Available online at www.sciencedirect.com Environment International 34 (2008) 756 – 764 www.elsevier.com/locate/envint Contamination of drinking water resources in the Mekong delta floodplains: Arsenic and other trace metals pose serious health risks to population Johanna Buschmann a,⁎, Michael Berg a,⁎, Caroline Stengel a , Lenny Winkel a , Mickey L Sampson b , Pham Thi Kim Trang c , Pham Hung Viet c a c 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 Center for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Hanoi, Vietnam Received 27 July 2007; accepted 22 December 2007 Available online 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 is b1 g L− Total 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− , with 37% of the studied wells exceeding the WHO guidelines of 10 µg L− arsenic 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 ~ 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 Introduction 1.1 Geographic, geologic and demographic overview of the Mekong delta The Mekong delta floodplain stretches over 52,000 km2 in 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 ⁎ 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 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 (b m above sealevel) 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 J Buschmann et al / Environment International 34 (2008) 756–764 sedimentation sequence in the delta started off with predominantly 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 sedimentation 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 sedimentation 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%) 757 (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 PO43− to be replaced by AsO43− 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) 1.2 Drinking water in the Mekong delta 1.4 Comprehensive groundwater survey 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 g L− 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 assessment of drinking water in Cambodia conducted in the year 2000 found 10 groundwater samples with arsenic levels N 10 µg L− (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− (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 This study provides a comprehensive overview of groundwater 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 b g L− TDS (see Fig 1) Family-based groundwater wells were sampled at locations presumably exhibiting b g L− TDS, and analyzed for 30 hydrogeochemical parameters The analytical 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 from Buschmann 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 1.3 Arsenic epidemiology Methodology 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 2.1 Groundwater salinity map Groundwater exhibiting a salinity of N g L− 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 758 J Buschmann et al / Environment International 34 (2008) 756–764 Fig 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 Fig 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 in N150 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 in Fig 1, groundwater 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 sample per 20 km2 and 30 km2 in 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 approximately 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 °C in the dark until analysis J Buschmann et al / Environment International 34 (2008) 756–764 2.3 Chemical analysis The chemical constituents in the groundwater samples were quantified in triplicates Arsenic concentrations were measured by AFS (Atomic Fluorescence Spectroscopy) Crosschecking 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; alkalinity by titration; and DOC with a TOC 5000 A analyzer Detailed information on quality assurance and further information on analytical methods are described elsewhere (Buschmann 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 a–c (and Figs SD a–c of the supplementary data) are simplified plots The maps were drawn using a nearest neighbor algorithm, a standard geostatistical technique in the 759 GIS program Arc GIS (Arc Map Version 9.1) They 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 of the supplementary data Results and discussion 3.1 Salinity As mentioned above, our groundwater survey focused on the region where salinity was predominantly below g L− TDS The boundary of this in-depth study area is depicted in Fig 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 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 760 J Buschmann et al / Environment International 34 (2008) 756–764 Table 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 Cambodia Parameter unit Astotal Ba Ca Fe K Mg Mn Na HCO−3 Cl− NH+4 –N NO−3 –N PO3− P H4SiO4–Si SO2− Cd Co Cr Cu Ni Se U DOC pH (field) Ec (field) Eh (field) T (field) Well depth −1 µg L µg L− mg L− mg L− mg L− mg L− mg L− mg L− mg L− mg L− mg L− mg L− mg L– mg L− mg L− µg L− µg L− µg L− µg L− µg L− µg L− µg L− mg L− µS cm− mV °C m Southern Vietnam North of Phnom Penh South of Phnom Penh (n = 108) (n = 132) a (n = 112) Average Median Range Average Median Range Average Median Range 70 280 53 3.5 4.4 22 0.4 81 380 35 3.3 2.0 0.3 24.9 18 b0.1 0.8 0.4 8.4 3.2 0.4 3.4 2.3 6.71 990 56 29.7 37 200 43 0.3 2.1 18 0.2 45 330 13 0.3 b0.2 b0.2 21.0 b5 b0.1 0.4 0.2 1.6 1.8 0.1 0.6 0.8 6.76 710 50 29.7 37 b1–1150 19–970 1–220 b0.05–32 0.4–100 0.2–140 b0.05–2.5 3–610 12–1500 2.3–360 b0.1–53 b0.2–43 b0.2–3.4 7.7–85 b5–210 b0.1–0.2 b0.1–6.3 b0.1–10 0.1–300 0.2–53 b0.1–15 b0.1–59 b0.1–21 4.05–8.54 34–15600 − 139–252 25.6–31.1 8–74 155 330 42 2.2 2.8 23 0.6 83 340 75 5.0 0.3 0.5 19.6 33 0.2 0.8 0.7 6.8 3.0 0.7 2.0 3.1 6.92 900 − 52 29.6 37 160 36 0.2 2.3 17 0.4 47 330 19 1.1 b0.2 0.2 18.6 b5 0.1 0.2 0.3 6.3 2.2 0.5 0.2 2.6 6.94 710 −29 29.5 36 b1–1300 12–4200 1.1–220 b0.05–15.5 0.4–24 0.6–150 b0.05–3.2 6–700 34–840 0.6–1200 b0.1–52 b0.2–22 b0.2–3.1 4.9–37 b5–1000 b0.1–2.3 b0.1–16 0.1–14 0.4–31 0.4–23 0.1–6.4 b0.1–32 b0.1–15 5.42–7.65 78–6150 −408–96 28.2–30.8 9–65 39 337 75 2.6 8.5 59 3.3 330 230 690 5.0 0.2 0.3 20.0 41 0.2 2.8 0.1 6.0 1.6 5.8 0.4 2.9 6.85 2500 14 29.6 69 b1 190 51 b0.05 5.4 34 