Arsenic in groundwaters of the Lower Mekong

Ó Springer 2005

Environmental Geochemistry and Health (2005) 27:341–357
DOI: 10.1007/s10653-005-3991-x

Arsenic in groundwaters of the Lower Mekong
Gordon Stanger1,6, To VanTruong2, K.S. Le Thi My Ngoc3, T.V. Luyen4 & Tuyen Tran Thanh5
1
Australia-based water resources consultant, currently with the UNDP, Box 551, Sanaa´, Yemen
2
Sub-Institute for Water Resources Planning, 253A An Duong Vuong, Quan 5, TP Ho Chi Minh, Vietnam
3
Sub Institute of Hydrometeorology of South Vietnam, Phu Trach Tram, Tram Thuc Nghiem, KTTVNN,
Dong Bang, Song Cuu Long, Vietnam
4
Centre for Nuclear Techniques, 217 Nguyen Trai St., Quan 1, TP Ho Chi Minh, Vietnam
5
Department of Environmental and Natural Resources Management, Can Tho University, 3/2 Street, TP Can
Tho, Vietnam
6
Author for correspondence (e-mail: )
Received 14 October 2004; Accepted 17 March 2005

Key words: arsenic, arsenicosis, Cambodia, Cuu Long Delta, groundwater, Mekong, Vietnam
Abstract
Increasing incidence and awareness of arsenic in many alluvial aquifers of South-east Asia has raised concern
over possible arsenic in the Lower Mekong Basin. Here, we have undertaken new research and reviewed many
previous small-scale studies to provide a comprehensive overview of the status of arsenic in aquifers of
Cambodia and the Cuu Long Delta of Vietnam. In general natural arsenic originates from the Upper Mekong
basin, rather than from the local geology, and is widespread in soils at typical concentrations of between 8 and
16 ppm (dry weight). Industrial and agricultural arsenic is localised and relatively unimportant compared to

the natural alluvial arsenic. Aquifers most typically contain groundwaters of no more than 10 lg L)1,
although scattered anomalous areas of 10 to 30 lg L)1 are also quite common. The most serious, but possibly
ephemeral arsenic anomalies, of up to 600 lg L)1, are associated with iron and organic-rich flood-plain
sediments subject to very large flood-related fluctuations in water level, resulting in transient arsenopyrite
dissolution under oxidizing conditions. In general, however, high-arsenic groundwaters result from the
competing interaction between sorption and dissolution processes, in which arsenic is only released under
reducing and slightly alkaline conditions. High arsenic groundwaters are found both in shallow water-tables,
and in deeper aquifers of between 100 and 120 m depth. There is no evidence of widespread arsenicosis, but
there are serious localised health-hazards, and some risk of low-level arsenic ingestion through indirect
pathways, such as through contaminated rice and aquaculture. An almost ubiquitous presence of arsenic in
soils, together with the likelihood of greatly increased groundwater extraction in the future, will require
continuing caution in water resources development throughout the region.

Introduction
In recent years, high-profile discussion of the
arsenic problems in Bangladesh, West Bengal and
Nepal has raised fears of impending arsenic
problems in other regions of comparable hydrogeology. High amongst these concerns is the Cuu
Long (Mekong) Delta in Southern Vietnam, and
adjacent areas of Cambodia where the Lower

Mekong and its tributaries connect with significant
alluvial aquifers deposited by the ‘palaeo-MekongÕ
and other rivers. The arsenic threat has been
recognised since the late 1990s, and many studies
have been undertaken by regional and central
government departments and concerned NGOs,
partly using off-the-shelf test kits, and partly utilising laboratory facilities in Phnom Penh, Can Tho
and Ho Chi Minh Cities. This paper is an attempt

342 gordon stanger et al.
to draw together the existing data from numerous
sources, supplemented by our own field and laboratory work, to ‘fill in the gapsÕ. The overall
finding is that of a less dire arsenic distribution
than in Bangladesh, Uttar Pradesh, Nepal and the
Red River Basin of Northern Vietnam. Nevertheless, there are localised arsenic ‘hot-spotsÕ, a more
general risk of low-level arsenic ingestion (arguably sub-clinical in effect), additional potential
pathways of arsenic exposure through the food
chain, and a potential for increasing arsenic concentrations in groundwater over time. In short,
there are a few sites requiring urgent attention, and
a generally low-level problem elsewhere. The latter
requires continuing vigilance, but is of relatively
minor importance compared to microbiological,
PAH1 and organophosphate pesticide contamination in Cambodia and Vietnam.

Geology, hydrogeology and drainage of the Lower
Mekong
The Lower Mekong in Cambodia and Vietnam
naturally divides into three geologically and
physiographically distinct areas; the main channel
area, the Tonle Sap, and the Cuu Long Delta (i.e.
the Mekong Delta).
Between the Lao-Cambodian border and
Phnom Penh the flood plain is narrow to completely absent, with heavily forested hills of up to
600 m in elevation constricting the Mekong River
on both banks. These hills are of Triassic age,
comprising partly basalts and partly sediments of
the Lower- to Mid-Indosinias group. In this area,
the Indosinias consists of andesitic and dacitic

lavas with predominantly continental sandstones,
silty shales, red shales, marls and conglomerates
with subordinate breccias. Gleyic and ferralic
cambisols, or rhodic and humic ferralsols have
developed on these sediments and volcanics, with
several localised low-yielding aquifers being
tapped by shallow wells. Six wells from this environment were tested but none exhibited significant
arsenic concentrations, (i.e. >10 lg L)1). Only
further downstream, where the flood plain begins
to broaden, does significant arsenic begin to
appear in groundwaters. This arsenic is restricted
1

