4 16.8 20.9 20.4 21.9 27.6 29.1 31.0 17.8 16.4 15.1 20.1 53.9 43.5 26.2 24.3 2.0 4.0 4.2 2.7 4.0 3.9 5.8 2.5 2.2 3.1 3.3 1.6 1.7 1.6 1.7 1.5 1.5 2.0 2.4 2.1 1.7 1.5 1.3 2.0 1.7 1.1 1.9 1.6 2.0 6.4 66.9 68.9 103 92.2 69.5 46.9 48.7 44.9 76.6 49.9 26.4 15.5 16.9 27.3 16.5 16.7 29.6 49.5 48.8 44.4 20.1 21.9 14.6 24.0 14.5 45.0 31.8 27.7 1.54 21.9 22.5 33.5 29.8 25.6 33.5 31.7 32.3 35.2 27.1 11.9 4.90 5.20 6.69 7.21 6.21 11.9 19.3 1.39 1.39 0.61 0.70 0.41 0.64 0.43 16.9 11.5 9.02 0.25 0.25 0.46 0.50 0.94 2.39 2.66 2.31 0.59 1.81 0.10 0.03 0.01 0.02 0.08 0.07 0.09 0.12 0.05 0.10 0.03 0.04 0.08 0.01 0.03 0.09 0.14 0.05 0.03 0.02 8.09 8.36 10.4 10.2 10.1 0.46 1.71 0.37 0.21 2.46 0.05 < 0.03 0.04 < 0.03 0.11 < 0.03 < 0.03 0.07 0.07 < 0.03 < 0.03 < 0.03 0.06 < 0.03 < 0.03 < 0.03 0.29 0.11 27 375 381 491 386 322 478 477 465 199 353 158 81 103 81 122 116 122 222 117 164 105 101 66 109 152 179 150 86 2.8 11.4 11.0 8.1 58.4 12.5 121 105 132 211 147 12.5 5.7 5.7 13.0 3.7 4.6 25.4 41.2 56.0 50.1 9.8 13.4 12.9 11.3 22.2 46.8 25.3 56.7 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 1.3 < 0.3 < 0.3 < 0.3 1.0 < 0.3 3.8 0.9 0.3 10.1 < 0.3 21.8 9.7 3.0 3.8 8.9 < 0.3 1.4 < 0.3 < 0.3 20.9 77 95 58 60 47 < 10 17 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 10 < 0.1 9.0 < 10 (continued on next page) < 0.1 < 0.1 39.7 0.7 34.4 < 0.1 < 0.1 3.3 8.8 5.8 34.4 0.8 0.6 0.7 < 0.1 1.9 14.6 7.2 31.8 15.5 0.6 0.1 0.3 3.3 2.8 31.6 9.9 19.5 DO2(%) Temp Na K Ca Mg Fe Mn HCO3 Cl SO4 NO3 As E.C (mS cmÀ1) (8C) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) Table Chemical composition of well waters from Bangladesha R.T Nickson et al / Applied Geochemistry 15 (2000) 403±413 405 RR ITW IRR PTW HTW PTW HTW PM PTW HTW PTW PTW HTW HTW HTW RTW Keraniganj Cornakhula Sakta No Harirampur H.C PTW H.C HTW T.H PTW T.H HTW Faridpur PTW 12 (after) PTW 12 (before) Jhiltuli HTW PTW 10 PTW 11 (new) HTW near PTW 11 Gopalganj HTW2 HTW3 HTW4 (new) 64 52 18 70 70 18 38 28 26 7.40 900 7.12 870 7.25 950 7.11 1010 7.07 980 7.03 1330 600 520 420 12 21 0 0 0 0 0 14 25.0 25.0 25.4 26.0 26.0 25.2 26.1 25.6 25.5 25.5 25.3 25.4 24.9 24.9 24.9 25.2 25.1 118 882 79.6 2.50 35.6 29.8 29.8 49.1 48.6 76.2 20.9 11.3 12.7 13.7 26.8 25.2 11.8 16.5 8.8 17.0 7.0 1.4 4.8 4.7 4.5 5.4 5.7 5.6 4.9 4.1 4.1 4.5 3.6 3.0 3.7 2.4 296 219 214 6.40 127 110 142 123 111 158 105 111 115 137 105 78.1 70.6 50.3 71.5 120 52.7 1.18 36.1 36.3 28.1 36.2 40.2 37.6 36.9 24.3 31.6 28.3 48.0 24.3 22.7 18.9 0.98 29.2 21.8 24.7 0.39 < 0.03 7.82 0.12 6.87 6.90 2.46 21.8 3.07 5.87 7.63 10.4 15.9 0.31 0.19 0.32 0.05 0.16 0.19 0.69 0.31 0.14 2.03 0.20 0.82 0.51 1.27 0.12 0.86 8.00 1.19 10.4 697 654 642 542 580 504 595 639 702 597 399 508 577 382 399 355 302 631 2380 493 6.2 22.1 14.2 47.7 37.2 18.0 101 9.0 4.3 4.9 3.3 61.4 40.9 10.8 7.8 < 0.1 < 0.1 1.8 1.6 0.4 < 0.1 < 0.1 < 0.1 < 0.1 40.4 0.2 35.7 1.0 < 0.1 55.6 16.9 0.2 0.2 < 0.3 7.7 1.8 0.8 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 1.8 < 0.3 < 0.3 < 0.3 178 332 118 < 10 42 220 26 191 268 49 159 107 164 152 < 10 49 34 14 E.C DO2(%) Temp Na K Ca Mg Fe Mn HCO3 Cl SO4 NO3 As (mS cmÀ1) (8C) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) (mg lÀ1) 18 6.69 2800 46 6.83 7890 6.63 1900 100 100 98 83 23 > 90 7.04 30 7.26 90 7.17 7.17 68 7.14 87 6.84 52 7.00 56 6.77 pH a Wells may contain several screened sections; values are top of highest screen and bottom of lowest screen Values for PTW 12 in Faridpur refer to before and after Fe