Department Water Resources and Drinking Water

Arsenic contamination of shallow groundwater is among the biggest health threats in the developing world. Targeting uncontaminated deep aquifers is a popular mitigation option although its long-term impact remains unknown. Here we present the alarming results of a large-scale groundwater survey covering the entire Red River Delta and a unique probability model based on three-dimensional Quaternary geology. Our unprecedented dataset reveals that ∼7 million delta inhabitants use groundwater contaminated with toxic elements, including manganese, selenium, and barium. Depth-resolved probabilities and arsenic concentrations indicate drawdown of arsenic-enriched waters from Holocene aquifers to naturally uncontaminated Pleistocene aquifers as a result of > 100 years of groundwater abstraction. Vertical arsenic migration induced by large-scale pumping from deep aquifers has been discussed to occur elsewhere, but has never been shown to occur at the scale seen here. The present situation in the Red River Delta is a warning for other As-affected regions where groundwater is extensively pumped from uncontaminated aquifers underlying high arsenic aquifers or zones.

Large alluvial deltas of the Mekong River in southern Vietnam and Cambodia and the Red River in northern Vietnam have groundwaters that are exploited for drinking water by private tube-wells, which are of increasing demand since the mid-1990s. This paper presents an overview of groundwater arsenic pollution in the Mekong delta: arsenic concentrations ranged from 1-1610 μg/L in Cambodia (average 217 μg/L) and 1-845 μg/L in southern Vietnam (average 39 μg/L), respectively. It also evaluates the situation in Red River delta where groundwater arsenic concentrations vary from 1-3050 μg/L (average 159 μg/L). In addition to rural areas, the drinking water supply of the city of Hanoi has elevated arsenic concentrations. The sediments of 12-40 m deep cores from the Red River delta contain arsenic levels of 2-33 μg/g (average 7 μg/g, dry weight) and show a remarkable correlation with sediment-bound iron. In all three areas, the groundwater arsenic pollution seem to be of natural origin and caused by reductive dissolution of arsenic-bearing iron phases buried in aquifers. The population at risk of chronic arsenic poisoning is estimated to be 10 million in the Red River delta and 0.5–1 million in the Mekong delta. A subset of hair samples collected in Vietnam and Cambodia from residents drinking groundwater with arsenic levels >50 μg/L have a significantly higher arsenic content than control groups (<50 μg/L). Few cases of arsenic related health problems are recognized in the study areas compared to Bangladesh and West Bengal. This difference probably relates to arsenic contaminated tube-well water only being used substantially over the past 7 to 10 years in Vietnam and Cambodia. Because symptoms of chronic arsenic poisoning usually take more than 10 years to develop, the number of future arsenic related ailments in Cambodia and Vietnam is likely to increase. Early mitigation measures should be a high priority.

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 <1 g L^{− 1} 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^{− 1}, with 37% of the studied wells exceeding the WHO guidelines of 10 µg L^{− 1} 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 ~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.

Groundwater contamination by arsenic in Vietnam poses a serious health threat to millions of people. In the larger Hanoi area, elevated arsenic levels are present in both, the Holocene and Pleistocene aquifers. Family-based tubewells predominantly tap the Holocene aquifer, while the Hanoi water works extract more than 600,000 m³/day of groundwater from the Pleistocene aquifer. Detailed groundwater and sediment investigations were conducted at three locations exhibiting distinct geochemical conditions, i.e., i) high levels of dissolved arsenic (av. 121 µg/L) at the river bank, ii) low levels of dissolved arsenic (av. 21 µg/L) at the river bank and, iii) medium levels of dissolved arsenic (60 µg/L) in an area of buried peat and excessive groundwater abstraction. Seasonal fluctuations in water chemistry were studied over a time span of 14 months.
Sediment-bound arsenic (1.3–22 µg/g) is in a natural range. Arsenic correlates with iron (r²>0.8) with variation related to grain size. Sediment leaching experiments showed that arsenic can readily be mobilized at each of the three locations. Low levels of arsenic in groundwater (<10 µg/L) generally exhibit manganese reducing conditions, whereas elevated levels are caused by reductive dissolution under iron- and sulphate reducing conditions. Average arsenic concentrations in groundwater are twofold higher at the river bank than in the peat area. The lower levels of arsenic contamination in the peat area are likely controlled by the high abundance of iron present in both the aqueous and sediment phases. With median molar Fe/As ratios of 350 in water and 8700 in the sediments of the peat area, reduced iron possibly forms new mineral phases that resorb (or sequester) previously released arsenic to the sediment. Despite similar redox conditions, resorption is much less significant at the river bank (Fe/As(aq)=68, Fe/As(s)=4700), and hence, arsenic concentrations in groundwater reach considerably higher levels.
Drawdown of Holocene water to the Pleistocene aquifer in the peat area, caused by the pumping for the Hanoi water works, clearly promotes reducing conditions in Pleistocene groundwater. This demonstrates that excessive abstraction of water from deep wells, i.e., wells tapping water below the arsenic burdened depth, can cause a downward shift of iron-reducing conditions and concurrently mobilize arsenic along the way.
Vertical migration of reduced groundwater may also impact aquifers under natural hydrological conditions. Seepage of DOC-enriched groundwater derived from degradation of organic matter in the clayey sediments at the river bank was observed to enhance (and maintain) iron-reducing conditions in the aquifer where organic matter is scarce. Once the aquifer becomes reduced, arsenic is released from the aquifer solid-hosts but additionally derives from the arsenic-enriched groundwater seeping from the clay into the aquifer. This behaviour is an important mechanism for arsenic contamination in aquifers that might not necessarily contain enough organic matter in their sediments to induce reducing conditions independently.