Polycyclic aromatic hydrocarbons, in this case mainly consisting of dioxin residues.

to areas where sedimentary deposition clearly
originates from the Mekong.
To the north-west of Phnom Penh the ‘reversible
tributaryÕ of the Mekong, the Tonle Sap River,
drains into, and out of, the Tonle Sap Lake (the
‘Great LakeÕ). Thirteen other river basins, comprising some 60% of Cambodia, also drain into
this lake from the highlands which form the
perimeter of the north-western provinces
(Figure 1). To the south-west of the Great Lake
the four main river basins consist of uninhabited
and densely forested mountains up to 1500 m
high. The highland headwaters of these catchments are of heterogeneous facies comprising
Cretaceous to Palaeocene sandstones and conglomerates of the Upper Indosinias, Triassic
sandstones, conglomerates, tuffs and shales of the
Mid-Indosinias, and Devonian quartzites, shales,

schists and gneiss. None of these yield sediments
containing arsenic-rich groundwaters. However,
the eastern part of the massif is dominated by the
‘Aoral graniteÕ and Triassic marine shales. The
latter are presumed to be marine in origin, and
possibly arsenic-rich, although suspected minefields prevented close inspection and sampling.
Northward drainage from this area has deposited
more than 150 m of low-lying intercalated ironrich clays and coarse quartz sands associated with
typical groundwater arsenic concentrations of
about 20 lg L)1. This anomaly is marked ‘AÕ in
Figure 2. It is a moot point whether the arsenic
originates from a palaeochannel of the Mekong or
from arsenious shales in the upper catchment.
Perhaps there is intercalation of sediments from
both sources, but drilling records suggest that a
relatively local provenance is more likely.
If this is so, then the ‘Pursat–Kampong ChhnangÕ arsenic anomaly, ‘AÕ, is the only known case
of natural non-Mekong arsenic throughout Cambodia and Southern Vietnam. Of the 40 wells tested in this area the highest arsenic concentration
was 30 lg L)1.
North-east of the Tonle Sap the hills are relatively low, occasionally reaching about 500 m
elevation, and merge eastwards with the forested
hill country of the Mekong proper. Likewise the
geology is of the more easterly Indosinias facies,
comprising quartzite, sandstone and basalt.
Groundwater sampling in this area was sparse,
and arsenic concentrations were all less than
10 lg L)1, with the majority being below the


arsenic in groundwater 343

Fig. 1. Geography, and distribution of actual and potential acid sulphate soils.

detection limit. Red clay and lateritic sediments
are widespread in this area, providing a potential
substrate for arsenic adsorption, but no evidence
was found for high arsenic within or downstream
of these northern hills.
Between the south-western mountains and
north-eastern hills lies the sedimentary depression
of the Great Lake and surrounding low-lying
alluvium. This basin is partially fault-bounded on
its south-western edge, but is of unknown thickness. A roughly concentric synclinal pattern of
sedimentation surrounds the lake, with older
coarser ferruginous silts, sands and grits around
the perimeter conformably overlain by younger

red-clayey and silty sediments towards the lake.
During the monsoon season, from about June to
October, flood waters from the Mekong back up
into the lake, which then resumes normal drainage
back into the Mekong between November and
March. This results in a seasonal lake-level fluctuation of between 6.7 and 8.5 m in amplitude.
Rainfall into and evaporation from the lake are
closely balanced so the dry-season outflow of the
Tonle Sap closely matches the combined inflow of
the Tonle Sap and the 13 other circum-lacustrine
river basins. The sediment flux appears to be less
well balanced, although available data is somewhat imprecise. Carbonnel and GuiscafreÕs

344 gordon stanger et al.

Fig. 2. Distribution of arsenic concentrations in groundwater.

estimates (1963) were an inflow of 4.6 million
tonnes of sediment, of which the Tonle Sap River
contributed 2.7 million tonnes (58%). A more
recent estimate by the Mekong River Commission
cites an average annual inflow to the ‘Great LakeÕ
of 5.7 million tonnes, of which the Tonle Sap
River remobilises a seasonal outflow of 4.7 million
tonnes (82%), thereby yielding between 0.10 and
0.23 mm of deposition per year, depending upon
the degree of compaction and effective area of
deposition. This compares with Carbonnel and

GuiscafreÕs estimates of 0.3 mm of deposition per
year, averaged over the previous 5000 years. Both
estimates indicate a net annual gain of Mekongderived argillaceous sediment, and hence the
possibility of ‘modernÕ arsenic importation of farupstream provenance. However, there is an
alternative
possibility.
Tectonic
evidence
from the ‘Golden TriangleÕ area (of Thailand–
Myanmar–Laos) suggests that the Mekong River
configuration has remained stable for about
5 million years, prior to which the palaeo-Mekong

arsenic in groundwater 345
entered the Gulf of Thailand somewhat to the east
of Bangkok. During the transition from the old to
the new channel configuration it is very likely that
it flowed through the north-west Cambodian/Thai
border, and thence along the current axis of the
‘Great LakeÕ and into the Cuu Long Delta. Whilst
details of this palaeo-channel are sparse, it
would explain the low relief along that part of the
Thai–Cambodian border, the slight arsenic
groundwater anomaly (marked ‘BÕ on Figure 2),
and the distribution of low concentrations of
arsenic throughout sediments of the northwest
Cambodian provinces.
Further downstream there is no clearly defined
apex to the Cuu Long Delta. Rather, the broad
floodplain downstream of Phnom Penh broadens
still further to become the Delta at about the
Cambodian–Vietnam border. All of the very high
arsenic concentrations encountered in this survey,
i.e. >100 lg L)1, occur within this floodplaindelta complex.
At 144 to 160 million tonnes per year the
Mekong has the seventh largest sediment load of
any river (Ta et al. 2002a/b). This heavy
sedimentary load has been deposited as the Cuu
Long Delta through numerous cycles of marine
regression and transgression, the latter corresponding to Pleistocene glacial and interglacial
sea-level stands, respectively. Hence, this alluvial
flat, the worldÕs third largest flood-plain delta, is

