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Statistical Modeling of Global Geogenic Arsenic Contamination in Groundwater

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Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
* Corresponding author e-mail: [email protected]; phone: +41 1 823-5465 ; fax: +41 1 823-5375.
Cite this: Environ. Sci. Technol. 2008, 42, 10, 3669–3675
Publication Date (Web):April 16, 2008
https://doi.org/10.1021/es702859e

Copyright © 2008 American Chemical Society. This publication is licensed under these Terms of Use.

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Abstract

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.

Synopsis

Probability maps of groundwater arsenic contaminaiton modeled on a global scale using statistical analysis based on measured arsenic data, physical variables, and known geochemical processes are presented.

Introduction

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In many regions around the world, arsenic-rich groundwater is used for drinking water and irrigation purposes (1, 2). Long-term exposure to arsenic can affect human health and is considered to be a significant environmental cause of cancer (2, 3). Exposure to arsenic may also cause skin pigmentation, hyperkeratosis, and cardiovascular disease, and may affect the mental development of children, among other possible adverse effects (1, 4, 5). Due to its high toxicity at low concentrations (in the µg L−1 range), arsenic is a cause for concern in many regions of the world (6-9). However, the worldwide scale of affected regions is still unknown because groundwater quality has only been determined in certain regions. Hence, a global map, showing the areas with high probability of contamination is of great value for civil authorities and aid agencies in preventing the increase in consumption of arsenic-contaminated waters.
Solid-phase arsenic is widely distributed in nature and can be mobilized in groundwaters through a combination of natural processes and/or anthropogenic activities (6-11). At the global scale, natural processes are predominantly responsible for the elevated arsenic concentrations, although anthropogenic sources of arsenic, such as leaching from mine tailings, can be very important on the local scale. Regardless of the source of arsenic, its solubility is mostly controlled by pH and Eh (6), which in turn are governed by geology, climate, drainage, and topography (7, 11, 12). Two important chemical processes lead to arsenic mobilization through desorption and dissolution. These processes take place under two different physicochemical conditions: first, highly reducing aquifers where arsenic is predominantly present in its reduced state As(III), and second, high-pH aquifers where arsenic is relatively soluble in its oxidized state As(V) (6, 10). We, henceforth, refer to regions with these conditions as “reducing” and “high-pH/oxidizing” process regions, respectively.
Reducing aquatic environments, where arsenic is most probably released by reductive dissolution (6), are typically poorly drained and rich in organic matter content making them conducive to high microbial activity and, hence, low oxygen concentrations (13). We acknowledge that in reducing regions with higher sulfate concentrations, dissolved arsenic could be low due to microbial sulfate reduction and subsequent precipitation of arsenic sulfides (14). Oxidizing environments occur in arid and semiarid regions, again with poor drainage conditions (e.g., closed hydrologic basins), which may result in high evaporation rates leading to high salinity and high pH (6, 7). In such conditions anions including arsenate (AsO43−) are poorly sorbed to mineral surfaces and are thus quite soluble and mobile (6).
This study aims to provide a global overview of arsenic-affected groundwaters. We use the digitally available information on the global scale such as geology, soil, climate, and topographic data to (1) delineate the world into reducing and high-pH/oxidizing regions, (2) model arsenic concentration in each region, and (3) produce probability maps for the occurrence of arsenic in concentrations greater than the WHO guideline value of 10 µg L−1 (15). For these purposes the existing geochemical knowledge of the arsenic-releasing processes is combined with statistical methods using fuzzy logic and fuzzy neural networks. The outcomes are predictive models, which were tested and subsequently used to develop global probability maps for groundwater arsenic contamination.

