NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 5 doc

Naturally occurring arsenic in groundwater of Terai region in Nepal and mitigation options Nirmal Tandukar Department of Water Supply & Sewerage (DWSS), Kathmandu, Nepal Prosun Bhattacharya, Gunnar Jacks & Antonio A. Valero Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden ABSTRACT: Natural arsenic (As) was detected in groundwaters in the Terai Alluvial Plain (TAP) in southern Nepal in the year 1999. By the end of March 2004, about 245,000 wells have been tested for As, out of which about 3% samples are found to have exceeded Interim Nepalese Standard of 50 ␮g/L. From the detail study conducted in hotspot district Nawalparasi, natural rocks are thought to be the sources of As that are leached mainly due to the weathering of As bearing minerals from the Himalayas towards the northern Nepal. In this paper, the chemistry of groundwater from highly arsenic affected Nawalparasi district in the central part of the TAP in southern Nepal has been presented. TAP groundwaters are found to be predominantly of reducing character with low SO 4 2Ϫ and NO 3 Ϫ , but high HCO 3 Ϫ concentrations. Total arsenic (As tot ) concentration in groundwater varied from 1.7␮g/L to as high as 404␮g/L. As(III) species is found to be predominant along with elevated levels of dissolved Fe and Mn. The correlation between DOC, HCO 3 Ϫ , Fe tot and As tot strongly supports the hypothesis of reductive dissolution of Fe-oxyhydroxides as the main mech- anism of mobilization of As in groundwater in TAP. Blanket testing by As-field test kits is the easi- est way to find out As free sources nearby for tubewell switching. In the absence of As-free source, the only available option is the treatment of water either at the point of entry or at the point of use to meet the drinking water standard. DWSS in collaboration with UNICEF and WHO is conducting blanket testing of As in 10 Terai districts. Based on the blanket test result, As treatment methods such as 3-gagri filter, arsenic biosand filter etc., which are simple, effective, affordable and socially acceptable will be provided as a short-term option to the affected communities in hotspot areas. 1 INTRODUCTION Extraction of groundwater in Nepal started during International Water Supply and Sanitation Decade about 30 years back by installing tubewells to provide microbially safe water. However the presence of natural arsenic (As) in groundwater has become a global problem especially in Asian continent. Arsenics was detected very recently in groundwaters in Terai Alluvial Plain (TAP) in southern Nepal (Tandukar 2000, Tandukar et al. 2001, Valero 2002, Bhattacharya et al. 2003). The population of Nepal is about 23.4 million among which about 47% live in the 20 Terai districts and about 90% of these people are dependent on groundwater for drinking and other purposes (Fig. 1). From the study conducted so far each Terai district has of the order of about 25,000 tubewells, out of this about 85% tubewells are privately owned. By the end of March 2004, about 245,000 tubewells have been tested for As, out of which about 3% samples are found to have exceeded Interim Nepalese Standard of 50 ␮g/L (NSCA 2001). This paper presents the chemistry of arsenic-rich groundwaters of Nawalparasi district in the central part of the TAP in southern Nepal. 