geologically very young. Little is known of the
sub-alluvial topography. Within the floodplain at
the Cambodian–Vietnam border the sediment is as
little as 40 m deep, whereas it is at least 650 m
thick in the Ben Tre area (the outer delta).
Attempts to rationalise the available hydrogeological data suggest that there are four accessible
silty and/or sandy aquifers sandwiched between
thicker clay-rich aquicludes. These are:
•
•
•
•

QR Recent thin shallow to surface sediments.
QIV Holocene, incised river silts and coastal
sand dunes up to 20 m thick.
QII–III Upper to mid Pleistocene, freshwater
to saline, 10–30 m thick, widely used.
QI Lower Pleistocene, freshwater to saline,
underlies about 90% of the delta.

Some arsenic is associated with all four of these
aquifers. Deeper Neogene aquifers exist but are
seldom exploited and nothing is known of their
arsenic risk. Neither the extent of lateral aquifer

continuity, nor the effectiveness of vertical
hydraulic separation between aquifers are
adequately known. Comparison with betterknown deltaic environments suggests that the
sedimentology is almost certainly more complex

than indicated on the available litho-stratigraphic
sections.
The current delta, from the surface to a depth of
between )10 and )20 m has all been deposited
during the past 5000 years (Ta et al. 2001; Tanabe
et al. 2003). Hence during the last glacial maximum high stand the coast was some 250 km
further inland, near the Cambodian border. Prior
to this, several cycles of erosion and deposition
have buried subaerial laterites and floodplain silts
in inland areas, together with beach sands and
mangrove swamps and incised channels along a
prograding shoreline. This has resulted in many
localised areas of oxidised palaeosols and more
widespread strongly reducing organic clayey
sediments, all of which potentially provide
substantial adsorption substrates for arsenic. An
association of finely disseminated pyrite within the
organic cambisols and gleysols has facilitated the
development of widespread ferruginous acid-sulphate soils, which have becoming a problem over
many parts of the delta (Figure 1).

Arsenic in soils
Industrial sources of arsenic in the Lower
Mekong are trivial in comparison to natural
sources. Huy et al. (2002) noted slightly elevated
arsenic concentrations in sediments of the Luong
Canal, which drains the industrial area of
northwest Ho Chi Minh City, but their maximum
observed enrichment factor relative to the background concentration was only 8.2 at a single
sampling point. A potential arsenic source is from

fertilisers, since arsenic is readily adsorbed onto
phosphate, and much of the phosphate fertiliser
used in the Cuu Long Delta is from the Red
River basin of Northern Vietnam, which is
known to be arsenic-rich. To investigate the
possibility of such artificial arsenic enhancement a
set of surface soils from an area of variable fertiliser use in rice paddies between OÕMon and
Can Tho was analysed by neutron activation. The
results are plotted in Figures 3 and 4. Apart from
a single unexplained phosphorus anomaly, the


346 gordon stanger et al.

P hosphate %

10.00

1.00

0.10

0.01
5

10

15

20

25

30

Arsenic (ppm)
Fig. 3. Phosphate versus As in surface soils from the OÕMon–Can Tho farming area.

lack of any correlation between arsenic and
phosphorus suggests that arsenic from fertiliser is
not a significant factor.
A better, albeit still weak relationship, exists
between total iron and arsenic, Figure 4. This
could be related to the natural presence of either
finely disseminated arsenious pyrite or to arsenic
adsorbtion onto an iron oxy-hydroxide substrate.
It is evident from the distribution of arsenic in
groundwater that the source of arsenic is more or
less ubiquitous throughout the soils and sediments
of the region. No primary source of arsenic mineralization has yet been identified, but we have
analysed major and trace element concentrations,

by neutron activation, from 129 soils and nearsurface sediments, including two modern suspended-sediment samples from the Bassac river,
the second largest of the eight distributaries of the
delta, (see Figure 1). The geographic distribution
of soil-arsenic concentrations within the delta is
fairly uniform with no discernible clusters of high
concentrations. The histogram of concentrations is
shown in Figure 5.
Arsenic is present in surface or near-surface soils

at concentrations varying from 3 to 47 ppm, but
the great majority of measurements lie within the
range of 8–16 ppm. Hence, aside from a few high
values of >25 ppm, the natural variation only

5.0

Fe (%)

4.0

3.0

2.0

1.0

0.0
5

10

15

20

25

30

Arsenic (ppm)
Fig. 4. Total iron versus arsenic in surface soils from the OÕMon–Can Tho farming area.


arsenic in groundwater 347

Percentage of total (N=129)

35
30
25
20
15
10
5
0
0 to 5

5 to 10

10 to 15

15 to 20

20 to 25

25 to 30

>30

Range (ppm)
Fig. 5. Frequency histogram of surface soil arsenic concentrations in the Cuu Long Delta and adjacent alluvial areas, (mg Kg)1
dry weight).