Arsenic contamination of groundwater resources threatens the health of millions of people worldwide, particularly in the densely populated river deltas of Southeast Asia. Although many arsenic-affected areas have been identified in recent years, a systematic evaluation of vulnerable areas remains to be carried out. Here we present maps pinpointing areas at risk of groundwater arsenic concentrations exceeding 10 g l^-1. These maps were produced by combining geological and surface soil parameters in a logistic regression model, calibrated with 1,756 aggregated and geo-referenced groundwater data points from the Bengal, Red River and Mekong deltas. We show that Holocene deltaic and organic-rich surface sediments are key indicators for arsenic risk areas and that the combination of surface parameters is a successful approach to predict groundwater arsenic contamination. Predictions are in good agreement with the known spatial distribution of arsenic contamination, and further indicate elevated risks in Sumatra and Myanmar, where no groundwater studies exist.

Arsenic removal efficiencies of 43 household sand filters were studied in rural areas of the Red River Delta in Vietnam. Simultaneously, raw groundwater from the same households and additional 31 tubewells was sampled to investigate arsenic coprecipitation with hydrous ferric iron from solution, i.e., without contact to sand surfaces. From the groundwaters containing 10-382 μg/L As, 99%, 90%, and 71%, respectively. The concentration of dissolved iron in groundwater was the decisive factor for the removal of arsenic. Residual arsenic levels below 50 Ag/L were achieved by 90% of the studied sand filters, and 40% were even below 10 Ag/L. Fe/As ratios of >= 50 or >= 250 were required to ensure arsenic removal to levels below 50 or 10 Ag/L, respectively. Phosphate concentrations > 2.5 mg P/L slightly hampered the sand filter and coprecipitation efficiencies. Interestingly, the overall arsenic elimination was higher than predicted from model calculations based on sorption constants determined from coprecipitation experiments with artificial groundwater. This observation is assumed to result from As(III) oxidation involving Mn, microorganisms, and possibly dissolved organic matter present in the natural groundwaters. Clear evidence of lowered arsenic burden for people consuming sand-filtered water is demonstrated from hair analyses. The investigated sand filters proved to operate fast and robust for a broad range of groundwater composition and are thus also a viable option for mitigation in other arsenic affected regions. An estimation conducted for Bangladesh indicates that a median residual level of 25 Ag/L arsenic could be reached in 84% of the polluted groundwater. The easily observable removal of iron from the pumped water makes the effect of a sand filter immediately recognizable even to people who are not aware of the arsenic problem.

Arsenic-rich groundwater is currently being used as drinking water by millions of households in different parts of the world. The problem of arsenic intoxication by contaminated drinking water emerged in the past two decades, when surface water and groundwater from open dug wells, formerly used to cover the drinking water supply in rural areas of many tropical regions, were abandoned for groundwater pumped through small-scale tubewells. As documented, chronic arsenic exposure can lead to severe health problems, such as skin lesions, hyperkeratosis, melanosis, skin cancer and cancer of internal organs.

Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl⁻ or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl⁻ or B as a tracer in the model. Average As concentrations ± standard error in the methanogenic zone were 182 ± 23 μg L⁻¹ (n = 50%), 41 ± 6 μg L⁻¹ (n = 43%), and 61 ± 20 μg L⁻¹ (n = 24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO₄-reducing and the Fe-reducing zones, where averages were 23 ± 7 μg L⁻¹ (n = 27%, zone I), 14 ± 3 μg L⁻¹ (n = 48%, zone S) and 26 ± 9 μg L⁻¹ (n = 64%, zone S). These results suggest that a sufficient supply of inhibits the release of As to groundwater and that reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater.