41 Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam (eds) © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Copyright © 2005 Taylor & Francis Group plc, London, UK 42 Figure 1. Map of Nepal showing the districts (marked with stars) with elevated As concentration in groundwater (based on Tandukar et al. 2001). Clay Sand Sand-silty clay Gravel Clay Gravel Sunawal Sunawal Silty sand-clay Sand-silty clay Gravel Coarse sand with gravel Sand Sand-silty clay Sukrauli Sukrauli Badera Badera Sand-silty clay Silty sand-clay Gravel mixed with silt Clay Rampurwa Rampurwa Gravel Clay Sand with gravel Clay Gravel Sand-silty clay 83.64 83.66 83.68 83.7 83.72 83.74 27.44 27.46 27.48 27.5 27.52 27.54 27.56 27.58 27.6 km 0 Rampurwa Kushma Bairihawa Hakui Sukrauli Baikunthapur Pokharapali Manari Ahirauli Kasipur Swathi Basahi Somnath Sunawal Khairani Choti Pratappur Badera Chowk Tilakpur Ghodpali Imlitole Thulo Kumuwar Magarmudha Radhanagar Kumuwar 0 30 m 2 4 Figure 2. Schematic lithology of the selected boreholes in Nawalparasi. Sampling locations are shown in the inset map. Copyright © 2005 Taylor & Francis Group plc, London, UK 2 LOCATION AND GEOLOGY OF THE STUDY AREA TAP is the northern extension of Indo-Gangetic Plain. It consists of 20 districts including Nawalparasi with a population of about 11.5 million. It has an average width of 30–40km and altitude ranging from 60–310 m above mean sea level (Anonymous, 2003). Nawalparasi district lies in the Western Development Region of Nepal occupying the total area of 2162 km 2 and has a population of about 0.56 million. The length of the highway linking the capital Kathmandu with the Ramgram Municipality (Parasi) is about 260 km. The average rainfall in the region is ca. 2381mm (1997–2001). In general TAP has geology, which is similar to Bengal Delta Plain (BDP) and is represented by thick clastic sequence of Holocene age comprising inter-locked alluvial deposit of the wider Ganges Plain (Bhattacharya, 2002; GWRDB-UNDP, 1989). The general flow of groundwater is from North to South. The lithology of the aquifers (Fig. 2) shows the sequence of gravel and sand-silty clay-clay sequence, which has been exploited for groundwater abstraction. 3MATERIALS AND METHODS 27 private and public tubewells extending to a depth of 7.6 to 54.9 m were sampled in the western part of Nawalparasi district. The well locations were marked using Global Positioning System (GPS). Water samples were collected following the procedures of Bhattacharya et al. (2002) which included: (i) filtered (using 0.45 ␮m filters) for the analysis of major anions (ii) filtered and acidified with supra pure HNO 3 for the analysis of cations and trace elements including As. Speciation of As(III) was carried out in the field using disposable cartridges following the method as described by Meng & Wang (1998) and Meng et al. (2001). Major cations and trace elements including As were analyzed by Varian Vista-PRO Simultaneous ICP-OES equipped with a SPS-5 autosampler. Major anions like Cl Ϫ , SO 4 2Ϫ were analyzed with a Dionex DX-120 ion chromatograph using an IonPac As14 column. NO 3 Ϫ and PO 4 3Ϫ were analyzed with Tecator Aquatec 5400 spectrophotometer using the wavelength of 540nm and 690nm respectively. 