exceeds the measurement error by a factor of
about four. The average inter-sampling distance in
this survey was about 30 km, which yielded no
discernible pattern of concentration across the
delta. About 35 soil samples were also taken from
north-east of the Mekong Delta, in and around Ho
Chi Minh City, where smaller rivers clearly derive
their sediments more locally (from Binh Phuoc and
Tay Ninh provinces). These showed a marked
contrast in soil-arsenic concentrations (Table 1).
Between the Mekong Delta sensuo stricto and
the north-eastern river basins lies the ‘Plain of
ReedsÕ, an area strongly affected by the MekongÕs
annual flood, but which may partially have derived
sediment from other sources in the past. One of the
key features of the Plain of Reeds is the high
prevalence of acid sulphate soils, with water pHs
as low as 2.9. The obvious presence of widespread
sulphides, oxidising at or near the surface as part
of a jurbanite-acidic-sulphate controlled equilibrium system2 raises the suspicion that aqueous
arsenic might be derived from the dissolution of
arsenious pyrite, and conditionally precipitated
with jarosite. Acid sulphate soils affect some 41%
of the Cuu Long Delta, and contain abundant
jarosite, (K,Na)Fe3Æ(SO4)2Æ(OH)6, in which arsenate anions are known to substitute for sulphate
(Savage et al. 2000). However, there is insufficient

evidence to conclude that arsenic concentrations
from acidic soils are significantly different from
2

Jurbanite, Al(SO4)ÆOHÆ5H2O is the phase controlling the
equilibrium water chemistry.

those from non-acidic areas of the delta. Rather, it
appears that all of the Mekong sediments, with or
without pyrite and jarosite, (i.e all parts of the
Delta), are naturally arsenic-rich.
With such large inter-sampling distances, the
scale of natural variation, and hence of the typicality of samples, was considered. To test this
variation, samples were taken from 18 farms in the
OÕMon area near Can Tho City, with an intersample distance of about 5 km. Unlike pointsampling from the main survey, composite soil
samples were taken at each farm, from predominantly rice paddies. These samples were homogenised prior to INAA analysis, to give an average
value over tens of hectares.
The results, plotted in Figure 6, are only slightly
more uniform than the main regional sampling.
This degree of local variation is probably
influenced by differential ‘dilutionÕ with organic
matter in such intensively cultivated top-soils.
Table 1. Contrast in soil-arsenic concentrations.
Mean sediment
concentrations, total
As ppm
58 samples from the
Mekong Delta
35 samples from river basins to
the north-east of the delta

5 samples from the Plain of
Reeds, and downstream
2 samples of suspended sediment
from the Bassac river

13.8
4.8
15.0
9.5


348 gordon stanger et al.

Fig. 6. Variation in soil–arsenic concentrations from the farmed area of OÕMon-Can Tho.

The arsenic distribution map, Figure 2, is based
upon 932 water quality analyses collected between
1998 and 2003 as part of 12 different sources and
studies3. Some of these were purely field studies
using ‘HachÕ field test kits, but in most instances
semi-quantitative field indications of greater than
10 lg L)1 (‘ppbÕ) were followed-up by more
accurate laboratory analysis upon HNO3)spiked,
freshly pumped groundwater samples. The classes
of groundwater arsenic concentrations depicted in
Figure 2 are based upon the WHO revised
‘acceptable limitÕ of 10 lg L)1. This limit arises
more from the constraints of realistic measurement
than from any proven limit of ‘no-adverse effectÕ,

3

These comprise (1) The AusAID-funded Vietnam–Australia
Water Resources Management Assistance Project, Component
3, 2003, (2) NWIS Project (ADB) TA-3758 Cambodia, 2002, (3)
The World Bank-funded SIWRP reconnaissance data for
arsenic in Groundwaters of the CLD, (4) Haskoning and
ARCADIS Euroconsult, Groundwater Study of the Mekong
Delta, (1999). (5) JICA, The Study of Groundwater Development in Central Cambodia, 2002, (6–9) Unpublished data from
the Pursat et al Meanchey Departments of Rural Water Supply,
Cambodia, (10) Unpublished data from the Cambodian Ministry of Rural Development, (11) the AusAID-funded Cuu
Long Delta Regional WSS Project, and (12) new data collected
and analysed by the authors of this paper.

although in practice concentrations below this
limit are probably safe.
The sampling distribution was uneven, being
pragmatically based upon the availability of
accessible dug wells and boreholes. As found in
numerous other studies, in Bangladesh, Assam
and Nepal, analyses from clusters of wells in
villages and towns do not yield consistent or
spatially uniform results. Rather, it was commonly
found that one or two groundwater samples
would yield greater than 10 lg L)1 total arsenic
within a local scatter of wells with less than
detectable concentrations4. That is within about
1 km2.
There were areas of very low groundwater
sampling density within the northern and western

Cuu Long Delta (in Vietnam). This is partly due to
difficulty of access, but mainly because the abundance of surface water, and the expense of sinking
wells, does not yet justify the development of
groundwater. Consequently there are no existing
wells to sample. However, as surface pollution and
dry-season water-demand increases, the future
4

For laboratory analyses the limit of detection was estimated at
1.4 lg L)1. For field kits the limit of detection is subjective, but
approximately 5 lg L)1 total arsenic.


arsenic in groundwater 349
demand for groundwater is likely to increase very
substantially, thus requiring a continuing vigilance
to deal with rising arsenic at an early stage.
In Cambodia there are few deep boreholes apart
from a few town water supplies although there is
an abundance of shallow dug wells in most provinces. This has facilitated a reasonably representative spatial sampling throughout the lowland
agricultural areas, and to a lesser extent, along the
Mekong riparian zone as far north as Kratie. The
only area with unsatisfactory coverage is in Siem
Reap, north of the ‘Great LakeÕ. Overall, the
average sampling densities were 1 per 101 km2 in
the Cuu Long Delta, and 1 per 111 km2 in the
inhabited parts of Cambodia. The total studied
area was 105 km2.

