Box 494, Phnom Penh, Cambodia c Center for Environmental Technology and Sustainable Development CETASD, Hanoi University of Science, Hanoi, Vietnam Received 27 July 2007; accepted 22 Dec
Trang 1Contamination of drinking water resources in the Mekong delta floodplains: Arsenic and other trace metals pose serious health risks to population
Johanna Buschmanna,⁎ , Michael Berga,⁎ , Caroline Stengela
, Lenny Winkela,
a Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 Dübendorf, Switzerland
b Resource Development International-Cambodia, RDIC, P.O Box 494, Phnom Penh, Cambodia
c Center for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Hanoi, Vietnam
Received 27 July 2007; accepted 22 December 2007
Available online 4 March 2008
Abstract
This study presents a transnational groundwater survey of the 62,000 km2 Mekong delta floodplain (Southern Vietnam and bordering Cambodia) and assesses human health risks associated with elevated concentrations of dissolved toxic elements The lower Mekong delta generally features saline groundwater However, where groundwater salinity isb1 g L− 1Total Dissolved Solids (TDS), the rural population
started exploiting shallow groundwater as drinking water in replacement of microbially contaminated surface water In groundwater used as drinking water, arsenic concentrations ranged from 0.1–1340 µg L− 1, with 37% of the studied wells exceeding the WHO guidelines of
10 µg L− 1arsenic In addition, 50% exceeded the manganese WHO guideline of 0.4 mg L− 1, with concentrations being particularly high in Vietnam (range 1.0–34 mg L− 1) Other elements of (minor) concern are Ba, Cd, Ni, Se, Pb and U Our measurements imply that
groundwater contamination is of geogenic origin and caused by natural anoxic conditions in the aquifers Chronic arsenic poisoning is the most serious health risk for the ~ 2 million people drinking this groundwater without treatment, followed by malfunction in children's development through excessive manganese uptake Government agencies, water specialists and scientists must get aware of the serious situation Mitigation measures are urgently needed to protect the unaware people from such health problems
Published by Elsevier Ltd
Keywords: Manganese; Trace elements; Salinity; Drinking water; Vietnam; Cambodia
1 Introduction
1.1 Geographic, geologic and demographic overview of the
Mekong delta
The Mekong delta floodplain stretches over 52,000 km2in
Southern Vietnam and some 10,000 km2 in neighboring
Cambodia The Mekong River originates in the Tibetan Plateau,
has a length of 4300 km and a catchment area of 520,000 km2 It
discharges great volumes of sediments (160 million t yr− 1) and
its dissolved salts contribute ca 30% of the world's input to the
oceans (Meybeck and Carbonnel, 1975) At Phnom Penh (capital of Cambodia), the Mekong River divides into two branches, the Mekong to the east and the Bassac to the south The present Mekong delta was formed during the last 6000– 10,000 years (Holocene) (Tamura et al., 2007) and consists of alluvial sediments of marine and fluvial origin (Nguyen et al.,
2000) The sediments were deposited in a north-south trending valley bordered by Pleistocene terraces About 60% of the present delta forms low-lying floodplains (b2 m above sea-level) with actual or potential acid sulphate soils (Ollson and Palmgren, 2001) The climate is monsoonal humid and tropical, with average temperatures of 27–30 °C The rainy season lasts from April to November (Giger et al., 2003)
In the last 6000 years the Mekong delta has prograded more than 200 km from around the Cambodian border to the present coastline in southern Vietnam (Tamura et al., 2007) The
Environment International 34 (2008) 756 –764
www.elsevier.com/locate/envint
⁎ Corresponding authors Berg is to be contacted at Tel.: +41 44 823 5078;
fax: +41 44 823 5028 Buschmann, Tel.: +41 44 823 5086; fax: +41 44 823 5028.
E-mail addresses: johanna.buschmann@eawag.ch (J Buschmann),
michael.berg@eawag.ch (M Berg).
