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.
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