For the purposes of this reconnaissance study,

localities in which at least one sample was greater
than 10 lg L)1 total arsenic are mapped at this
concentration, even though the same area may
have contained a majority of samples at less than
10 lg L)1. Three factors, the widespread occurrence of aqueous arsenic concentrations at close to
the limit of detection, the local soil and groundwater variation in arsenic concentrations, and the
consistency of the fluvial source area, all suggest
that the concentrations of arsenic are governed
more by processes of arsenic mobilisation than by
its geochemical availability.
Literature from better known and longer studied areas (BGS, 1999); McArthur et al. 2004) seem
to have reached a consensus that there are two

Fig. 7. Distribution of the annual flooding depth of the Mekong–Tonle Sap system.


350 gordon stanger et al.
In response to this flooding, there is an annual
cycle in groundwater levels, which varies from at
least 6 m in some riparian environments, to only
centimetres in more distant parts of the floodplain.
The sediments are mostly organic-rich, originating
from both former reeds and mangroves, and
modern cultivation, with anoxic preservation at
greater depth. Therefore, there is likely to be an
annual cycle of near-surface oxidising and reducing conditions, more or less corresponding to the
dry and wet seasons. The oxidising conditions
would, of course, be consistent with pyrite oxidation and partial dissolution. Against this lies two
strands of evidence regarding redox, and potential
substrates.

Firstly, Figure 9 illustrates the relationship
between redox potential and groundwater arsenic
concentrations. At low concentrations of up to
10 lg L)1, the arsenic appears to be stable across
virtually the entire redox spectrum – a range of
some 500 mV. On the other hand, high arsenic
concentrations are obviously dominated by
reducing, or only slightly oxidizing conditions.
Given the availability of arsenic in virtually
all soils, Figure 9 suggests that some other major
co-variable is required to mobilise high concentrations of arsenic. Curiously the iron–redox
relationship was much less clear-cut, with 1 to
10 mg L)1 total iron across the same redox

main processes of arsenic mobilization in shallow
non-thermal groundwater environments, namely
(a) oxidation of arsenious pyrite from microscopic
pyrite/arsenopyrite framboids, and (b) release of
adsorbed arsenic through the process of reductive
dissolution of FeOÆOH. In Bangladesh, Bengal,
Nepal and the Red River basin of Northern Vietnam the second of these processes is now regarded
as by far the more important (Berg et al. 2001;
Smedley 2003; McArthur et al. 2004). In the Lower
Mekong, however, there is circumstantial evidence
to suggest that oxidation has at least a contributing influence upon the highest arsenic anomalies in
groundwater, measured in hundreds of micrograms per litre, and may also be a secondary
process contributing to the episodic release and
continuing mobility of arsenic over wide areas at
concentrations of tens of micrograms per litre.
Comparison of Figures 2 and 7 show that there

is some correspondence between the depth of
annual flooding and the occurrence of arsenic
groundwater anomalies. In particular, the annual
monsoonal flood of the Mekong results in a stage
amplitude variation of between 8 and 11 m at
Phnom Penh, decreasing to about 3 m, some
125 km further downstream at Chau Doc, i.e. at
the deltaÕs apex, just south of the CambodiaVietnam border. Typical hydrographs are shown
in Figure 8.

6.0

4.0

3.0

2.0

1.0

Month

Fig. 8. Typical annual stage hydrographs of the Mekong river at Chau Doc.

-D
ec
31

-N
ov

30

31
-O
ct

-S
ep
30

ul

-A
ug
31

31
-J

ul
1J

un
1J

1M
ay

Ap
r

1-

1M
ar

31
-J
an

an

0.0
1J

S tage (metres)

5.0


arsenic in groundwater 351
700

Total Arsenic (ppb)

600

500

400
300

200

100

0

-200

-100

0

100

200

300

400

Redox (mV)
Fig. 9. The redox control upon arsenic in Southern Cambodian groundwaters.

spectrum – a point which would doubtless be
clarified with further iron-species-specific data.
In addition, sulphate concentrations, relative to
other major anions, were generally very low
throughout the flood-plain (mean, 30 and 70& of
27, 7 and 0 mg L)1 SO42) respectively). This

strongly suggests a widespread occurrence of
sulphate reduction, and hence of predominantly

reducing conditions in most groundwaters for
most of the time.
Secondly, Figures 10 and 11 show an almost
identical correlation between arsenic-iron and
arsenic-aluminium in soils.
Had the arsenic been predominantly associated
with arsenious pyrite there should be no arsenicaluminium relationship of any significance.

50
45

Correlation coefficient of main cluster = 0.88

40

Arsenic (ppm)

35
30
25
20
15
10
5
0
0

2

4

6

8

10

12

14

16

Total Iron (%)
Fig. 10. Arsenic versus total iron in soils of the Cuu Long Delta.

18

20


352 gordon stanger et al.
50
45
Correlation coefficient of main cluster = 0.81
40

Arsenic , ppm.

35
30
25
20
15
10
5
0
0

5

10

15

20

25

30

Aluminium (Al2O3%)
Fig. 11. Arsenic versus aluminium in soils of the Cuu Long Delta.

