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 Australia-based water resources consultant, currently with the UNDP, Box 551, Sanaa´, Yemen Sub-Institute for Water Resources Planning, 253A An Duong Vuong, Quan 5, TP Ho Chi Minh, Vietnam Sub Institute of Hydrometeorology of South Vietnam, Phu Trach Tram, Tram Thuc Nghiem, KTTVNN, Dong Bang, Song Cuu Long, Vietnam Centre for Nuclear Techniques, 217 Nguyen Trai St., Quan 1, TP Ho Chi Minh, Vietnam Department of Environmental and Natural Resources Management, Can Tho University, 3/2 Street, TP Can Tho, Vietnam Author for correspondence (e-mail: hydrodocgeo@yahoo.co.uk) 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 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 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 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 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 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 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 and Apart from a single unexplained phosphorus anomaly, the 346 gordon stanger et al P hosphate % 10.00 1.00 0.10 0.01 10 15 20 25 30 Arsenic (ppm) Fig 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 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 Arsenic is present in surface or near-surface soils at concentrations varying from 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 10 15 20 25 30 Arsenic (ppm) Fig 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 0 to 5 to 10 10 to 15 15 to 20 20 to 25 25 to 30 >30 Range (ppm) Fig 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 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 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 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 samples from the Plain of Reeds, and downstream samples of suspended sediment from the Bassac river 13.8 4.8 15.0 9.5 348 gordon stanger et al Fig 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 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, 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 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 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 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 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 per 101 km2 in the Cuu Long Delta, and 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 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 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 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 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 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 and 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 and 11 m at Phnom Penh, decreasing to about 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 6.0 4.0 3.0 2.0 1.0 Month Fig 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 -200 -100 100 200 300 400 Redox (mV) Fig 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, and 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 0 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 0 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 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 pH Fig 12 The aqueous arsenic–pH relationship 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 or 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 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 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 [...]... 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... 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... 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... 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... 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... 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... 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. .. 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, ... 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... 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