4GROUNDWATER CHEMISTRY Groundwater samples were near neutral to alkaline with the pH in the range between 6.1 to 8.1. Field measured redox potential varied in the range between Ϫ0.197 to Ϫ0.105 V, which suggest fairly reduced condition in the aquifer. The concentration of SO 4 2Ϫ (0–133 mg/L) and NO 3 Ϫ (up to 10.8 mg/L) were low. Total arsenic (As tot ) concentration were found in the range 1.7–404 ␮g/L with 79–99.9% as As(III) species. Concentration of total Fe (Fe tot ) and Mn ranged between 0.11–16.4 mg/L and 0.01–1.95 mg/L respectively. Levels of DOC ranged between 15.2–31.9mg/L (Table 1). The groundwaters were predominantly of Ca-Mg-HCO 3 type with HCO 3 Ϫ as the principal anion with concentration ranging between 332–549 mg/L (Fig. 3). Total iron (Fe tot ) concentrations in these groundwaters were positively correlated with As tot (R 2 ϭ 0.59) and DOC (R 2 ϭ 0.56) especially at depths below 20m (Fig. 4). A positive correlation was observed between As total and HCO 3 (R 2 ϭ 0.54). Likewise, a strong correlation was observed between DOC and HCO 3 (R 2 ϭ 0.68). A strong correlation was noted between As(III) and NH 4 ϩ (R 2 ϭ 0.89) and DOC (R 2 ϭ 0.79). The concentration of As exceeding the Interim Nepalese standard of 50 ␮g/L was found in the depth range of 7–35 m. 5 DISCUSSION The hydrogeochemical data for groundwater of the TAP aquifer suggest a predominantly reducing character with high HCO 3 Ϫ , low SO 4 2Ϫ , and NO 3 Ϫ concentrations. This is further supported by the 43 Copyright © 2005 Taylor & Francis Group plc, London, UK 44 Table 1. Geochemical characteristics of the groundwater samples from Na walparasi district, Nepal. Sample Latitude Longitude Depth pH Eh HCO 3 Ϫ Cl Ϫ NO 3 Ϫ SO 4 2Ϫ Na ϩ K ϩ Mg 2ϩ Ca 2ϩ DOC NH 4 ϩ As tot As(III) As(III) Fe tot Mn (deg. N) (deg. E) (m) (V) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (␮g/L) (␮g/L) (%) (mg/L) (mg/L) N-1 27.438 83.711 24.4 7.46 Ϫ0.179 369 3.74 0.0 0.1 18.5 4.2 18.5 98.7 31.9 0.7 78.0 77.9 99.96 4.03 0.04 N-2 27.474 83.689 13.0 6.88 Ϫ0.120 521 21.5 5.4 73.7 51.7 87.6 37.9 145.6 19.4 0.0 2.5 2.0 80.81 0.11 0.32 N-3 27.520 83.674 13.7 7.36 Ϫ0.149 332 0.8 bdl bdl 11.8 1.1 14.5 93.5 28.1 0.2 20.0 19.0 95.12 1.94 0.09 N-4 27.538 83.624 15.2 6.85 Ϫ0.159 427 32.0 bdl 0.4 38.1 1.6 29.1 89.8 28.2 1.6 265.4 262.5 98.88 3.28 0.03 N-5 27.522 83.627 16.8 7.29 Ϫ0.105 440 1.0 0.1 bdl 24.0 1.4 30.5 91.9 25.1 0.9 34.9 33.7 96.55 3.30 0.07 N-6 27.534 83.661 17.4 6.65 Ϫ0.174 476 3.4 1.2 2.3 57.8 2.0 28.8 68.0 27.6 1.7 272.3 270.6 99.37 1.91 0.02 N-7 27.537 83.664 16.8 6.09 Ϫ0.119 871 42.0 0.4 59.6 30.6 155.5 49.4 147.0 25.6 0.0 3.1 3.0 95.83 0.35 1.55 N-8 27.542 83.695 18.3 6.62 Ϫ0.136 505 9.2 bdl bdl 60.7 2.2 41.1 66.2 25.5 2.4 409.4 397.8 97.18 2.05 0.01 N-10 27.537 83.691 19.8 7.40 Ϫ0.129 428 11.0 1.3 2.5 53.3 1.7 32.1 69.0 25.5 0.6 385.1 303.8 78.88 0.78 0.07 N-11 27.528 83.688 16.8 6.50 Ϫ0.141 465 1.4 0.1 2.8 36.4 2.1 21.8 102.4 22.7 3.2 120.4 120.1 99.78 3.82 0.08 N-12 27.539 83.670 16.8 6.49 Ϫ0.131 525 18.0 0.2 0.1 53.6 1.6 29.4 96.9 21.2 1.4 120.3 115.9 96.33 2.01 0.10 N-13 27.544 83.676 19.8 6.73 Ϫ0.131 408 0.6 bdl bdl 41.0 1.2 20.6 80.3 21.1 0.7 65.7 64.8 98.62 1.77 0.12 N-14 27.545 83.668 245.0 6.97 Ϫ0.125 444 1.9 bdl 0.5 91.1 1.3 10.2 35.5 20.2 0.1 19.0 15.5 81.57 0.23 0.18 N-15 27.535 83.715 19.8 6.61 Ϫ0.147 508 10.5 0.4 3.1 34.5 1.8 25.7 117.6 22.7 2.5 153.9 151.4 98.35 1.94 0.06 N-16 27.544 83.723 7.6 6.24 Ϫ0.169 502 261.0 bdl 133.0 77.7 6.2 42.9 226.4 18.0 1.9 81.9 81.3 99.31 8.41 0.14 N-17 27.550 83.726 29.0 6.81 Ϫ0.168 453 1.1 bdl bdl 80.5 0.9 19.9 49.2 16.4 0.3 118.0 108.1 91.61 1.45 0.08 