Rather, the aluminium is present either as
Gibbsite, Al(OH)3, as mixed-layer clays (hydrous
alumino-silicates), or as Jurbanite, Al(SO4)Æ

(OH)Æ5H2O. Gibbsite would present a potentially
adsorbing substrate for arsenic (V) anions, as
would the isostructural Goethite, a FeOÆOH. The
clay is not likely to be significant, and there is no
available data upon Jurbanite adsorption characteristics. The aqueous arsenic–pH relationship of
Figure 12 suggests that any large-scale desorption

occurs at a pH of between 6.5 and 7.5. Low aqueous
concentrations of arsenic under acidic conditions
may be regarded as the corollary of strongly adsorbed arsenic at a pH of less than 7. This is pleasingly consistent with the arsenic (V) adsorption
schemes upon both ferrihydrite (Pierce & Moore
1982), and upon a-Al2O3 (Halter & Pfeifer 2001).
We consider that the most likely explanation for
these relationships is one of competing processes.
It is known that finely disseminated sulphide is

600

Arsenic ( p p b )

500
400
300
200
100
0
4

5

6

pH

7

Fig. 12. The aqueous arsenic–pH relationship.

8

9


arsenic in groundwater 353
widely dispersed within arsenic- and organic-rich
sediments, and that these near-surface sediments
are subject to cycles of oxidation and reduction.
Under such circumstances it is hard to see how
arsenic mobility through pyritic oxidation and/or
dissolution could be unimportant. Once in solution the arsenic either remains mobile in a reducing
and neutral to alkaline (pH 6.5–8.0) environment,
or undergoes anionic adsorbtion onto a suitable
substrate, under acid conditions. It is not clear
what the favoured substrate would be, although
any iron phase would appear to be ferric rather
than ferrous. The weakness of anionic sorbtion
could account for the very widespread occurrence
of residual arsenic at low concentrations.
The wet tropical climate of the lower Mekong,
combined with an abundance of organic humus

are ideal conditions for a strongly biological role
in the soil chemistry. However, the extent to which
the above processes are mediated by bacterial
activity is currently unclear. More detailed studies
(Chapelle 2000); Akai et al. 2004; McArthur et al.
2004) have concluded that bacterially mediated
non-equilibrium desorbtion processes are crucial
to a full understanding of the observed arsenic
distribution.
An areal comparison of aqueous concentration
ranges is summarised by the histograms in
Figure 12. To some extent the slightly differing

frequencies of concentration between the Cuu
Long Delta, North-western Cambodia and
Southern Cambodia may be artefacts of different
analytical accuracy and slightly varied sampling
strategies in each area. Nevertheless, overall, there
is little difference in the distribution of concentrations throughout the Lower Mekong, whereas
there is an enormous contrast with much worse
groundwater contamination within the Red River
and Ganges–Brahmaputra Basins, (of Northern
Vietnam and Bangladesh, respectively). Within the
lower Mekong only 5.7% of all groundwater
samples exceeded the former WHO standard of
50 lg L)1, whilst 12.9% exceeded the more
stringent recent limit of 10 lg L)1.
There were insufficient deep wells in Cambodia
to discern other than near-surface (water-table)
arsenic concentrations. In the Cuu Long Delta

nearly 300 boreholes yield the depth versus concentration relationship shown in Figure 14. Some
caution is needed to interpret this scattergram
since the indicated total depth of the well does not
necessarily correspond to either the most productive horizon, or to the horizon of highest arsenic
concentration. It is quite likely that some nearsurface arsenic-rich groundwaters seep through
the gravel pack into more deeply set well-screens,
especially under rapid drawdown conditions at the
commencement of pumping. Nevertheless, two

Fig. 13. Relative distributions of arsenic concentrations (as lg L)1) in the Lower Mekong, and comparisons with the Red river
and Bangladesh arsenic-affected areas.


354 gordon stanger et al.

Fig. 14. The apparent depth–concentration relationship in aquifers of the Cuu Long Delta.

conclusions are drawn; firstly that arsenic in excess
of the 10 lg L)1 acceptable limit can occur at any
depth, and secondly that there are two horizons in
which arsenic is particularly prevalent. As one
might expect, the upper of these horizons is the
water-table aquifer, QR. The 100–120 m aquifer
could correspond with either the Qiv aquifer in the
outer delta, or the Qii–iii aquifer further inland.
Temporal variation
Long-term variations in arsenic concentrations
have yet to be monitored, but a seasonal study in
Tra Vinh province was undertaken in 2003 to
compare the effects of dry- and rainy-season

arsenic concentrations in 18 shallow wells from
three contrasting micro-environments: a nearcoastal acidic sandy area (Truong Long Hoa
village, Duyen Hai district), a non-acidic older
sedimentary area (Long Son village, Cau Ngang
district), and the more inland tidal riparian area of
Tra Vinh town. The dry season most typically ends
in April, (Figure 15), to be followed by 6 or
7 months of heavy rain, during which water tables
in Tra Vinh province typically rise between 2.4 and
3.3 m. Hence there is a dry season in which pyrite
and organics in the unsaturated zone are exposed
to oxidation, followed by a wet season in which
oxidation products enter the saturated zone under
more reducing conditions.
Arsenic concentrations from these environments are shown in Figure 16. In 10 wells the
arsenic fluctuations were consistently less than

10 lg L)1 throughout both seasons. In seven
wells there was a substantial decrease in arsenic
concentration from dry to wet season, but in one
well there was 25-fold increase in concentration.
The predominant tendency for up to a 20-fold
reduction in arsenic concentration in the wet
season is contrary to purely pH–redox based
expectations. It could be due to simple dilution of
the near-surface groundwaters by direct recharge,
or there may be a sorption-related explanation.
Given the sluggish kinetics of arsenic chemistry,
the possibility of delayed recharge through the
unsaturated zone, and the single case of increasing concentration during the wet season, the

causative processes governing the seasonal variations are currently ambiguous. Nevertheless, on a
purely empirical basis, these results indicate that
no single sample, in any season, can be guaranteed to disclose a seasonal or ephemeral arsenic
anomaly, but that a mid-to-end of dry season
sampling re´gime is more likely to yield high
arsenic concentrations than a mid-to-end of wet
season re´gime.