Materials and Methods

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Database Compilation

We compiled around 20,000 data points of measured arsenic concentrations in groundwaters from around the world and in addition, natural factors related to climate, geology, hydrology, soil properties, land use, and elevation on a global scale from various Internet sources (see Supporting Information, Tables S1 and S2). The latter data were brought in the same projection in a GIS environment (ArcGIS, ver. 9.1) and were used as proxies to model the subsurface conditions.
Arsenic concentrations are point measurements within vertical depths of the wells of 10−100 m, while the other information have coarser spatial resolution being generally greater than 30 arc seconds (∼1 km at the equator). The point measurements were aggregated vertically and horizontally to the same spatial resolution as the geology map. The geometric mean of the arsenic measurements was used to aggregate the arsenic data falling in the same pixels of the geological map units. Using this approach the number of arsenic data points was decreased to about 6000 pixel-based values. Proxy variables, as listed in Table 1, were deduced from the above-mentioned natural factors and used to model “reducing” and “high-pH/oxidizing” regions as discussed below.
Table 1. Definition of Relevant Variables in the Global Databasea
variable definition-unit type-format resolution
arsenic concentration (µg L−1) points  
elevation (m) continuous raster 30 (arc second)
slope (degree) continuous raster 30 (arc second)
geology age (million years) continuous raster 5 arc minute
ET evapotranspiration (mm year−1) continuous raster 0.5 degree
P precipitation (mm year−1) continuous raster 0.5 degree
ET/P   continuous raster 0.5 degree
runoff (mm year−1) continuous raster 0.5 degree
T temperature (°C) continuous raster 0.5 degree
irrigation irrigated areas (%) continuous raster 5 arc minute
Sol_CbN1 topsoil C/N ratio continuous raster 5 arc minute
clay1 topsoil clay content (%) continuous raster 5 arc minute
silt1 topsoil silt content (%) continuous raster 5 arc minute
sand1 topsoil sand content (%) continuous raster 5 arc minute
Sol_CbN2 subsoil C/N ratio continuous raster 5 arc minute
clay2 subsoil clay content (%) continuous raster 5 arc minute
silt2 subsoil silt content (%) continuous raster 5 arc minute
sand2 subsoil sand content (%) continuous raster 5 arc minute
CEC_S subsoil cation exchange capacity ranked raster 1:5000000
Drain_Code soil drainage code ranked raster 1:5000000
N_S subsoil nitrogen content ranked raster 1:5000000
OC_S subsoil organic carbon content ranked raster 1:5000000
pH_S subsoil pH ranked raster 1:5000000
Dist_Volc distance from volcanoes continuous raster 0.5 degree
Dist_Volc2 distance from volcanic rocks continuous raster 0.5 degree
Dist_Riv distance from rivers continuous raster 0.5 degree
a

CEC ranks from 10 (<20 meq/100 g clay) to 43 (>100 meq/100 g clay); Drain_Code ranks from 10 (extremely drained) to 87 (very poorly drained) (seeTable S3); N_S ranks from 10 (<0.02%) to 53 (>0.5%); OC_S ranks from 10 (<0.2%) to 54 (> 2%) (see Table S4); pH_S ranks from 10 (<4.5) to 54 (>8.5) (see Table S5). Bold variables were used as delineating variables. Topsoil refers to a depth of 0−30 cm, while subsoil is from 30 to 100 cm.

Rule Development and Process Region Delineation

In order to separately model the two conditions defined as “reducing” and “high-pH/oxidizing”, it was necessary to define regions in which either of these conditions were likely to prevail. Details of the delineation procedure are given in the Supporting Information.
In the first step, we defined what we call “delineating variables” that represent the critical conditions for identifying the type of process regions, using geochemical knowledge and statistical analysis. The information gained from geochemical expertise and literature review (6-11) is given in Table 2. Every variable and combination of all variables was statistically checked using supervised clustering and regression analysis for their significance in delineating process regions. The most significant variables, e.g., ratio of evapotranspiration over precipitation, drainage conditions, topographic slope, soil organic carbon content, and soil pH (Table 1) were used to develop rules to delineate different process regions.
Table 2. Characteristics of Reducing and Oxidizing Regions Based on Expert Opinions (4-13)
variablea reducing oxidizing
ET/P <1 >1
drainage condition imperfect to poor imperfect to poor
hydrologic basin deltas closed
slope flat flat
organic carbon high  
salinity low high
temperature high  
pH   high
geology young sediment young sediment
a

ET = evaptranspiration, P = precipitation.

In the second step, the qualifiers such as “high”, “low”, or “poor” (Table 2) were handled as fuzzy sets in a fuzzy inference system (FIS). FIS uses membership functions, fuzzy logic operators, and if−then rules to map a given input to an output (16-19). Fuzzy sets are sets whose elements have degrees of membership defined with membership functions valued in the real unit interval [0,1]. Rules were defined by continuous fuzzy sets for each of the delineating variables (ET/P < 1), (poor drainage condition), (high organic carbon), and (high pH). These sets were then used to calculate the maps of membership degree for each rule in the world. The overall membership for the combined set of rules is that of the rule that has the minimum membership value. This then determines the “possibility” of a pixel (10 km × 10 km) belonging to a reducing or a high-pH/oxidizing region. Using the developed FIS, we then constructed possibility maps of reducing and high-pH/oxidizing regions (see details of the procedure in Supporting Information).