0160-4120/$ - see front matter Published by Elsevier Ltd.
doi: 10.1016/j.envint.2007.12.025
Trang 2sedimentation sequence in the delta started off with
predomi-nantly fluvial deposits at the transition of Pleistocene to
Holocene during sea level rise Then during the early Holocene,
depositional environments shifted from tidal to fluvial
sedi-mentation in the coastal region, resulting in a seaward movement
of the coastline As the sea level rise subsequently decelerated,
the depositional environment shallowed and resulted in the
accumulation of peat in marshes Finally, floodplain sediments
constitute the uppermost layer Consequently, the high
sedi-mentation rates of young and organically-rich sediments in the
Mekong delta promoted anoxic conditions which lead to the
reductive dissolution of iron(hydr)oxides and the release of
arsenic
An estimated 17 million Vietnamese and 2.4 million
Cambodians live in the Mekong delta The infant mortality
rate in Vietnam is 30 per 1000 live births and the life expectancy
is significantly higher with 68 years for males and 73 years for
females compared to Cambodia with 56 and 61 years,
respectively (http://www.nationmaster.com) In Cambodia,
where 85% of the population have their homes in rural areas,
the infant mortality rate is 74 per 1000 live births caused by
contaminated water among other factors (http://worldfacts.us/
Cambodia.htm) This ratio of 7.4% is 18 times higher than in
Europe (0.4%)
1.2 Drinking water in the Mekong delta
Over the past decade, groundwater has become an important
source of drinking water in the Mekong delta and it is tapped
wherever the high salinity is not compromising its use (i.e below
1 g L− 1 TDS, Total Dissolved Solids) Groundwater arsenic
contamination has been documented for the Red River delta in
Northern Vietnam (Berg et al., 2001; Berg et al., in press), but no
comprehensive groundwater quality survey has been carried out
so far in Southern Vietnam However, a chemical quality
assess-ment of drinking water in Cambodia conducted in the year 2000
found 10 groundwater samples with arsenic levelsN10 µg L− 1
(Feldman et al., 2007) Elevated arsenic levels in Cambodia were
associated with Holocene alluvial sediments (Polya et al., 2005)
Moreover, in the Cambodian floodplain south of PP (Phnom
Penh) highly anoxic shallow aquifers were identified where 48%
of the studied wells had arsenic concentrations N10 µg L− 1
(Buschmann et al., 2007) Since the aquifers of Cambodia
stretch downstream across the border into Vietnam there is
an urgent need to thoroughly survey groundwater quality over
the whole Mekong Delta, particularly in the large Vietnamese
floodplain
1.3 Arsenic epidemiology
Arsenic is a systemic toxicant known to induce cardiovascular
diseases, neurological disorders, diabetes, gastrointestinal and
renal disorders (Ratnaike, 2006) Moreover, chronic arsenic
exposure has been associated with a variety of cancers (bladder,
kidney, skin and liver) (Tchounwou et al., 2003; Lamm and Kruse,
2005) The adverse health effects are related to the speciation of
As, where inorganic arsenic is more toxic than organic arsenicals
(Le et al., 2004) In groundwater, arsenic is primarily found in its inorganic forms, either As(III) or As(V) Both inorganic forms are toxic for the human body where As(V) is reduced to As(III) The mechanisms causing toxic effects are based on the inhibition of various mitochondrial enzymes by As(III) and the uncoupling of oxidative phosphorylation The affinity of As(III) for sulfhydryl groups of enzymes and the chemical similarity of As and phosphorus which allows PO4 −to be replaced by AsO4 −lead to these toxic effects (Scott et al., 1993)
A reconnaissance study of arsenic levels in hair conducted in
2004 in two villages of the Vietnamese Mekong delta for the first time revealed that people in Southern Vietnam are exposed
to high levels of arsenic (Berg et al., 2007) Several cases of arsenic-related skin lesions were observed in Cambodia in autumn 2006 (M Sampson, personal communication) Since the daily use of groundwater as drinking water has become popular in the Mekong Delta only during the last 10–15 years, it
