Naturally occurring arsenic in groundwater of Terai region in Nepal and mitigation options Nirmal Tandukar Department of Water Supply & Sewerage DWSS, Kathmandu, Nepal Prosun Bhattachary
Trang 1Naturally 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
50g/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
SO4 and NO3 , but high HCO3 concentrations Total arsenic (Astot) concentration in groundwater varied from 1.7g/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, HCO3 , Fetotand Astot
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
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 50g/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
Trang 2Figure 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
Silty sand-clay
Sand-silty clay
Gravel
Coarse sand
with gravel
Sand
Sand-silty clay
Sukrauli
Badera
Sand-silty clay
Silty sand-clay
Gravel mixed with silt Clay
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
Kushma
Bairihawa Hakui
Sukrauli
Baikunthapur
Ahirauli Kasipur
Swathi
Basahi
Somnath Sunawal
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.
Trang 32 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 30–40 km 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 km2and 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 2381 mm (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
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 0.45m filters) for the analysis of major anions (ii) filtered and acidified with supra pure HNO3for 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, SO4were analyzed with a Dionex DX-120 ion chromatograph using an IonPac As14 column NO3 and PO4 were analyzed with Tecator Aquatec 5400 spectrophotometer using the wavelength of 540 nm and 690 nm respectively
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 between0.197 to 0.105 V, which suggest fairly reduced condition in the aquifer The concentration of SO4 (0–133 mg/L) and NO3 (up to 10.8 mg/L) were low Total arsenic (Astot) concentration were found in the range 1.7–404g/L with 79–99.9% as As(III) species Concentration of total Fe (Fetot) and Mn ranged between 0.11–16.4 mg/L and 0.01–1.95 mg/L respectively Levels of DOC ranged between 15.2–31.9 mg/L (Table 1) The groundwaters were predominantly of Ca-Mg-HCO3type with HCO3 as the principal anion with concentration ranging between 332–549 mg/L (Fig 3)
Total iron (Fetot) concentrations in these groundwaters were positively correlated with Astot
(R2 0.59) and DOC (R2 0.56) especially at depths below 20 m (Fig 4) A positive correlation was observed between Astotaland HCO3(R2 0.54) Likewise, a strong correlation was observed between DOC and HCO3 (R2 0.68) A strong correlation was noted between As(III) and NH4
(R2 0.89) and DOC (R2 0.79)
The concentration of As exceeding the Interim Nepalese standard of 50g/L was found in the depth range of 7–35 m
The hydrogeochemical data for groundwater of the TAP aquifer suggest a predominantly reducing character with high HCO3 , low SO4 , and NO3 concentrations This is further supported by the
43
Trang 4Note: bdl – below detection limit.
Copyright © 2005 Taylor & Francis Group plc, London, UK
Trang 5SO
4 + Cl
Ca + Mg
HCO 3 + CO 3
Na + K
20 20
40 40
60 60
80 80
Figure 3 Piper diagram showing the dominance of Ca-Mg-HCO3 water type in TAP groundwaters in Nawalparasi.
y = 0.0069x + 0.9303
R 2 = 0.5883
0
1
2
3
4
5
As tot (µg/L)
y = 0.1408x - 0.8508
R2 = 0.5638
0 1 2 3 4 5
DOC (mg/L)
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
HCO 3 (mg/L)
y = 0.578x - 158.95
R 2 = 0.5393
0
50
100
150
200
250
300
HCO 3 (mg/L)
Figure 5 Plots showing the relationship between: (a) HCO 3 and As
tot ; and (b) HCO 3 and DOC in TAP groundwaters in Nawalparasi.
Trang 6presence of Fe and Mn at elevated concentration, together with the predominance of As(III) in the groundwater Elevated HCO3 concentrations result primarily due to the oxidation of organic mat-ter (Mukherjee & Bhattacharya 2001, Bhattacharya et al 2002), while low SO4
2 concentrations result due to reduction of sulfate Strong correlation between DOC and HCO3indicate 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
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
y = 0.0346x + 15.397
R 2 = 0.7301
10 15 20 25 30 35
As(III) (µg/L) As(III) (µg/L)
Figure 6 Bivariate plots showing the relationship of As(III) with NH 4 and DOC in TAP groundwaters in Nawalparasi.
0 10 20 30 40 50 60
Fe tot , Mn (mg/L)
Fe tot Mn
b
0
10
20
30
40
50
60
As tot (µg/L)
WHO Safe Drinking Water Limit Interim Nepalese Drinking Water Standard
a
Figure 7 Variation with depth the concentration of (a) Astotaland (b)Fetotal and Mn in TAP groundwaters in Nawalparasi.
Trang 7and 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 50g/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
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, FeTotal and AsTotal
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
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