Modeling Arsenic in each Process Region

In the third step, to develop a predictive model for arsenic in each process region, the following procedures were carried out: (i) the existing data set was split by stratified random sampling into two subsets for model training (85% of data) and model testing in each region, (ii) stepwise regression was used to identify the significant variables including those used to delineate process regions (see Table 1), and (iii) Adaptive Neuro-Fuzzy Inference System (ANFIS) was applied to relax the linearity assumption of the stepwise regression (see details of ANFIS in Supporting Information).
In the fourth and final step, the ANFIS models were linked to a Latin hypercube sampling (20) method to propagate the uncertainty of the model parameters on the results. This allowed the construction of the cumulative distribution function of arsenic concentrations in each pixel and, consequently, calculation of the probability of arsenic concentration exceeding the WHO guideline (15) at each pixel.

Results and Discussion

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Process Region Delineation

Based on the statistical analysis the following rules were found to delineate the process regions as shown in Figure S3.
• If (ET/P < 1) then a region is considered humid, else the region is arid. Where ET is evapotranspiration (mm y−1), and P is precipitation (mm y−1).
• If (Drain_Code ≥ 60) then drainage condition is poor, else drainage condition is good. Where Drain_Code is the drainage condition ranging from 10 (extremely drained) to 87 (very poorly drained) (see Table S3). In this rule, poor drainage includes mostly Histosols, Gleysols, and Fluvisols (according to FAO soil Map (21)) which implies gentle slopes and the presence of recent (Quaternary) sediments.
• If (OC_S ≥ 30) then subsoil (30−100 cm) organic carbon content is high, else it is low. Where OC_S is the organic carbon content of the subsoil ranging from 10 (<0.2 kg m−3) to 54 (>2 kg m−3) (see Table S4).
• If (pH_S ≥ 40) then pH is high, else it is low. Where pH_S is the soil pH rank where ≥40 is greater than 7.2 (see Table S5).
Based on the above, the process regions were defined as follows:
• If (region is humid) and (drainage condition is poor) and (subsoil organic content is high) then the condition is considered as reducing.
• If (region is arid) and (drainage condition is poor) and (subsoil pH is high) then the condition is considered as high-pH/oxidizing.

Influencing Variables and Arsenic Model Results

Table 3 shows the relative significance of the variables in the arsenic model. The occurrence of arsenic under reducing aqueous conditions is most closely correlated to climatic parameters (ET/P and T), geological parameters (Drain_Code, Dist_Volc, Dist_Volc2), and drainage conditions (Drain_Code), followed by soil parameters (Silt, pH_S, and Sol_CbN2) and proximity to surface water (Dist_Riv). The occurrence of arsenic under high-pH/oxidizing aqueous conditions is most closely correlated to soil parameters (clay and silt), and drainage condition (Drain_Code), followed by climatic conditions (precipitation and temperature), topography (elevation), and geological factors (Dist_Volc and Drain_Code).
Table 3. Significant Variables for Developing Arsenic Predictive Model for Process Regionsa
reducing condition high-pH/oxidizing condition
variable t-value p-value variable t-value p-value
ET −17.88 <10−40 clay2 −13.56 <10−40
T 13.53 <10−40 silt1 10.41 <10−40
ET/P 7.08 <10−40 clay1 7.34 <10−40
Dist_Volc 5.8 <10−40 Drain_Code −6.67 <10−40
Drain_Code 5.5 <10−40 T 5.55 <10−40
silt2 5.31 <10−40 Dist_Volc 5.19 <10−40
silt1 −5.13 <10−40 elevation 4.82 <10−40
Dist_Volc2 −4.06 <10−40 P 3.57 <10−40
Sol_CbN2 3.23 <10−40 irrigation 2.56 0.01
pH_S −2.93 <10−40      
Dist_Riv −2.6 0.01      
a

ET = evapotranspiration; T = temperature; P = precipitation; Dist_Volc = distance from volcanoes; Drain_Code = drainage code; silt2 = subsoil silt content; silt1 = topsoil silt content; Dist_Volc2 = distance from volcanic rocks; Sol_CbN2 = carbon to nitrogen ration of subsoil; pH_S = subsoil soil pH; clay1 = topsoil clay content; clay2 = subsoil clay content; t-value indicates the relative importance of a variable (the larger in absolute values the more important a variable), and p-value indicates if a variable was rated important by chance or not (a value closer to zero is more desirable).