This is the first publication on arsenic contamination of the Red River alluvial tract in the city of Hanoi and in the surrounding rural districts. Due to naturally occurring organic matter in the sediments, the groundwaters are anoxic and rich in iron. With an average arsenic concentration of 159 μg/L, the contamination levels varied from 1 to 3050 μg/L in rural groundwater samples from private smallscale tubewells. In a highly affected rural area, the groundwater used directly as drinking water had an average concentration of 430 μg/L. Analysis of raw groundwater pumped from the lower aquifer for the Hanoi water supply yielded arsenic levels of 240-320 μg/L in three of eight treatment plants and 37-82 μg/L in another five plants. Aeration and sand filtration that are applied in the treatment plants for iron removal lowered the arsenic concentrations to levels of 25-91 μg/L, but 50% remained above the Vietnamese Standard of 50 μg/L. Extracts of sediment samples from five bore cores showed a correlation of arsenic and iron contents (r² = 0.700, n = 64). The arsenic in the sediments may be associated with iron oxyhydroxides and released to the groundwater by reductive dissolution of iron. Oxidation of sulfide phases could also release arsenic to the groundwater, but sulfur concentrations in sediments were below 1 mg/g. The high arsenic concentrations found in the tubewells (48% above 50 μg/L and 20% above 150 μg/L) indicate that several million people consuming untreated groundwater might be at a considerable risk of chronic arsenic poisoning.

Arsenic enrichment of groundwater in the Red River (Song Hong) delta in Vietnam was discovered in 1998. Several studies performed in this area found concentrations of As exceeding the WHO-guideline of 10 μg/L. This study focuses on an area south of Hanoi city, Nam Du, where a new well field came into operation in 2004. The new well field is situated on the bank along the Red River in order to facilitate induced infiltration. The Nam Du area receives surface water with a high load of nutrients and organic matter from the Hanoi sewage system, and is subject to recently increased groundwater extraction from the Pleistocene aquifer system. The objective of the study was (1) to assess the situation in the Nam Du area by mapping the distribution of As, (2) to identify possible sources of As in the groundwater and (3) to investigate the mobilisation processes releasing As into the groundwater. Two main field campaigns were carried out, in 2006 and 2007, both during the dry season. Groundwater and surface water levels were measured and water- and sediment samples were collected. The water in the Pleistocene aquifer shows the same water-level variations as the Red River at a distance of 2.5 km from the riverbank, while the Holocene aquifer heads are recharged by surface water ponds and show less seasonal variation. The concentration of As in the groundwater in Nam Du exceeded the WHO provisional guideline value at all sampled locations. The main conclusions are summarised as (i) the distribution of As is highly variable but the zones with the highest concentrations of As are near the Red River in the Holocene aquifer and just down gradient from this in the Pleistocene aquifer, (ii) the sediments within the aquifers are considered to be the source of the As, where the Holocene aquifer is believed to act as the main source of As into the Pleistocene aquifer as reduced groundwater containing As from the Holocene aquifer is flowing downwards due to the downward gradient, and (iii) two different processes appear to take part in the mobilisation process. In the Holocene aquifer, reductive dissolution of FeOOH and the release of adsorbed As appear to be the main mobilisation processes. In the Pleistocene, however, mobilisation of adsorbed As due to competition from HCO₃^– ions for surface sites on FeOOH may be a major mechanism of As mobilisation. It is suggested that the drinking water supplier undertakes the following actions to ensure acceptable levels of As in the treated drinking water: (a) to implement a long-term monitoring program, (b) implement alternative treatment technologies; and (c) to possibly consider an alternative drinking water source.

The spatial variability of As concentrations in aquifers of the Red River Delta, Vietnam, was studied in the vicinity of Hanoi. Two sites, only 700 m apart but with very different As concentrations in groundwater (site L: <10 μg/L vs. site H: 170–600 μg/L) in the 20–50 m depth range, were characterized with respect to sediment geochemistry and mineralogy as well as hydrochemistry. Sequential extractions of the sediment were carried out in order to understand why As is released to groundwater at one site and not the other. No major differences were observed in the bulk mineralogy and geochemistry of the sediment, with the exception of the redox state of Fe oxyhydroxides inferred from sediment colour and diffuse spectral reflectance. At site H most of the As in the sediment was adsorbed to grey sands of mixed Fe(II/III) valence whereas at site L As was more strongly bound to orange-brown Fe(III) oxides. Higher dissolved Fe and low dissolved S concentrations in groundwater at site H (~14 mg Fe/L, <0.3 mg S/L) suggest more strongly reducing conditions compared to site L (1–2 mg Fe/L, <3.8 mg S/L). High concentrations of NH₄⁺ (~10 mg/L), HCO₃^– (500 mg/L) and dissolved P (600 mg/L), in addition to elevated As at site H are consistent with a release coupled to microbially induced reductive dissolution of Fe oxyhydroxides. Other processes such as precipitation of siderite and vivianite, which are strongly supersaturated at site H, or the formation of amorphous Fe(II)/As(III) phases and Fe sulfides, may also influence the partitioning of As between groundwater and aquifer sands.
The origin of the redox contrast between the two sites is presently unclear. Peat was observed at site L, but it was embedded within a thick clayey silt layer. At site H, instead, organic rich layers were only separated from the underlying aquifer by thin silt layers. Leaching of organic matter from this source could cause reducing conditions and therefore potentially be related to particularly high concentrations of dissolved NH₄⁺, HCO₃^–, P and DOC in the portion of the aquifer where groundwater As concentrations are also elevated.