treatment HTW=hand pumped water supply tubewell; PTW=water supply tubewell with electric pump; IRR=irrigation well; ITW=pumped tubewell for industrial water supply; HC=health complex; TH=Thana headquarters Rainwater IRR Saturia Well 58 top base Type Screen (m) Dhamrai Well 74 Location Table (continued ) 406 R.T Nickson et al / Applied Geochemistry 15 (2000) 403±413 R.T Nickson et al / Applied Geochemistry 15 (2000) 403±413 407 Table Chemical parameters of sediments from Bangladesh Sample Dark grey clay Grey clay Grey clayey silt Grey silty sand Grey sand Brown clay Grey clay Grey silty clay Grey silt Grey silty sand Depth mbgl Total Diagenetically available Fe (%) As (ppm) Fe (%) As (ppm) Al (%) S (%) 3.0 4.6 6.1 7.6 9.1 1.8 2.1 4.3 5.2 7.6 3.15 3.26 3.07 2.69 1.47 3.93 1.81 3.42 2.73 3.11 24 28 26 17 28 12 26 25 26 3.12 3.19 2.72 2.60 1.46 3.74 1.55 3.30 2.59 2.91 24 26 22 17 26 24 21 22 2.51 2.92 1.49 1.56 0.58 0.71 0.56 1.96 1.25 1.76 0.17 0.16 0.21 0.16 0.09 0.14 0.17 0.11 0.12 0.17 Pyrite (equiv %) Total C (%) Org C (%) 0.32 0.29 0.39 0.29 0.18 0.26 0.33 0.21 0.23 0.33 0.63 6.21 0.71 0.59 0.65 0.48 6.20 0.61 0.47 0.18 À Fig Chemistry of Bangladesh well water Relation of (a) As to dissolved O2; (b) As to NOÀ ; (c) As to Fe; (d) As to HCO3 408 R.T Nickson et al / Applied Geochemistry 15 (2000) 403±413 Tract and from 28 wells that tap the alluvial aquifers comprised of the late Pleistocene-Holocene sediments of the Brahmaputra and Ganges Rivers These latter wells were sited within 50 km of Dhaka City at Dhamrai, Faridpur, Harirampur, Keraniganj, Manikganj, Narayanganj, Savar, Saturia and at Tungipara, district of Gopalganj, which is 100 km further to the southwest; locations are shown in Fig and well details are given in Table Water samples were ®ltered on site using 0.45 mm membrane ®lters Samples for cation analysis were acidi®ed to pH 2, those used for anion analysis were not acidi®ed Measurements of dissolved O2, conductivity and alkalinity were made at the well head With some wells, measurement of dissolved O2 was a€ected by contamination with atmosphere and values for such wells are therefore spuriously high Alkalinity is reported as equivalent HCOÀ and is corrected for acidity produced by oxidation of Fe(II) during the titration, as many samples precipitated Fe oxyhydroxides soon after exposure to atmosphere Sediment samples were collected from two borehole cores taken in the late PleistoceneHolocene sediments at Gopalganj, 100 km SW of Dhaka (Fig 1) For waters, cation analysis was done using ICP-AES and anion analysis was done using ion chromatography Concentrations of As were measured on acidi®ed samples using graphite-furnace AAS (detection limit 10 mg lÀ1) The amount of diagenetically-available Fe, As, Al and S, in sediments was determined by extraction with hot concentrated HCl acid (Raiswell et al., 1994) followed by analysis of extracts with ¯ameAAS for Fe and Al, graphite-furnace AAS for As and ion chromatography for SO2À For the determination of total Fe, As, S and Al, samples were fused with lithium metaborate and the fusion dissolved in dilute acid for analysis by ICP-AES and graphite-furnace AAS (for As) Analyses for organic C and total C were done with a LECO C/S 125 Analyser; for organic C, samples were pretreated with 10% v/v HCl to remove inorganic carbonate Chemical data are given in Table Analytical precision was [...]... 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