1.0 220 190 370 1.4 b0.2 b0.2 18.4 15 0.1 0.8 0.1 0.5 0.9 2.8 0.1 1.0 6.81 1700 24 29.4 52 b1–850 1–2900 0.5–620 b0.05–56 1.2–92 0.2–440 1.0–34 11–4000 19–790 2.1–8600 b0.1–35 b0.2–4.4 b0.2–5.3 b0.1–39 b5–360 0.1–5.0 0.1–44 0.1–0.5 0.2–480 0.1–10 b0.1–64 b0.1–5.1 1.0–58 4.99–9.31 224–18000 − 303–625 28.4–33.9 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) significantly higher in Southern Vietnam compared to Cambodia reflecting seawater intrusion (Table 1) This trend is also mirrored in a pronounced increase of average chloride concentration from 35 mg L− north of PP to 690 mg L− in 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 concentrations N10 µg L− (WHO guideline) and 26% actually had arsenic levels N 50 µg L− (Table 2) The average concentration was 92 µg L− (range b 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− in 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 contaminated by arsenic, which has been found to be co-incident Table 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 Mn As and/or Mn Ba Cd Co Cr Cu Ni Pb Sb Se Tl U Zn Fe Ec TDS 10 400 see above 700 50 50 2000 20 10 20 10 no criteria 15 5000 EPA secondary criteria: 300 3000 µS cm− 1.8 g L− 37 50 71 11 0.3 0 1.4 1.1 7.1 all samples b0.25 µg L− 3.1 42 12 12 J Buschmann et al / Environment International 34 (2008) 756–764 with the topography (Buschmann et al., 2007) featuring rather dry terrains of a few meters higher elevation (compare elevation isolines in Figs and 2a) A correlation coefficient matrix of all groundwater parameters is provided as supplementary data (Table SD 2) The positive correlation of total As and Fe, PO43−, NH4+ and DOC, as well as the negative correlation with the redox potentials, typically characterizes anoxic aquifers where reductive dissolution 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 in the supplementary data) Whereas PCA factors and depict major ions and trace metals, respectively, factor 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 PO43− 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 PO43− (r2 = 0.46) supports a release mechanism caused by reductive dissolution of sediment minerals because PO43− and AsO43− 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% N 0.4 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 manganese 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 Bangladesh Other studies also support these findings (Bhattacharya et al., 2002; Ahmed et al., 2004; Jakariya et al., 2007; von Bromssen et al., 2007) 761 Fig Arsenic versus manganese concentration in Vietnam (○) and Cambodia ( ) 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 BaSO4 reduction 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 762 J Buschmann et al / Environment International 34 (2008) 756–764 Table Elements that exceed the WHO guidelines and their specific health threats Element Health threat Remarks Reference 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) As skin damage, cardiovascular disease, neurological disorders, cancer Particularly harmful for newborns and children Neither mutagenic nor carcinogenic effects Lung, prostate and kidney cancer (Tchounwou et al., 2003) 3.6 Antagonistic health effects Mn Cancer, skin damage Ba Neurological disorders Hypertension Cd Cancer Ni Cancer Skin damage Pb U (Wasserman et al., 2006) (http://www.rense.com/ general21/tox.htm) (Bertin and Averbeck, 2006) (Stohs and Bagchi, Increased risk of respiratory cancer due to 1995; Denkhaus and Salnikow, 2002) chronic inhalation of fumes or fine particles when exposure is to known carcinogenic forms like nickel oxide; asthma, nasal and sinus problems Hematological Nausea, abdominal pain, (Stohs and Bagchi, and neurological irritability, insomnia, 1995) problems excess lethargy, hyperactivity or hypertension Kidney damage Carcinogenic effects of (Craft et al., 2004) decay product radon caused 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 N 10 µg L− had Sb concentrations N µg L− 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 N 10 µg L− are simultaneous contamination by Ni N µg L− (29% of all samples), Cd N0.15 µg L− (4%), Co N2.5 µg L− (2%) or Cr N 2.5 µg L− (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 DOC N mg L− and 293 samples (83%) had DOC N1 mg L− 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- 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 concentrations N 10 µg L− 1, only 22 samples (17%) had Se N1 µg L− 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 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 concentrations N 0.1 mg L− 1, with a maximum of 2.3 mg L− Thus, the daily consumption of L of groundwater without any additional uptake of Zn by other means would not help to mitigate the adverse health effects caused by arsenic 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 (N g L− TDS) everywhere else The serious groundwater contamination in the area of the Mekong delta with drinkable groundwater (TDS b 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, especially if one considers co-contamination involving multiple toxic elements Concentrations of elements potentially mitigating 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 J Buschmann et al / Environment International 34 (2008) 756–764 Therefore, 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 As Ahmed 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 acknowledged for excellent analytical support Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.envint.2007.12.025 References Ahmed KM, Bhattacharya P, Hasan MA, Akhter SH, Alam SMM, Bhuyian MAH, et al Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview Appl Geochem 2004;19(2):181–200 Ahmed MF, Ahuja S, Alauddin M, Hug SJ, Lloyd JR, Pfaff A, et al Epidemiology — ensuring safe drinking water in Bangladesh Science 2006;314(5806):1687–8 Bailly R, Lauwerys R, Buchet JP, Mahieu P, Konings J Experimental and human studies on antimony metabolism — their relevance for the biological monitoring of workers exposed to inorganic antimony Br J Ind Med 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