Note that some of these variables act as surrogates for some important but as yet unavailable influencing variables. For example, poorly drained areas (Drain_Code ≥ 60) correlate well with young sediments and flat areas (slope <1%), while areas of good drainage (Drain_Code < 60) correlate with volcanic rocks, or old sediments (see Table S7). Thus, Drain_Code is used as a proxy for a geological condition. Similarly, silt represents to some extent the transport and deposition of fresh materials in the river basin. As silt is produced only by mechanical weathering, it is highly reactive and therefore provides active absorption sites for arsenic species. Also distance to river (Dist_Riv) acts as an indicator for the deposition of fresh sediments. The soil parameter pH_S is important in the model since it has an influence on the redox potential of As-bearing species (22) and can facilitate higher arsenic occurrences (23). Climatic variables are also quite important in the reducing region. Evapotranspiration is negatively correlated with arsenic because larger ET indicates areas of larger precipitation or irrigation leading to greater groundwater recharge and smaller arsenic concentration, while ET/P, which is positively correlated with arsenic, indicates a smaller groundwater recharge. In high-pH/oxidizing region precipitation and temperature have similar effects where both seem to increase groundwater pH leading to larger arsenic concentration. Clay content appears prominently in the oxidizing model; which may modify drainage and hence leaching to groundwater. However, its exact role is not quite clear. Use of easily available information (surrogates) to estimate hard-to-obtain variables, often referred to as pedotransfer functions, is widely practiced in natural sciences (24).
The results of the arsenic model training and testing for both process regions are shown in Figure 1. In the reducing region, 77% of the variation in groundwater concentration in the training data set was explained using ANFIS, compared to 63% based on linear regression. For the test data set these were 65% and 59%, respectively. In high-pH/oxidizing region, however, ANFIS explained 68% of the variation in the training set (51% linear regression), while 65% was explained for the test data set (56% for linear regression).

Figure 1

Figure 1. Scatter plot of measured versus predicted arsenic concentration in (a) reducing and (b) high-pH/oxidizing process regions for training and testing of the neuro-fuzzy model.

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Probability Maps

Probability maps in Figure 2a and b depict the probability of the occurrence of groundwater total arsenic concentrations exceeding the WHO guideline in reducing and high-pH/oxidizing regions, respectively. It is important to note that low-probability-areas do not exclude existence of arsenic contamination. Similarly, in areas with high probability of contamination clean aquifers may also be found. Groundwaters with a high probability of arsenic contamination appear to be widespread across the world. The predicted areas resulting from “reducing” and “high pH/oxidizing” conditions are reported in Table 4 along with the references that have reported similar results.

Figure 2

Figure 2. Modeled global probability of geogenic arsenic contamination in groundwater for (a) reducing groundwater conditions, and (b) high-pH/oxidizing conditions where arsenic is soluble in its oxidized state.

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Table 4. State of Arsenic Contamination in Groundwaters in Different Countries of the World
predicted contaminated regions with reported contamination predicted contaminated regions with no measurements or reported contamination
country condition % areab country condition % areab
Bangladesh (2) reducing 35.4 Estonia reducing 37.2
Cambodia (25, 26) reducing 45.8 Amazon basina reducing 32.6
Vietnam (8, 9, 25) reducing 15.8 Lithuania both 35.0
Taiwan (27) reducing 8.2 Congo reducing 30.1
Nepal (28) reducing 3.2 Russia both 14.8
Romania (29) reducing 3.5 Myanmar both 9.2
USA (7, 10, 23) both 8.3 Poland both 8.8
Argentina (30) oxidizing 4.9 Cameroon both 14.0
India (31) both 6.4 Ukraine oxidizing 7.0
China (4, 32) both 2.5 Byelarus oxidizing 3.3
Hungary (29) reducing 7.4 Zambia oxidizing 7.0
Finland (33) unknown 34.7 Nigeria oxidizing 9.0
Greece (34) unknown 0.1 Angola oxidizing 5.5
      Kenya oxidizing 2.4
      Ethiopia oxidizing 5.3
a

Average values for Peru, Brazil, and Colombia.

b

% Area in each country with probability of arsenic contamination >0.75.