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.

A long-term education and research partnership has been established between the Swiss Federal Institute for Environmental Science and Technology (EAWAG) and two university institutes in Hanoi. Here we give a summary report on environmental analytical investigations conducted in cooperation with the Hanoi University of Science focusing on (i) arsenic contamination in ground and drinking water, (ii) volatile organic compounds (VOCs) including disinfection by-products and chlorination practice in drinking water, (iii) analysis and occurrence of organophosphorus pesticides in rice growing areas, and (iv) chlorinated phenols and other chlorinated pollutants in wastewater of a pulp and paper mill. Arsenic concentrations ranged from 1 to 3050 μg/l (average 159 μg/l) in groundwater from the city of Hanoi and surrounding rural areas. The high arsenic levels indicate that several million people consuming untreated groundwater might be at a considerable risk of chronic arsenic poisoning. Water produced by the Hanoi waterworks is partly affected by arsenic, but VOCs and disinfection by-products were below international guideline limits. However, the current chlorination practice was found to be critical regarding water quality issues. Chlorinated pollutants were particularly abundant in wastewater effluents of pulp bleaching, suggesting that point-of-source treatment options should be implemented. The high pesticide concentrations measured in rice fields (>500 μg/l) were rapidly flushed into ambient surface waters, where beneficial organisms could be affected.

The occurrence, temporal trend, sources and toxicity of PCBs and organochlorine pesticides were investigated in sediment samples from the sewer system of Hanoi City, including the rivers Nhue, To Lich, Lu, Set, Kim Nguu and the Yen So Lake. In general, the concentrations of the pollutants followed the order DDTs > PCBs > HCHs (β-HCH) > HCB. However, the pollution pattern was different for the DDTs and PCBs when the sampling locations were individually evaluated. The concentrations of the DDTs, PCBs, HCHs, and HCB ranged from 4.4 to 1100, 1.3 to 384, <0.2 to 36 and <0.2 to 22 ng/g d.w., respectively. These levels are higher than at any other location in Vietnam. Compared to measurements from 1997, the DDTs, PCBs, β-HCH and HCB levels show an increasing trend with DDT/DDE ratios, indicating very recent inputs into the environment although these persistent compounds are banned in Vietnam since 1995.

The occurrence and the fate of trihalomethanes (THMs) in the water supply system of Hanoi City, Vietnam was investigated from 1998 to 2001. The chlorination efficiency, THM speciation, and, THM formation potential (THMFP) was determined in the water works and in tap water. With regard to THM formation, three types of groundwater resources were identified: (I) high bromide, (II) low bromide, and (III) high bromide combined with high ammonia and high dissolved organic carbon (DOC) concentrations. Under typical treatment conditions (total chlorine residual 0.5-0.8mg/L), the total THM formation was always below WHO, EU, and USEPA drinking water standards and decreased in the order type I>type II>type III, although the THMFP was >400μg/L for type III water. The speciation showed >80% of bromo-THMs in type I water due to the noticeable high bromide level (≤140μg/L). In type II water, the bromo-THMs still accounted for some 40% although the bromide concentration is significantly lower (≤30μg/L). In contrast, only traces of bromo-THMs were formed (∼5%) in type III water, despite bromide levels were high (≤240μg/L). This observation could be explained by competition kinetics of chlorine reacting with ammonia and bromide. Based on chlorine exposure (CT) estimations, it was concluded that the current chlorination practice for type I and II waters is sufficient for ≥2-log inactivation of Giardia lamblia cysts. However, in type III water the applied chlorine is masked as chloramine with a much lower disinfection efficiency. In addition to high levels of ammonia, type III groundwater is also contaminated by arsenic that is not satisfactory removed during treatment. N-nitrosodimethylamine, a potential carcinogen suspected to be formed during chloramination processes, was below the detection limit of 0.02μg/L in type III water.