Health implications
Throughout this survey several hundred groundwater users were superficially observed for signs
of chronic arsenicosis, such as skin keratosis
(‘raindrop pigmentationÕ) on hands, face and feet,
and corneal ulceration. Such symptoms are
commonly observed in Bangladesh (Milton et al.


arsenic in groundwater 355

Fig. 15. Median monthly rainfall in Tra Vinh province.

2003; Saha 2003), but no such symptoms were
observed or reported throughout this study. A
few older people displayed substantial skin spotting comparable to incipient keratosis, but this is
more likely to be caused by sun damage as a
consequence of decades of unprotected UV
exposure during rice farming. Similarly, there
were no reports of lassitude, as typically experienced by arsenicosis victims in Bangladesh.
However, it would be prudent to undertake a
more detailed health survey in areas of high
arsenic concentration, on both sides of the

Cambodia–Vietnam border.

Complaints about water quality were common
in some areas, but none of the internal symptoms
reported were typical of arsenic contamination.
Apart from enteric illness, associated with bacterially contaminated shallow groundwaters (some
water tables were as shallow as half a metre), the
most frequent water quality complaint was
the bad taste, associated with iron. Throughout
the delta, in almost all cases of high iron concentration, (i.e. >5 mg L)1), this iron-tainted
groundwater was exclusively used for non-potable
purposes, such as washing, farming, irrigation
and a huge range of small-scale industrial uses. In

Fig. 16. Seasonal sampling from 18 shallow wells in Tra Vinh province.


356 gordon stanger et al.
view of the high iron-arsenic correlation, this
preference for iron-free water has probably prevented most, if not all, cases of toxic ingestion
from high-arsenic waters.
However, the practice of using moderate- to
high-arsenic groundwater for chicken farming,
aquaculture and livestock may, locally, be
regarded as cause for concern in view of the
potential for arsenicÕs bio-concentration within
the food chain. In particular, the highest arsenic
anomaly in the delta, of 400 lg L)1 total arsenic,
was used exclusively for aquaculture comprising
fish and ‘freshwater prawnsÕ (crayfish?) cultivation, for which there appears to be no speciesspecific data on bio-concentration. Groundwater

is seldom used for irrigation in the Lower
Mekong, so the risk of arsenic accumulation in
rice is minimal. A much greater risk occurs from
cooking the rice in contaminated water (Meharg
& Rahman 2003). Arsenic concentrations of up
to 60 lg L)1 occur in the orchard and mixed
farming areas of Beˆn Tre and M~
y Tho, but neither the extent of groundwater usage in these
areas, nor the bio-accumulation of arsenic in
fruit, is currently known.
Although low-level arsenic groundwaters are
widespread, in fact almost ubiquitous, throughout
Cambodia and the Mekong Delta, the physiological response to this form of contamination is currently sub-clinical to absent. Nevertheless there is a
high-risk area, around the apex of the delta, where
particular care needs to be exercised if any future
groundwater development is to proceed.
Other forms of water contamination, such as
from toxic farm pesticides, notably organo-phosphate pesticides and herbicides, have a much
greater impact upon health, whilst both arsenic
and pesticides pale into insignificance compared to
the health threat from the microbiological contamination of surface water.

Conclusions
As a welcome change from much of the gloomy
literature on the Asian arsenic crisis, this study
concludes that the health threat posed by arsenic
in the Lower Mekong is not as great as some had
feared. There is no evidence of acute arsenicosis
occurring within the Mekong Delta, and the
number of wells or boreholes in which arsenic

concentrations exceed the new stringent WHO
drinking water standard of 10 lg L)1 is only
12.9% of the total. In all such cases the bad taste
of the associated high iron and aluminium concentrations results in alternative drinking water
supplies, mainly rainwater, being utilised. However, as the rapidly growing population and effects
of increasing coastal salinization both increase, an
inevitably increased dependence upon groundwater from the first and second confined aquifers
(QIV and QII–III) will occur, and hence the potential for more widespread and higher arsenic concentrations is real. The geochemical distribution of
high arsenic concentrations (>10 ppm) includes
virtually all the fluvial sediments of the Mekong
flood plain and delta, as well as some of the
MekongÕs important tributaries, such as the Tonle
Sap.
Very large seasonal fluctuations in groundwater arsenic concentration betoken serious problems for sampling strategy in more detailed
future studies, since no single sample can be
guaranteed as typical over an inter-seasonal
period. In particular, mid- to late-rainy season
sampling is least likely to identify seasonally high
arsenic anomalies.
There is a need for an urgent study of the upper
part of the delta and floodplain, on both sides of
the border, and especially in riparian areas of large
stage fluctuations. This would benefit from more
detailed process studies, the collection of more
detailed time-series data, with particular attention
to the indirect ingestion pathways of aquaculture
and agriculture.
Acknowledgements
Of the numerous people and organizations who

have assisted this reconnaissance study the
authors would particularly like to thank the
following: John Cantor and Graham Jackson (of
different AusAID projects in southern Vietnam),
Ha and Hien of the SIWRP, and the Ca Mau
peopleÕs committee for provision of boat transport and other very helpful logistical support.
References cited
Akai J. et al. 2004 Mineralogical and geomicrobiological
investigations on groundwater arsenic enrichment in Bangladesh. Appl Geochem 19(2), 163–260.