The well-known contaminated regions in South East Asia, such as Bangladesh and India (6, 31), Nepal (28), Cambodia and Vietnam (8, 9, 12, 25, 26), China (4, 32), and Taiwan (27), correspond well with the predicted regions of high arsenic concentrations. Other predicted areas with high probability of arsenic such as those in Brazil, Bolivia, Ecuador, Costa Rica, Guatemala, and Nicaragua, for which no measurements were available to us, are reported to have high arsenic contamination (35). A large area in Amazon River Basin is predicted to have large probability of contamination, but in this region the risk to human health is small as groundwater is not a major source of water use. The predicted regions with high probability of arsenic contamination in Mexico, Chile, and Argentina (6), and the southwest United States (7, 10) are in agreement with the reported literature. Our predicted maps also show a high probability of arsenic contamination in Northern European countries such as Russia, Ukraine, Poland, Belarus, Estonia, and Lithuania. To our knowledge there is no report on the contamination of groundwaters with arsenic in Russia and Ukraine, although in surface waters of the Barents region elevated concentrations of arsenic have been observed (36). In addition, in the Siberian Arctic elevated concentrations of arsenic are reported in the rivers and suspended particulate (37) and in soils, vegetation, and fish (38). The risk of geogenic groundwater contamination to human health is probably very small in the Barents region as people do not depend on groundwater as a source of drinking water (36).
A closer comparison between the modeled and measured data points with arsenic concentrations exceeding the WHO guideline is shown in Figure 3 for the United States and in Figure 4 for Bangladesh. There are regions such as the areas of southern New Hampshire and Maine, where the probability map underestimates contamination. There are, however, other detailed studies (23) that have reported small probabilities for this region of the U.S.

Figure 3

Figure 3. Probability map of arsenic concentration exceeding guideline value of 10 µg L−1 in the United States. Black dots indicate a pixel (10 km × 10 km) with mean measured arsenic concentration greater than 10 µg L−1

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Figure 4

Figure 4. Probability map of arsenic concentration exceeding the guideline value of 10 µg L−1 in Bangladesh. Black dots indicate a pixel (10 km × 10 km) with mean measured arsenic concentration greater than 10 µg L−1.

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Implications and Reliability

It is important to note that validating a probability map without extensive additional measurements is not possible. However, we compared our results with as many studies as we could find in the published literature and found a quite good correspondence between the model prediction and measurements. One such example is based on a recent publication (32) where reported cases of arsenicosis (10,096 cases) in eight Chinese provinces (Supporting Information, Figure S6) were correlated with the percentage of contaminated wells (445,638 tested wells). In Figure S7 the percentage of arsenicosis and also the percentage of contaminated wells are plotted against the probability of arsenic contamination greater than 40% where a positive correlation can be observed. This comparison is important because it is a successful model verification that links our predictions to cases of arsenicosis in a location for which we had no data in our database.
As there are few regional or national arsenic maps around, we believe our global maps have the credibility of having a large database, physical variables, and a sound statistical analysis behind it. The weaknesses of the modeling work, perhaps not surprisingly, also lie in the size and quality of the databases. The measured arsenic points come from different sources and were measured with different accuracies; they were taken at different depths and are not distributed equally around the world. The geological information, which is important to geogenic processes, was too coarse and did not provide detailed (three-dimensional) information. For example, although it is known that arsenic contamination in reducing conditions in Asia is related to very young Holocene sediments (<104 yr), the available geological database used in this study does not distinguish rocks less than 65 million years (Cenozoic period). For this reason, geological variables were mostly furnished through surface information proxies. Despite these shortcomings, the arsenic models in both reducing and high-pH/oxidizing conditions performed quite well in training and testing, indicating it is possible to capture most of the variation in groundwater arsenic with the proxy variable used in this study. Probability maps give an overview of the spatial pattern of the likely contaminated and uncontaminated areas. They are not meant to represent arsenic concentrations of individual groundwater wells. The maps developed here are useful to policy makers, global organizations, local governments, and NGOs. The maps send a message that many untested areas may be subject to arsenic contamination. Although such information may be available in some countries, in most of the world arsenic is not routinely measured in groundwaters. This is particularly important as the risk of contamination to human health will increase as a result of increasing groundwater demand.