Occurrence and behavior of fluoroquinolone antibacterial agents (FQs) were investigated in hospital wastewaters in Hanoi, Vietnam. Hospital wastewater in Hanoi is usually not treated and this untreated wastewater is directly discharged into one of the wastewater channels of the city and eventually reaches the ambient aquatic environment. The concentrations of the FQs, ciprofloxacin (CIP) and norfloxacin (NOR) in six hospital wastewaters ranged from 1.1 to 44 and from 0.9 to 17 μg l^–1, respectively. Total FQ loads to the city sewage system varied from 0.3 to 14 g d^–1. Additionally, the mass flows of CIP and NOR were investigated in the aqueous compartment in a small wastewater treatment facility of one hospital. The results showed that the FQ removal from the wastewater stream was between 80 and 85%, probably due to sorption on sewage sludge. Simultaneously, the numbers of Escherichia coli (E. coli) were measured and their resistance against CIP and NOR was evaluated by determining the minimum inhibitory concentration. Biological treatment lead to a 100-fold reduction in the number of E. coli but still more than a thousand E. coli colonies per 100 ml of wastewater effluent reached the receiving water. The highest resistance was found in E. coli strains of raw wastewater and the lowest in isolates of treated wastewater effluent. Thus, wastewater treatment is an efficient barrier to decrease the residual FQ levels and the number of resistant bacteria entering ambient waters. Due to the lack of municipal wastewater treatment plants, the onsite treatment of hospital wastewater before discharging into municipal sewers should be considered as a viable option and consequently implemented.

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 <1 g L^{− 1} 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^{− 1}, with 37% of the studied wells exceeding the WHO guidelines of 10 µg L^{− 1} 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 ~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.

Arsenic contamination of groundwater resources threatens the health of millions of people worldwide, particularly in the densely populated river deltas of Southeast Asia. Although many arsenic-affected areas have been identified in recent years, a systematic evaluation of vulnerable areas remains to be carried out. Here we present maps pinpointing areas at risk of groundwater arsenic concentrations exceeding 10 g l^-1. These maps were produced by combining geological and surface soil parameters in a logistic regression model, calibrated with 1,756 aggregated and geo-referenced groundwater data points from the Bengal, Red River and Mekong deltas. We show that Holocene deltaic and organic-rich surface sediments are key indicators for arsenic risk areas and that the combination of surface parameters is a successful approach to predict groundwater arsenic contamination. Predictions are in good agreement with the known spatial distribution of arsenic contamination, and further indicate elevated risks in Sumatra and Myanmar, where no groundwater studies exist.

Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl⁻ or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl⁻ or B as a tracer in the model. Average As concentrations ± standard error in the methanogenic zone were 182 ± 23 μg L⁻¹ (n = 50%), 41 ± 6 μg L⁻¹ (n = 43%), and 61 ± 20 μg L⁻¹ (n = 24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO₄-reducing and the Fe-reducing zones, where averages were 23 ± 7 μg L⁻¹ (n = 27%, zone I), 14 ± 3 μg L⁻¹ (n = 48%, zone S) and 26 ± 9 μg L⁻¹ (n = 64%, zone S). These results suggest that a sufficient supply of inhibits the release of As to groundwater and that reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater.

Arsenic contamination of groundwater has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water needs. We conducted a comprehensive groundwater survey in the Mekong River floodplain, comprising an area of 3700 km² (131 samples, 30 parameters). Seasonal fluctuations were also studied. Arsenic ranged from 1 to 1340 μg L^-1 (average 163 μg L^-1), with 48% exceeding 10 μg L^-1. Elevated manganese levels (57% >0.4 mg L^-1) are posing an additional health threat to the 1.2 million people living in this area. With 350 people km^-2 potentially exposed to chronic arsenic poisoning, the magnitude is similar to that of Bangladesh (200 km^-2). Elevated arsenic levels are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers (Kandal Province). Arsenic in this province averaged 233 μg L^-1 (median 100 μg L^-1), while concentrations to the west and east of the rivers were <10 μg L^-1. Arsenic release from Holocene sediments between the rivers is most likely caused by reductive dissolution of metal oxides. Regions exhibiting low and elevated arsenic levels are co-incident with the present low relief topography featuring gently increasing elevation to the west and east of a shallow valley - understood as a relict of pre-Holocene topography. The full georeferenced database of groundwater analysis is provided as Supporting Information.