is expected that in the near future victims suffering from chronic arsenic poisoning will also be identified in Southern Vietnam Manganese is another hazardous groundwater contaminant (Huang et al., 1989; Ono et al., 2002; Yazbeck et al., 2006) Its toxicity is particularly harmful for newborns and children (Wasserman et al., 2006) Exposure to elevated manganese levels in drinking water during pregnancy may hamper the intellectual development of the child (Wasserman et al., 2006) 1.4 Comprehensive groundwater survey
This study provides a comprehensive overview of ground-water quality in the Mekong delta comprising the floodplains of Southern Vietnam and neighboring Cambodia Since large proportion of the Holocene aquifers in the Vietnamese delta part exhibit a groundwater salinity that is unsuitable for drinking, detailed analysis of groundwater was focused on areas where the salinity is b1 g L− 1 TDS (see Fig 1) Family-based groundwater wells were sampled at locations presumably exhibitingb1 g L− 1TDS, and analyzed for 30 hydrogeochem-ical parameters The analythydrogeochem-ical results of 220 samples collected
in Vietnam and Cambodia are presented in a fully georeferenced database and are joined with an additional 132 samples of Cambodia fromBuschmann et al (2007)(supplementary data) The main geochemical triggers leading to groundwater contamination are evaluated and statistically verified Health risks related to the elevated levels (above WHO guidelines) of
As, Mn, Ba, Cd, Ni, Pb, Se and U as well as to multi-metal contamination are discussed Finally, groundwater components that aggravate arsenic toxicity such as Sb and DOC and antidotes (Se and Zn) are considered
2 Methodology 2.1 Groundwater salinity map Groundwater exhibiting a salinity of N1 g L− 1 TDS is generally disfavored for drinking which causes the people to use surface water Our in-depth study consequently focused on the regions where groundwater salinity is below this level (see
Trang 3Fig 1) The map of Holocene groundwater salinity in the
Mekong Delta was obtained from the Vietnam Department of
Geology and Mineralogy (DGMV), Southern Hydrogeological
and Engineering Geological Division (SHEGD) It is derived
from TDS measurements conducted in 2004 inN150 wells of
the national groundwater observation network The salinity
contours were established with MapInfo software using the
nearest neighbor algorithm
2.2 Groundwater sampling
Within the in-depth study areas depicted inFig 1,
ground-water from family-based tube-wells was collected at 112
locations in Southern Vietnam and at 108 locations north of
PP (Cambodia), whereas additional 132 samples from the south
of PP (Cambodia) were taken from Buschmann et al (2007)
Sampling locations were randomly selected in accessible areas with a sampling density of 1 sample per 20 km2and 30 km2in Vietnam and Cambodia, respectively Generally, tube-well depths varied within 10 to 70 m, with 12 samples in Vietnam exceeding 100 m depth (Table SD 1) Groundwater was taken at the tube by a hand or electrical pump The wells were purged for
10 minutes prior to measurement of redox potential and other on-site parameters such as pH, temperature, oxygen and conductivity The samples were filled in polypropylene bottles
An aliquot (60 mL) for the analysis of metals, ammonium and phosphate was 0.45 µm filtered and acidified with approxi-mately one milliliter of concentrated nitric acid to reach a pH b2 Anions (chloride, nitrate, phosphate and sulphate), alkalinity and DOC were determined in non-acidified and non-filtered water (120 mL) The samples were shipped to Switzerland and stored at 4 °C in the dark until analysis
Fig 1 Map of the Mekong delta depicting groundwater salinity in the Holocene aquifers Sampling locations for in-depth groundwater analysis (n = 352) are indicated
by red dots The contour plot shows the salinity distribution The salinity data was obtained from the DGMV (Ho Chi Minh City, Vietnam) The flat topography is indicated by elevation lines calculated with ArcGIS from the digital elevation model Gtopo30.