N-18 27.553 83.742 29.0 6.74 Ϫ0.197 504 45.6 bdl 6.2 64.1 1.3 34.5 90.8 19.5 0.9 177.8 170.2 95.72 2.64 0.23 N-20 27.574 83.734 43.9 6.81 Ϫ0.168 407 0.7 bdl 1.4 70.7 1.0 17.9 51.8 16.0 0.5 33.9 33.2 97.73 1.13 0.23 N-21 27.592 83.704 35.1 6.89 Ϫ0.168 355 0.4 bdl bdl 39.5 1.1 18.4 70.8 15.2 0.6 100.1 92.2 92.15 1.47 0.04 N-22 27.591 83.688 10.7 6.86 Ϫ0.178 460 25.1 bdl bdl 11.1 2.1 39.7 116.4 17.6 2.1 314.2 313.9 99.93 16.4 0.46 N-23 27.613 83.651 27.4 7.16 Ϫ0.154 407 0.9 1.9 bdl 36.8 1.4 20.7 84.8 17.7 1.2 91.8 87.9 95.76 1.73 0.08 N-24 27.607 83.646 54.9 7.12 Ϫ0.144 381 0.6 0.1 1.3 23.5 2.0 21.6 86.4 15.3 0.1 11.1 10.2 91.90 1.23 0.24 N-25 27.597 83.649 10.7 6.80 Ϫ0.188 459 33.4 bdl 0.3 15.4 2.0 34.3 116.5 19.5 2.9 67.6 62.2 92.01 12.13 0.13 N-26 27.577 83.661 10.7 7.37 Ϫ0.131 478 76.6 2.6 27.6 37.1 1.7 12.9 172.1 18.6 0.0 1.7 1.4 82.35 1.07 0.98 N-27 27.559 83.670 10.7 7.75 Ϫ0.175 353 1.8 0.1 0.1 24.2 0.8 15.4 83.8 16.8 0.7 75.3 69.0 91.61 4.51 0.11 Note: bdl – below detection limit. Copyright © 2005 Taylor & Francis Group plc, London, UK 45 Ca Na HCO 3 Cl SO 4 + Cl Ca + Mg Mg SO 4 HCO 3 + CO 3 Na + K 20 20 20 20 20 20 40 40 40 40 40 40 60 60 60 60 60 60 80 80 80 80 80 80 Figure 3. Piper diagram showing the dominance of Ca-Mg-HCO 3 water type in TAP groundwaters in Nawalparasi. y = 0.0069x + 0.9303 R 2 = 0.5883 0 1 2 3 4 5 0 100 200 300 400 As tot (µg/L) Fe tot (mg/L) Fe tot (mg/L) y = 0.1408x - 0.8508 R 2 = 0.5638 0 1 2 3 4 5 10 15 20 25 30 35 DOC (mg/L) ab Figure 4. Plots showing the relationship between: (a) As tot , and Fe tot and (b) DOC and Fe tot in TAP ground- waters in Nawalparasi. y = 0.0281x + 5.4257 R 2 = 0.6763 10 15 20 25 30 200 300 400 500 600 HCO 3 (mg/L) DOC (mg/L) y = 0.578x - 158.95 R 2 = 0.5393 0 50 100 150 200 250 300 200 300 400 500 600 HCO 3 (mg/L) As tot (µg/L) ab Figure 5. Plots showing the relationship between: (a) HCO 3 Ϫ and As tot ; and (b) HCO 3 Ϫ and DOC in TAP groundwaters in Nawalparasi. Copyright © 2005 Taylor & Francis Group plc, London, UK presence of Fe and Mn at elevated concentration, together with the predominance of As(III) in the groundwater. Elevated HCO 3 Ϫ concentrations result primarily due to the oxidation of organic mat- ter (Mukherjee & Bhattacharya 2001, Bhattacharya et al. 2002), while low SO 4 2Ϫ concentrations result due to reduction of sulfate. Strong correlation between DOC and HCO 3 indicate the abun- dance of degradable organic matter (Bhattacharya 2002, Bhattacharya et al. 2004). The presence of high DOC levels coupled with dominance of As(III) in groundwater suggest strong anoxic con- ditions caused by microbially mediated reduction of organic matter. These co-relations strongly support the hypothesis of reductive dissolution of Fe-Oxyhydroxides as the main mechanism of mobilization of As in groundwater. 6 MITIGATION OPTIONS To co-ordinate and streamline all the activities related to As of different agencies under single umbrella ‘The National Steering Committee on Arsenic (NSCA)’ has been formed with 20 mem- bers representing different governmental, non-governmental and donor agencies working in the field of water, sanitation and health sector. Information on As should be disseminated properly to avoid imminent danger. Therefore to provide uniform flow of information, IEC and training mater- ials that are suitable in the context of Nepal have already been printed and are being distributed in the