The As concentration in shallow groundwater in Cambodia was estimated using 1329 georeferenced water samples collected during the period 1999–2004 from wells between 16–100 m depth. Arsenic concentrations were estimated using block regression-kriging on the log transformed As measurements. Auxiliary raster maps (DEM-parameters, remote sensing images and geology) were converted to 16 principal components that were used to explain the distribution of As over the study area. The regression-kriging model was validated using an external set of 276 samples, and the results were compared to those obtained by ordinary block kriging. The regression analysis revealed that there is a good correlation between topographic environmental variables and the content of As in groundwater. This result is broadly consistent with the findings of previous studies and is not unexpected given models of microbial mediated As mobilization in recent low lying sediments. Kândal, Prey Vêng and Kâmpóng Cham are the provinces with the highest potential As hazard, indicating the requirement for development and implementation of policy control measures. The regression-kriging model explained 48% of the variability in the validation set. However, the model does not show good results for the prediction of high As concentration. This points to the existence of local environmental factors, not captured by this model, that highly influence the mobilization of As in groundwater. Even if the results of the validation of regression-kriging and ordinary kriging are similar, the regression kriging approach provides a more realistic description of the distribution of As since it also captures the large-scale variation of As in the study area.

Groundwater resources in the Pannonian Basin (Hungary, Romania, Croatia and Serbia) are known to contain elevated naturally occurring As. Published estimates suggest nearly 500,000 people are exposed to levels greater than the EU maximum admissible concentration of 10μg/L in their drinking water, making it the largest area so affected in Europe. In this study, a variety of groundwaters were collected from Romania and Hungary to elucidate the general geochemistry and identify processes controlling As behaviour. Concentrations ranged from <0.5 to 240μg/L As(tot), with As predominantly in the reduced As(III) form. Using cluster analysis, four main groups of water were identified. Two groups (1 and 2) showed characteristics of water originating from reducing aquifers of the area with both groups having similar ranges of Fe concentrations, indicating that Fe-reduction occurs in both groups. However, As levels and other redox characteristics were very different. Group 1, indicative of waters dominated by methanogenesis contained high As levels (23-208μg/L, mean 123μg/L), with group 2 indicative of waters dominated by SO42--reduction containing low As levels (<0.5-58μg/L, mean 11.5μg/L). The remaining two groups were influenced either by (i) geothermal and saline or (ii) surface contamination and rain water inputs. Near absence of As in these groups, combined with positive correlations between δ⁷Li (an indicator of geothermal inputs) and As(tot) in geothermal/saline influenced waters indicate that elevated As is not from an external input, but is released due to an in-aquifer process. Geochemical reasoning, therefore, implies As mobilisation is controlled by redox processes, most likely microbially mediated reductive dissolution of As bearing Fe-oxides, known to occur in sediments from the area. More important is an overlying retention mechanism determined by the presence or absence of SO₄^2-. Ongoing SO₄^2--reduction will release S^2-, removing As from solution either by the formation of As-sulfides, or from sorption onto Fe-sulfide phases. In methanogenic waters, As released by reductive dissolution is not removed from solution and can rise to the high levels observed. Levels of organic C are thought to be the ultimate control on the redox conditions in these 2 groups. High levels of organic C (as found in group 1) would quickly exhaust any SO₄^2- present in the waters, driving the system to methanogenesis and subsequent high levels of As. Group 2 has much lower concentrations of organic C and so SO₄^2- is not exhausted. Therefore, As levels in waters of the Pannonian Basin are controlled not by release but by retention mechanisms, ultimately controlled by levels of TOC and SO₄^2- in the waters.δD and δ¹⁸O analysis showed that groundwaters containing elevated As dated mostly from the last ice-age, and are sourced from Late Pliocene to Quaternary aquifers. The importance of TOC and retention capabilities of SO₄^2--reduction have only previously been suggested for recent (Holocene) sediments and groundwater, most notably those in SE Asia as these are the most likely to contain the right combination of factors to drive the system to a redox situation leading to high aqueous As concentrations. In contrast, it is shown here that a much older system containing As bearing Fe-oxides, also has the potential to produce elevated levels of As if the TOC is suitable for the microbial population to drive the system to the correct redox situation and SO₄^2- is either absent or wholly consumed.