arsenic in groundwater 357
Asia Development Bank–Pacific Consultants International
2003 Northwest Irrigation Sector Project. Final Report
Annex A: Climate, Hydrology, Hydrogeology and Hydrochemistry .
Berg M. 2001 Arsenic contamination of groundwater and
drinking water in Vietnam: a human health threat. Environ
Sci Technol 35(13), 2621–2626.
British Geological Survey with Mott MacDonald Ltd, (UK).
1999 Groundwater Studies for Arsenic Contamination in
Bangladesh. Phase 1 Final Report, Main Report. 96 pp.
Carbonnel GP, Guiscafre´ J. 1963 Grand Lac du Cambodge:
Sedimentologie et Hydrologie. Final Report, Paris.
Chapelle FH. 2000 The significance of microbial processes in
hydrogeology and geochemistry. Hydrogeol J (8), 41–46.
Dept. of Geology Mapping Office, (undated) 1:200,000 Geological Maps of Pursat, Sisophon and Siem Reap. General
Department of Mineral Resources, Toul Kouk, Phnom
Penh.
Halter WE, Pfeifer HR. 2001 Arsenic (V) Adsorption onto
a-Al2O3 between 25 and 70°C. Appl Geochem 16, 793–802.

Haskoning BV. 1999 Consulting Engineers and Architects, in
association with the Division of Hydrogeology and Engineering Geology for the south of Vietnam, and Arcadis
Euroconsult, Binh An Ward. District 2, TP HCM, Viet Nam.
Ground-water Study: Mekong Delta. Assessment of Arsenic
Pollution in Groundwater of (the) Mekong Delta. 28 pp.
Huy NQ, Luyen TV, Phe TM, Mai NV. 2002 Toxic elements
and heavy metals in sediments in Tham Luong Canal, Ho
Chi Minh, Vietnam. Environmen Geol. doi 10.1007/s00254–
002-0699-4.
JICA. 2002 The Study on Groundwater Development in
Central Cambodia, Draft Final Report. Kukusai Kogyo
Co., Ltd.
McArthur J.M., et al, – 13 authors, (2004), Natural organic
matter in sedimentary basins and its relations to arsenic in
anoxic ground water: the example of West Bengal and its
worldwide implications. Appl Geochem 19, 1255–1293.
Meharg AA, Rahman Md M. 2003 Arsenic contamination of
Bangladeshi paddy field soils: implications for rice
contribution to arsenic consumption. Environ Sci Technol
37(2), 229–234.

Milton AH, Hassan Z, Rahman A, Rahman M. 2003 Non
cancer effects of chronic arsenicosis in Bangladesh: Preliminary results. J Environ Sci Health A 38(1), 301–305.
Pierce ML, Moore CB. 1982 Adsorption of Arsenite and
Arsenate on Amorphous Iron hydroxide. Water Res 16,
1247–1253.
MRC/UNDP. 1998 Sectorial Studies Vol. 2, (B), Natural
Resources Based Development Strategy for the Tonle Sap
Area, Cambodia, CMB/95/003. Nedeco/Midas .
Saha KC. 2003 Diagnosis of keratosis. J Environ Sci Health A

38(1), 255–272.
Savage KS, Tingle TN, OÕDay PA, Waychunas GA, Bird DK.
2000 Arsenic speciation in pyrite and secondary phases,
Mother Lode Gold District, Tuolumne County, California.
Appl Geochem 15, 1219–1244.
Smedley PL. 2003 Arsenic in Groundwater – South and East
Asia. In Welch AH, Stollenwerk KG (eds).Arsenic in Ground
Water: Geochemistry and Occurrence. Chap. 7. Massachusetts, USA: Kluwer Academic Publishers, pp. 179–209.
Tanabe S, Ta TKO, Nguyen Lap Van, Tateishi M, Kobayashi
I, Saito Y 2003. Delta evolution model inferred from the
holocene Mekong Delta Southern Viet Nam, , pp. 175–188
Sidi , Tropical Deltas of Southeast Asia – Sedimentology,
Stratigraphy and Petroleum Geology, Society for Sedimentary Geology, Tulsa, OK USASpecial Publication No 76 Ed.
Book.
Thi Kim Oanh Ta Lap Nguyen Van, Tateishi M, Kobayashi I,
Saito Y, Nakamura T. 2002a Sediment facies and later
holocene progradation of the Mekong River Delta in Bentre
Province, Southern Vietnam: an example of evolution from a
tide-dominated to a tide- and wave-dominated delta. Sediment Geol 152, 313–325.
Thi Kim Oanh Ta Lap Nguyen Van, Tateishi M, Kobayashi I,
Tanabe S, Saito Y. 2002b Holocene delta evolution and
sediment discharge of the Mekong river, Southern Vietnam.
Quater Sci Rev 21, 1807–1819.
Thi Kim Oanh Ta Lap Nguyen Van, Tateishi M, Kobayashi I,
Saito Y. 2001 Sedimentary facies, diatom and foraminifera
assemblages in a late Pleistocene–holocene incised-valley
sequence from the Mekong River Delta, Bentre province,
Southern Vietnam: the BT2 core. J Asian Earth Sci 20, 83–94.