Trang 42.3 Chemical analysis
The chemical constituents in the groundwater samples were
quantified in triplicates Arsenic concentrations were
mea-sured by AFS (Atomic Fluorescence Spectroscopy)
Cross-checking with HR ICP-MS (High Resolution Inductively
Coupled Plasma-Mass Spectrometry, Element 2, Thermo
Fisher, Spectronex, Basel) agreed within 5% (Buschmann
et al., 2007) Cd, Co, Cr, Cu, Ni, Pb, Sb, Se and Zn were
measured by HR ICP-MS; Ba, Ca, Fe, K, Mg, Mn and Na
concentrations were measured by ICP-OES (Inductively
Coupled Plasma-Optical Emission Spectroscopy, Spectro,
Kleve, Germany); ammonium and phosphate by photometry;
nitrate, chloride and sulphate by ion chromatography;
alkali-nity by titration; and DOC with a TOC 5000 A analyzer
Detailed information on quality assurance and further
informa-tion on analytical methods are described elsewhere (
Busch-mann et al., 2007) The georeferenced data base of the 30
hydrogeochemical parameters analyzed in the 352 samples is
provided as supplementary data (Table SD 1)
2.4 Limitation of contour maps
It has to be emphasized that the contour maps shown in
Figs 1, and 2a–c (and Figs SD 1 a–c of the supplementary
data) are simplified plots The maps were drawn using a nearest
neighbor algorithm, a standard geostatistical technique in the
GIS program Arc GIS (Arc Map Version 9.1) They do show trends and are used for visualization of the situation Because the contaminant concentrations can vary significantly among neighboring wells, these contour maps have to be interpreted with care i.e., it is likely that some groundwater wells exhibit concentrations below the given thresholds, although they are located in areas depicted with high groundwater concentrations and vice versa
2.5 Statistical analysis PCA (Principal Component Analysis) was performed for the whole data set and the three regions (upstream and north of PP, downstream and south of PP, Cambodia, and Southern Vietnam)
in order to identify parameter associations The results are summarized in Table SD 3 of the supplementary data
3 Results and discussion 3.1 Salinity
As mentioned above, our groundwater survey focused on the region where salinity was predominantly below 1 g L− 1 TDS The boundary of this in-depth study area is depicted
in Fig 1 The concentrations of major cations and anions change drastically from a Ca–Mg–HCO3 type in the north towards a Na–Mg–Cl type in the south The salinity becomes
Fig 2 Maps of the Mekong Delta in-depth study area showing contour plots of (a) arsenic, (b) manganese and (c) iron concentrations measured in groundwater The maps were drawn using a nearest neighbor algorithm, a standard geostatistical technique For the limitations of the contour maps, see Limitation of contour maps section.
Trang 5significantly higher in Southern Vietnam compared to
Cam-bodia reflecting seawater intrusion (Table 1) This trend is
also mirrored in a pronounced increase of average chloride
concentration from 35 mg L− 1north of PP to 690 mg L− 1in
Vietnam (Table 1)
3.2 Arsenic contamination and its source