High concentrations of As in groundwater frequently occur throughout the world. The dissolved concentration, however, is not necessarily determined by the amount of As in the ambient sediment but rather by the partitioning of As between different minerals and the type of fixation. Sequential extractions are commonly applied to determine associations and binding forms of As in sediments. Due to the operational nature of the extracted fractions, however, the results do not provide insight into how and where precisely As is bound within mineral grains and no information about elemental associations or involved mineral phases can be gained. Furthermore, little is known about possible geochemical alterations that actually occur within a single grain during sequential extraction. Therefore, micro-synchrotron X-ray fluorescence analysis was applied to study the micro-scale distribution of As and other elements in single sediment grains.
Arsenic was found to be mainly enriched in Fe oxy-hydroxide coatings along with other heavy metals resulting in high correlations. Phosphate leached 34–66% of As from the studied grains. The release of As in this leaching step was accompanied by the disappearance of correlations between As and Fe as well as by a higher Fe/As ratio compared to untreated samples. During the Fe-leaching step the coatings were largely dissolved leading to much lower concentrations of As and Fe. The correlation between As and Fe was preserved only in association with K, indicating the presence of both elements in silicate structures.
Several distinctive features were observed such as the release of Fe, Mn and Cr during phosphate leaching as well as the lowering of mean K concentrations due to the Fe-leaching which indicates that not only target mineral phases were dissolved in these extraction steps. The importance of re-precipitation processes during sequential extraction was indicated by a consistently observed increase of the Fe/As ratio from the untreated to the Fe-leached samples.

Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl⁻ or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl⁻ or B as a tracer in the model. Average As concentrations ± standard error in the methanogenic zone were 182 ± 23 μg L⁻¹ (n = 50%), 41 ± 6 μg L⁻¹ (n = 43%), and 61 ± 20 μg L⁻¹ (n = 24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO₄-reducing and the Fe-reducing zones, where averages were 23 ± 7 μg L⁻¹ (n = 27%, zone I), 14 ± 3 μg L⁻¹ (n = 48%, zone S) and 26 ± 9 μg L⁻¹ (n = 64%, zone S). These results suggest that a sufficient supply of inhibits the release of As to groundwater and that reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater.

Arsenic contamination of groundwater resources threatens the health of millions of people worldwide, particularly in the densely populated river deltas of Southeast Asia. Although many arsenic-affected areas have been identified in recent years, a systematic evaluation of vulnerable areas remains to be carried out. Here we present maps pinpointing areas at risk of groundwater arsenic concentrations exceeding 10 g l^-1. These maps were produced by combining geological and surface soil parameters in a logistic regression model, calibrated with 1,756 aggregated and geo-referenced groundwater data points from the Bengal, Red River and Mekong deltas. We show that Holocene deltaic and organic-rich surface sediments are key indicators for arsenic risk areas and that the combination of surface parameters is a successful approach to predict groundwater arsenic contamination. Predictions are in good agreement with the known spatial distribution of arsenic contamination, and further indicate elevated risks in Sumatra and Myanmar, where no groundwater studies exist.

Contamination of groundwaters with geogenic arsenic poses a major health risk to millions of people. Although the main geochemical mechanisms of arsenic mobilization are well understood, the worldwide scale of affected regions is still unknown. In this study we used a large database of measured arsenic concentration in groundwaters (around 20,000 data points) from around the world as well as digital maps of physical characteristics such as soil, geology, climate, and elevation to model probability maps of global arsenic contamination. A novel rule-based statistical procedure was used to combine the physical data and expert knowledge to delineate two process regions for arsenic mobilization: “reducing” and “high-pH/oxidizing”. Arsenic concentrations were modeled in each region using regression analysis and adaptive neuro-fuzzy inferencing followed by Latin hypercube sampling for uncertainty propagation to produce probability maps. The derived global arsenic models could benefit from more accurate geologic information and aquifer chemical/physical information. Using some proxy surface information, however, the models explained 77% of arsenic variation in reducing regions and 68% of arsenic variation in high-pH/oxidizing regions. The probability maps based on the above models correspond well with the known contaminated regions around the world and delineate new untested areas that have a high probability of arsenic contamination. Notable among these regions are South East and North West of China in Asia, Central Australia, New Zealand, Northern Afghanistan, and Northern Mali and Zambia in Africa.

Groundwater conditions in the lowlands of Sumatra, where peat swamps are the dominant landscape, were investigated. Based on topography, soil and geological surface properties, this large area (about 100,000 km²) is vulnerable to groundwater As enrichment under reducing aquifer conditions. The reconnaissance groundwater survey was conducted in the province of South Sumatra, covering both presumed low- and high-risk areas of As enrichment. Five distinct types of groundwater were recognized, reflecting a variety of geological sources and chemical conditions which are understood to be typical for the whole east coast of Sumatra. Groundwater collected from tubewells in the youngest (Holocene) swamp deposits had elevated As concentrations (>10 μg L⁻¹) with a maximum of 65 μg L⁻¹. Other elements exceeding the WHO drinking water guideline values include B, Mn, and Se. In contrast to large deltas of continental South and SE Asia, significantly lower sediment loads are transported by the rivers of Sumatra. The organic-rich Holocene sediments are hence relatively thin. Tubewells tapping the oldest geological formations of the study area (middle Miocene to Pliocene) have a broad range of redox conditions reflecting variations in aquifer geochemistry. This group is generally characterized by alkaline pH conditions and high contents of Na, B, Se, and Sr. Oxic groundwaters were found in regions elevated above 20 m a.s.l. and are characterized by low concentrations of dissolved solids and acidic pH values (average 5.1). To date, groundwater data for the increasingly populated island of Sumatra are non-existent in the international literature and this study thus provides a basis for future in-depth groundwater studies. The complete georeferenced database of groundwater analysis is provided as supplementary material.