hot spot areas. Since training is an effective way to disseminate information on As, a network of trainers has already been established by imparting training to more than 300 staffs of DWSS 46 y = 0.0056x + 0.2476 R 2 = 0.8946 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 100 200 300 400 500 NH 4 + (mg/L) y = 0.0346x + 15.397 R 2 = 0.7301 10 15 20 25 30 35 0 100 200 300 400 500 As(III) (µg/L)As(III) (µg/L) DOC (mg/L) ab Figure 6. Bivariate plots showing the relationship of As(III) with NH 4 ϩ and DOC in TAP groundwaters in Nawalparasi. 0 10 20 30 40 50 60 051015 20 Fe tot , Mn (mg/L) Depth (m) Depth (m) Fe tot Mn b 0 10 20 30 40 50 60 0 100 200 300 400 500 As tot (µg/L) WHO Safe Drinking Water Limit Interim Nepalese Drinking Water Standard a Figure 7. Variation with depth the concentration of (a) As total and (b)Fe total and Mn in TAP groundwaters in Nawalparasi. Copyright © 2005 Taylor & Francis Group plc, London, UK and about 140 members from other organizations engaged with arsenic mitigation activities. These trainees, especially the frontline workers will go to the affected areas to create awareness on As problem and deal with mitigation options. Blanket testing by As-field test kit is the easiest way for screening to find out As free sources nearby for tubewell switching. Hence, DWSS in collabor- ation with UNICEF and WHO is conducting blanket testing program in 10 arsenic affected dis- tricts of TAP. In the absence of As free source nearby, the only available option is the treatment of water either at the point of entry or at the point of use to meet the drinking water standard. After getting the complete blanket test result, As treatment methods, which are simple, effective, affordable and socially acceptable treatment options will be provided to the affected communities in hotspot areas. A few institutions have already studied the simple options namely 3-gagri filter, arsenic biosand filter etc. These filters use locally available materials. However, such treatment options should be used for short-term remediation only. In long term plan the affected people should be provided with As free water. In Bangladesh, the study conducted by JICA/AAN shows that 23 out of 51 dugwells and 38 out of 243 deep tubewells were found to have arsenic concentration exceeding the limit of 50␮g/L (JICA/AAN 2004a, b). It shows that deep tubewell and dugwell waters are not necessarily always safe hence these wells should compulsorily be tested before recommending them as a safe source. It may be equally applicable in the context of Nepal also. 7 CONCLUSIONS The detection of As in groundwater of TAP in southern Nepal has raised concern about health risk for about one fourth million people. Positive correlation between DOC, HCO 3 Ϫ , Fe Total and As Total in groundwater indicate that As is mobilized primarily due to the reductive dissolution of Fe- oxyhydroxide in the presence of organic matter in the sediments of TAP. Blanket As testing by field kit is the easiest way to find out As free source nearby for tubewell switching. In the absence of As free source, the only available option is the treatment of water either at the point of entry or at the point of use to meet the drinking water standard. Treatment methods namely 3-gagri filter and arsenic biosand filters can be installed in the hotspot areas as short-term remedial options. However, in the long-term plan