Fig 2a shows a contour plot of the arsenic distribution in the
detailed study area where 37 % of the samples had arsenic
concentrationsN10 µg L− 1(WHO guideline) and 26% actually
had arsenic levels N50 µg L− 1 (Table 2) The average
concentration was 92 µg L− 1 (range b1 to 1340 µg L− 1)
Groundwater arsenic contamination is obviously less severe in
Vietnam (22% above 10 µg L− 1) than in Cambodia (44% above
10 µg L− 1), but concentrations still reached up to 850 µg L− 1in
one of the scattered hot spots of Vietnam (vicinity of Cao Lanh,
Fig 2a) The arsenic distribution is more homogenous in
Cambodia, but restricted to the floodplains along the Mekong,
Tonle Sap and Bassac rivers Correspondingly, the wells located
further away from the rivers are less anoxic and not
con-taminated by arsenic, which has been found to be co-incident
Table 1
Average concentrations (arithmetic mean), medians and ranges of groundwater parameters analyzed in the “upstream and north of PP”, the “downstream and south of
PP, Cambodia ” and the “Southern Vietnam” part of the Mekong delta floodplain
North of Phnom Penh South of Phnom Penh (n = 108) (n = 132)a (n = 112) Parameter unit Average Median Range Average Median Range Average Median Range
Ba µg L−1 280 200 19 –970 330 160 12 –4200 337 190 1 –2900
Ca mg L−1 53 43 1 –220 42 36 1.1 –220 75 51 0.5 –620
Fe mg L− 1 3.5 0.3 b0.05–32 2.2 0.2 b0.05–15.5 2.6 b0.05 b0.05–56
K mg L−1 4.4 2.1 0.4 –100 2.8 2.3 0.4 –24 8.5 5.4 1.2 –92
Mg mg L−1 22 18 0.2 –140 23 17 0.6 –150 59 34 0.2 –440
Mn mg L−1 0.4 0.2 b0.05–2.5 0.6 0.4 b0.05–3.2 3.3 1.0 1.0 –34
Na mg L−1 81 45 3 –610 83 47 6 –700 330 220 11 –4000 HCO 3 − mg L−1 380 330 12 –1500 340 330 34 –840 230 190 19 –790
Cl− mg L−1 35 13 2.3–360 75 19 0.6–1200 690 370 2.1–8600
NH 4+–N mg L−1 3.3 0.3 b0.1–53 5.0 1.1 b0.1–52 5.0 1.4 b0.1–35
NO 3 − –N mg L−1 2.0 b0.2 b0.2–43 0.3 b0.2 b0.2–22 0.2 b0.2 b0.2–4.4
PO 4 − P mg L–1 0.3 b0.2 b0.2–3.4 0.5 0.2 b0.2–3.1 0.3 b0.2 b0.2–5.3
H 4 SiO 4 –Si mg L−1 24.9 21.0 7.7 –85 19.6 18.6 4.9 –37 20.0 18.4 b0.1–39
SO 4 − mg L−1 18 b5 b5–210 33 b5 b5–1000 41 15 b5–360
Cd µg L−1 b0.1 b0.1 b0.1–0.2 0.2 0.1 b0.1–2.3 0.2 0.1 0.1 –5.0
Co µg L−1 0.8 0.4 b0.1–6.3 0.8 0.2 b0.1–16 2.8 0.8 0.1 –44
Cr µg L−1 0.4 0.2 b0.1–10 0.7 0.3 0.1 –14 0.1 0.1 0.1 –0.5
Cu µg L−1 8.4 1.6 0.1 –300 6.8 6.3 0.4 –31 6.0 0.5 0.2 –480
Ni µg L−1 3.2 1.8 0.2 –53 3.0 2.2 0.4 –23 1.6 0.9 0.1 –10
Se µg L−1 0.4 0.1 b0.1–15 0.7 0.5 0.1 –6.4 5.8 2.8 b0.1–64
U µg L−1 3.4 0.6 b0.1–59 2.0 0.2 b0.1–32 0.4 0.1 b0.1–5.1 DOC mg L−1 2.3 0.8 b0.1–21 3.1 2.6 b0.1–15 2.9 1.0 1.0 –58
pH (field) 6.71 6.76 4.05 –8.54 6.92 6.94 5.42 –7.65 6.85 6.81 4.99 –9.31
E c (field) µS cm−1 990 710 34 –15600 900 710 78 –6150 2500 1700 224 –18000
E h (field) mV 56 50 −139–252 −52 −29 −408–96 14 24 −303–625
T (field) °C 29.7 29.7 25.6 –31.1 29.6 29.5 28.2 –30.8 29.6 29.4 28.4 –33.9 Well depth m 37 37 8 –74 37 36 9 –65 69 52 10 –420 The full georeferenced data base of 30 parameters measured in 352 samples is provided as supplementary data ( Table SD 1).