The fate of arsenic in the aquatic environment is influenced by dissolved natural organic matter (DOM). Using an equilibrium dialysis method, conditional distribution coefficients (D_om) for As(III) and As(V) binding onto two commercial humic acids were determined at environmentally relevant As/dissolved organic carbon (DOC) ratios and as a function of pH. At all pH values, As(V) was more strongly bound than As(III). Maximum binding was observed around pH 7, which is consistent with H⁺ competition for binding sites at low pH values and OH^- competition for the arsenic center at high pH. For both oxidation states, D_om values increased with decreasing As/DOC ratios. D_om values were fitted as a function of the As/DOC ratio for As(III) and As(V). Compared to the aquatic humic acid, the terrestrial humic acid had a higher affinity for arsenic binding with 1.5-3 times higher D_om values under the same conditions. Al³⁺ in excess to arsenic successfully competed for strong binding sites at low As/DOC ratios. Under environmentally relevant conditions, about 10% of total As(V) may be bound to DOM, whereas >10% of As(III) is bound to DOM at very low As/DOC ratios only. Binding of arsenic to DOM should be considered in natural systems.

Kinetics and mechanisms of As(III) oxidation by free available chlorine (FAC-the sum of HOCl and OCl^-), ozone (O₃), and monochloramine (NH₂Cl) were investigated in buffered reagent solutions. Each reaction was found to be first order in oxidant and in As(III), with 1:1 stoichiometry. FAC-As(III) and O₃-As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, k″_app, of 2.6 (±0.1) × 10⁵ M ^-1 s^-1 and 1.5 (±0.1) × 10⁶ M ^-1 s^-1 at pH 7, whereas the NH₂Cl-As(III) reaction was relatively slow (k″_app = 4.3 (±1.7) × 10^-1 M^-1 s^-1 at pH 7). Experiments conducted in real water samples spiked with 50 μg/L As(III) (6.7 × 10 ^-7 M) showed that a 0.1 mg/L Cl₂ (1.4 × 10 ^-6 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 μg/L As(III) within 10 s of oxidant addition to waters containing negligible NH₃ concentrations and DOC concentrations <2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 × 10^-5 M) as NH ₃, >75% As(III) oxidation could be achieved within 10 s of dosing 1-2 mg/L Cl₂ (1.4-2.8 × 10^-5 M) as FAC. As(III) residuals remaining in NH₃-containing waters 10 s after dosing FAC were slowly oxidized (t_1/2 ≥ 4 h) in the presence of NH ₂Cl formed by the FAC-NH₃ reaction. Ozonation was sufficient to yield >99% depletion of 50 μg/L As(III) within 10 s of dosing 0.25 mg/L O₃ (5.2 × 10^-6 M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O₃ (1.7 × 10^-5 M) was sufficient for a water containing 5.4 mg-C/L of DOC. NH₃ had negligible effect on the efficiency of As(III) oxidation by O₃, due to the slow kinetics of the O₃-NH₃ reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination and ozonation of real waters were accurately modeled using the rate constants determined in this investigation.

In this study, we report the first ever large-scale environmental validation of a microbial reporter-based test to measure arsenic concentrations in natural water resources. A bioluminescence-producing arsenic-inducible bacterium based on Escherichia coli was used as the reporter organism. Specific protocols were developed with the goal to avoid the negative influence of iron in groundwater on arsenic availability to the bioreporter cells. A total of 194 groundwater samples were collected in the Red River and Mekong River Delta regions of Vietnam and were analyzed both by atomic absorption spectroscopy (AAS) and by the arsenic bioreporter protocol. The bacterial cells performed well at and above arsenic concentrations in groundwater of 7 μg/L, with an almost linearly proportional increase of the bioluminescence signal between 10 and 100 μg As/L (r² = 0.997). Comparisons between AAS and arsenic bioreporter determinations gave an overall average of 8.0% false negative and 2.4% false positive identifications for the bioreporter prediction at the WHO recommended acceptable arsenic concentration of 10 μg/L, which is far better than the performance of chemical field test kits. Because of the ease of the measurement protocol and the low application cost, the microbiological arsenic test has a great potential in large screening campaigns in Asia and in other areas suffering from arsenic pollution in groundwater resources.