affected communities should be provided with As free water by tapping sources from springs, rain water harvesting, treatment of water from rivers etc. ACKNOWLEDGEMENTS This study was carried out as a part of M.Sc. thesis with financial support by KTH. We acknow- ledge DWSS, His Majesty’s Government of Nepal for providing all the logistic support during the field work. We would like to thank Ann Fylkner, Monica Löwen (at the laboratories of Land and Water Resources Engineering, Royal Institute of Technology) and Joyanto Routh, Thomas Hjorth (Stockholm University) for their help in doing chemical analysis. We would also thank D. Chandrashekharam for his constructive comments on an earlier draft of this manuscript. REFERENCES Bhattacharya, P. 2002. Arsenic contaminated groundwater from the sedimentary aquifers of South-East Asia. Groundwater and Human Development, Proc. XXXII IAH and VI ALHSUD Congress, Mar del Plata, Argentina, E. Bocanegra, D. Martinez and H. Massone, 357–363. Bhattacharya, P., Jacks, G., Ahmed, K.M., Khan, A.A. & Routh, J. 2002. Arsenic in groundwater of the Bengal Delta Plain aquifers in Bangladesh. Bull. Env. Cont. Toxicol. 69: 538–545. Bhattacharya, P., Tandukar, N., Neku, A., Valero, A.A., Mukherjee, A.B. & Jacks, G. 2003. Geogenic arsenic in groundwaters from Terai alluvial plain of Nepal. Jour. de Physique IV France 107: 173–176. 47 Copyright © 2005 Taylor & Francis Group plc, London, UK Bhattacharya, P., Ahmed, K.M., Broms, S., Fogelström, J., Jacks, G., Sracek, O., von Brömssen, M. & Routh, J. 2004. Mobility of arsenic in groundwater in a part of Brahmanbaria district, NE Bangladesh. Managing Arsenic in the Environment: From Soil to Human Health, R. Naidu, E. Smith, L. Smith, J. Smith, and P. Bhattacharya (eds), CSIRO Publishing, Melbourne, (in press). GWRDB-UNDP 1989. Shallow groundwater exploration in the Terai. Nawalparasi District (West). Technical Report No. 5, Kathmandu, Nepal, 21p. JICA/AAN 2004a. Japan International Cooperation Agency/Asia Arsenic Network Arsenic Mitigation Project. Arsenic contamination of Deep Tubewells in Sharsha Upazila, Bangladesh, Report 1. JICA/AAN 2004b. Japan International Cooperation Agency/Asia Arsenic Network Arsenic Mitigation Project. 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Preliminary assessment of arsenic contamination in groundwater in Nepal. Book of Abstracts, Arsenic in the Asia-Pacific Region Workshop, CSIRO, Adelaide, Australia, 103–105. Valero, A.A. 2002. Arsenic in groundwater of alluvial aquifers in Nawalparasi and Kathmandu districts of Nepal: Extent of contamination, genesis and aspects of remediation. TRITA-LWR Master Thesis, 02-12, ISSN 1651-064X, KTH, Stockholm, Sweden, 57p. 48 Copyright © 2005 Taylor & Francis Group plc, London, UK . 83.668 2 45. 0 6.97 Ϫ0.1 25 444 1.9 bdl 0 .5 91.1 1.3 10.2 35. 5 20.2 0.1 19.0 15. 5 81 .57 0.23 0.18 N- 15 27 .53 5 83.7 15 19.8 6.61 Ϫ0.147 50 8 10 .5 0.4 3.1 34 .5 1.8 25. 7 117.6 22.7 2 .5 153 .9 151 .4 98. 35 1.94. 42.0 0.4 59 .6 30.6 155 .5 49.4 147.0 25. 6 0.0 3.1 3.0 95. 83 0. 35 1 .55 N-8 27 .54 2 83.6 95 18.3 6.62 Ϫ0.136 50 5 9.2 bdl bdl 60.7 2.2 41.1 66.2 25. 5 2.4 409.4 397.8 97.18 2. 05 0.01 N-10 27 .53 7 83.691. Nawalparasi. y = 0.0281x + 5. 4 257 R 2 = 0.6763 10 15 20 25 30 200 300 400 50 0 600 HCO 3 (mg/L) DOC (mg/L) y = 0 .57 8x - 158 . 95 R 2 = 0 .53 93 0 50 100 150 200 250 300 200 300 400 50 0 600 HCO 3 (mg/L) As tot