a Data from Buschmann et al (2007)
Table 2 Risk-based drinking water criteria recommended by the WHO and percentage of groundwater samples exceeding the guidelines
Parameter Risk-based drinking water
criteria (µg L−1) (WHO)
Percentage of groundwater samples exceeding WHO guideline
As and/or Mn see above 71
Tl no criteria all samples b0.25 µg L − 1
Fe EPA secondary criteria: 300 42
E c 3000 µS cm− 1 12 TDS 1.8 g L− 1 12
Trang 6with the topography (Buschmann et al., 2007) featuring rather
dry terrains of a few meters higher elevation (compare elevation
isolines inFigs 1 and 2a)
A correlation coefficient matrix of all groundwater
para-meters is provided as supplementary data (Table SD 2) The
positive correlation of total As and Fe, PO4 −, NH4and DOC, as
well as the negative correlation with the redox potentials,
typically characterizes anoxic aquifers where reductive
dissolu-tion of iron phases and release of surface bound arsenic (and
phosphate) is the principal source of dissolved arsenic in
groundwater of the Mekong delta Moreover, PCA supports
these findings (Table SD 3 in the supplementary data) Whereas
PCA factors 1 and 3 depict major ions and trace metals,
respectively, factor 2 delineates the release of arsenic under
reducing conditions It includes the negative correlation of As
and the redox potential, Eh, and the positive correlation of As
with NH4
+
, N-total, DOC and PO4
3 − Although correlation does not imply causation, these findings indicate that arsenic release
is likely promoted during microbial metabolization of dissolved
organic compounds where NH4 is produced as a reduction
product of organically bound nitrogen and/or dissimilatory
nitrate reduction (Tyrovola et al., 2006) Under reducing
conditions As(V) may be reduced to As(III), and/or minerals
such as goethite that exhibit binding sites for As may be reduced
and dissolved, which triggers the release of As (McArthur et al.,
2001) The positive correlation of As and PO4 − (r2= 0.46)
supports a release mechanism caused by reductive dissolution
of sediment minerals because PO4 − and AsO4 − have similar
chemical structures and therefore tend to bind to (and be
released from) the same mineral surface sites
3.3 Manganese concentrations in groundwater
Manganese concentrations above the WHO guideline
(0.4 mg L− 1) were present in 50% of the samples (Table 2),
hence manganese has to be considered as the second most
important groundwater contaminant in the Mekong delta The
distribution of Mn is by no means homogeneous (Fig 2b):
Southern Vietnam (69%N0.4 mg L− 1) and the areas west of the
Bassac River (72% N0.4 mg L− 1) are highly contaminated
Regions of uncontaminated wells are only present in the east of
the Mekong River (Prey Vêng Province, Cambodia) and along
the Tonle Sap River
Our results reveal that 71% of the studied wells are
contaminated with either As and/or Mn Many groundwater
wells have low As levels but high Mn because arsenic is less
mobilized under Mn reducing conditions (Fig 3) Other
samples show the opposite relation: low Mn and high As
concentrations A combination of high arsenic and/or
manga-nese was also reported in a regional study in Araihazar,
Bangladesh (Cheng et al., 2004), where only 11% and 16% of
629 samples met the WHO guidelines for arsenic and
manganese, respectively The authors mentioned that their
analyses were consistent with nationwide surveys in
Bangla-desh Other studies also support these findings (Bhattacharya
et al., 2002; Ahmed et al., 2004; Jakariya et al., 2007; von
Bromssen et al., 2007)
3.4 Other groundwater contaminants Besides As and Mn, the following toxic elements exceeded the WHO health-based guidelines (Table 2): Ba (11% of all samples), Se (7.1%), U (3.1%), Ni (1.4%), Pb (1.1%) and
Cd (0.3%) For Ba, Se and U, contour plots are provided as supplementary data (Figs SD 1–3) Barium hot spots are found
in Vietnam around the city of Cao Lanh Elevated Ba levels may result from terrestrial and/or marine inputs In reducing aquifers,
Ba is released during BaSO4reduction Consumption of Ba at the chronic dose level increases the risk for hypertension, however, neither mutagenic nor carcinogenic impacts have been reported (http://www.rense.com/general21/tox.htm)
The heavy metals Cd, Ni, Pb and U are known to have a number of negative impacts on human health, such as DNA damage, cancer and damage of the central nervous system (Stohs and Bagchi, 1995) (Table 3) Because Ni, Pb and Cd exceeded the WHO guidelines in only ~1 % of the samples, these heavy metals should actually not be considered as having
a high impact on the disease burden of people living in the Mekong delta Although minor in number, one should be aware
of some uranium hot spots in the Mekong delta (supplementary data Fig SD 1c) Uranium leads to kidney damage and is deposited at bone surfaces, where alpha radiation is emitted (Incorporated, 2002; Porter et al., 2004) and exposure to some
of its decay products, especially radon, does pose a significant health threat (Craft et al., 2004)
It is important to note that the impact of groundwater contamination by Cd, Ni, Pb and U on human health seems to
be outweighed by water related infectious diseases such as diarrhea since 60–70% of the rural population is still consuming surface water which is often microbially contaminated 3.5 Synergistic health effects
Apart from As and Mn, the percentage of other elements exceeding the WHO guideline values is rather small (Table 2) However, our findings raise concerns related to health issues
Fig 3 Arsenic versus manganese concentration in Vietnam ( ○) and Cambodia ( ).
Trang 7caused by multi-metal effects (Frisbie et al., 2002; Steenkamp
et al., 2002; Tsai et al., 2004; Hasgekar et al., 2006)
It is reported that Sb aggravates As toxicity with respect to
genotoxicity and metabolism (Bailly et al., 1991; Gebel, 1997)
The coexposure of Sb and As has been studied in Bangladesh
(McCarty et al., 2004) In our study, 9% of the samples with As
N10 µg L− 1 had Sb concentrations N1 µg L− 1 Thus, one
should be aware that co-contamination of As and Sb could
increase the arsenic-related disease burden in the studied area
Other examples of heavy metal co-contamination with As
N10 µg L− 1are simultaneous contamination by NiN1 µg L− 1
(29% of all samples), CdN0.15 µg L− 1(4%), CoN2.5 µg L− 1
(2%) or Cr N2.5 µg L− 1 (0.3%) The concentration of the
second contaminant considered here as potentially harmful has
been set to 1/20 of the corresponding WHO guideline assuming
that in the presence of one contaminant a second contaminant
aggravates its toxicity already at lower levels (Escher and
Hermens, 2002) These results clearly raise concerns about
potential multi-metal effects
Humic substances might also aggravate As toxicity (Lamm
and Kruse, 2005) By complexing inorganic arsenic (Buschmann
et al., 2006), humic acids– once consumed with drinking water –
release arsenic in the gastrointestinal tract where it is absorbed
(Tseng, 2005) In our study, 51 samples (14%) had DOCN5 mg
L− 1 and 293 samples (83%) had DOC N1 mg L− 1 Besides
complexing arsenic, humic acids exhibit different capabilities in
causing mutation (associated with BFD (Black Foot Disease)) or
lipid peroxidation (associated with arteriosclerosis and
throm-boangitis) (Xu, 2001) Although the etiology of BFD still remains controversial, arsenic combined with humic acids is the most probable cause for BFD (Lu et al., 1991; Tseng, 2005)
3.6 Antagonistic health effects Another groundwater contaminant worthwhile considering is selenium Selenium and arsenic act antagonistic (Biswas et al.,
1999) Significant reduction of arsenic toxicity through dietary intervention by Se has been reported (Gailer, 2002) Among the
131 wells with As concentrationsN10 µg L− 1, only 22 samples (17%) had SeN1 µg L− 1 By comparing the spatial distribution maps of As and Se (Fig SD 2a and SD 1b of the supplementary data), it is obvious, however, that elevated Se concentrations are found predominantly in regions with low levels of As (Southern Vietnam) (Table 1) This is supported by the poor 0.082 correlation coefficient (Table SD 2 in the supplementary data) Thus, the role of Se as a natural antidote against As toxicity seems to be negligible in the Mekong delta
Zinc was also reported to reduce arsenic toxicity It has been shown that marginal dietary zinc intake plays a role in severe vascular manifestations of chronic arsenic exposure caused by the indirect competition of zinc with arsenic in proteins containing dithiols (see (Engel et al., 1994) and references therein) The WHO recommends a daily intake of 15 mg zinc, which is normally achieved by uptake of food proteins (meat) If the diet includes beans, lentils, yeast and nuts, zinc deficiency should be no problem Zinc uptake through drinking water, however, would need high zinc concentrations in order to be sufficient for the daily needs and particularly in order to mitigate adverse health effects caused by arsenic In the study presented here, only 40 samples (11.4%) had zinc concentrationsN0.1 mg
L− 1, with a maximum of 2.3 mg L− 1 Thus, the daily consumption of 2 L of groundwater without any additional uptake of Zn by other means would not help to mitigate the adverse health effects caused by arsenic
4 Conclusions and recommendations The salinity in the Mekong delta increases significantly from north to south In Southern Vietnam, groundwater is used for drinking purposes only in the proximity of the rivers Bassac and Mekong due to elevated salinity (N1 g L− 1 TDS) everywhere else The serious groundwater contamination in the area of the Mekong delta with drinkable groundwater (TDS b1 g L− 1) requires urgent attention With 71% of the studied wells being contaminated by As and/or Mn in an area of ~ 8000 km2, this is
an alarming result In addition, several trace metals exceeded the WHO drinking water guidelines: Ba (11.0%), Se (7.1%), U (3.1%), Ni (1.4%), Pb (1.1%) and Cd (0.3%) Such findings should raise awareness about potential health impacts, espe-cially if one considers co-contamination involving multiple toxic elements Concentrations of elements potentially mitigat-ing arsenic toxicity such as Se and Zn are low or absent where
As is high so that negligible mitigating effects are expected Finally, co-contamination of As/Sb and As/DOC may lead to the aggravation of the toxic effects caused by arsenic
Table 3
Elements that exceed the WHO guidelines and their specific health threats
Element Health threat Remarks Reference
As Cancer,
skin damage
skin damage, cardiovascular disease, neurological disorders, cancer
( Tchounwou et al.,
2003 )
Mn Neurological
disorders
Particularly harmful for newborns and children
( Wasserman et al.,
2006 )
Ba Hypertension Neither mutagenic nor
carcinogenic effects
( http://www.rense.com/
general21/tox.htm )
Cd Cancer Lung, prostate and
kidney cancer
( Bertin and Averbeck,
2006 )
Ni Cancer
Skin damage Increased risk of
respiratory cancer due to chronic inhalation of fumes or fine particles when exposure is to known carcinogenic forms like nickel oxide;
asthma, nasal and sinus problems
( Stohs and Bagchi, 1995; Denkhaus and Salnikow, 2002 )
Pb Hematological
and neurological
problems
Nausea, abdominal pain, irritability, insomnia, excess lethargy, hyperactivity or hypertension
( Stohs and Bagchi,
1995 )
U Kidney damage Carcinogenic effects of
decay product radon
( Craft et al., 2004 )
Trang 8Therefore, mitigation efforts must be undertaken to provide safe
drinking water — and these mitigation actions should not be
limited to arsenic which is unquestionably the most significant
health risk, but they should also address Mn and several trace
metals Policy makers must become aware of the serious situation
and the governments and local agencies ought to test sources of
drinking water periodically Should sophisticated analytical
equipment not be readily available, chemical field test kits (Van
Geen et al., 2005) or inexpensive bioassays can be applied (Trang
et al., 2005) Where tube-well water has been tested, households
have to be informed about contaminant levels and – in case
of contamination – be encouraged to use a safe well in the
neighborhood AsAhmed et al (2006)pointed out, well switching
had the highest impact on arsenic mitigation in Bangladesh (29%),
while the drilling of deep tube-wells was proposed as the second
best option (12%) Other mitigation actions on the household level
include rainwater collection, dug wells, sand filters (Berg et al.,
2006) or SONO filters with a composite iron matrix (http://en
wikipedia.org/wiki/Sono_arsenic_filter) Treatment of surface
water by ceramic filters is another alternative which is currently
applied in the Kandal Province of Cambodia (http://www.rdic
org) Mitigation measures need to be urgently implemented to
protect people from health problems
Acknowledgements
Financial support was received from the Wolfermann–
Nägeli foundation (Switzerland) and the Swiss Agency for
Development and Cooperation (SDC, ESTNV project) We are
indebted to Do Tien Hung, Nguyen Kim Quyen and Nguyen
Trac Viet from the Vietnam Southern Hydrogeological and
Engineering Geological Division for providing the groundwater
salinity map Bui Hong Nhat, Vi Mai Lan, Moniphea Leng,
Mengieng Ung, Samreth Sopheap, Kagna Ouch, Um Rachana
and Vong Sovathana participated in field work Madeleine
Langmeier, David Kistler and Adrian Ammann are
acknowl-edged for excellent analytical support
Appendix A Supplementary data
Supplementary data associated with this article can be found,
in the online version, atdoi:10.1016/j.envint.2007.12.025
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