Supporting Information

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Additional data, tables, and figures. Supporting material is available free of charge via the Internet at http://pubs.acs.org.

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
    • C. Annette Johnson - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland;  Email: [email protected]
  • Authors
    • Manouchehr Amini - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Karim C. Abbaspour - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Michael Berg - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Lenny Winkel - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Stephan J. Hug - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Eduard Hoehn - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland
    • Hong Yang - Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland

Acknowledgment

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We are grateful to all the colleagues who have provided us with arsenic data. A list of their contributions appears in the Supporting Information, Table S1.

References

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    Applied Geochemistry (2002), 17 (5), 517-568CODEN: APPGEY; ISSN:0883-2927. (Elsevier Science Ltd.)
    A review. The range of As concns. found in natural waters is large, ranging from less than 0.5 μg l-1 to more than 5000 μg l-1. Typical concns. in freshwater are less than 10 μg l-1 and frequently less than 1 μg l-1. Rarely, much higher concns. are found, particularly in groundwater. In such areas, more than 10% of wells may be 'affected' (defined as those exceeding 50 μg l-1) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concns. is greatest in the Bengal Basin with more than 40 million people drinking water contg. 'excessive' As. These large-scale 'natural' As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geol. young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localized scale, in areas of mining activity and where oxidn. of sulfide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1-20 mg kg-1. There appear to be two distinct 'triggers' that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evapn. rates. This pH change leads either to the desorption of adsorbed As (esp. As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, esp. Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissoln. of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO4 concns. are small (typically 1 mg l-1 or less). Large concns. of phosphate, bicarbonate, silicate and possibly org. matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concns. in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concn. of As in a particular well from the results of neighboring wells and means that there is little alternative but to analyze each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concns. of dissolved Fe.
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  7. 7
    Welch, A. H.; Westjohn, D. B.; Helsel, D. R.; Wanty, R. B. Arsenic in ground water of the United States: occurrence and geochemistry Ground Water 2000, 38, 589– 604
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    Arsenic in ground water of the United States: occurrence and geochemistry
    Welch, Alan H.; Westjohn, D. B.; Helsel, Dennis R.; Wanty, Richard B.
    Ground Water (2000), 38 (4), 589-604CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
    Concns. of naturally occurring As in groundwater vary regionally due to a combination of climate and geol. Although slightly less than half of 30,000 As analyses of groundwater in the US were ≤1 μg/L, ∼10% exceeded 10 μg/L. At a broad regional scale, As concns. >10 μg/L appear to be more frequently obsd. in the western US than in the eastern half. Arsenic concns. in groundwater of the Appalachian Highlands and the Atlantic Plain generally are very low (≤1 μg/L). Concns. are somewhat greater in the Interior Plains and the Rocky Mountain System. Studies of groundwater in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that As concns. >10 μg/L are more widespread and common than previously recognized. Arsenic release from Fe oxide appears to be the most common cause of widespread As concns. >10 μg/L in groundwater. This can occur in response to different geochem. conditions, including release of As to groundwater through reaction of Fe oxide with either natural or anthropogenic (i.e., petroleum products) org. C. Fe oxide also can release As to alk. groundwater, such as that found in some felsic volcanic rocks and alk. aquifers of the western US. Sulfide minerals are both a source and sink for As. Geothermal water and high evapn. rates also are assocd. with As concns. ≥10 g/L in groundwater and surface water, particularly in the west.
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  8. 8
    Berg, M.; Tran, H. C.; Nguyen, T. C.; Pham, H. V.; Schertenleib, R.; Giger, W. Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat Environ. Sci. Technol. 2001, 35 (13) 2621– 2626
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    Arsenic Contamination of Groundwater and Drinking Water in Vietnam: A Human Health Threat
    Berg, Michael; Tran, Hong Con; Nguyen, Thi Chuyen; Pham, Hung Viet; Schertenleib, Roland; Giger, Walter
    Environmental Science and Technology (2001), 35 (13), 2621-2626CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    This paper deals with As contamination of the Red River alluvial tract in the city of Hanoi and in the surrounding rural districts. Due to naturally occurring org. matter in the sediments, the groundwaters are anoxic and rich in Fe. With an av. As concn. of 159 μg/L, the contamination levels were 1-3050 μg/L in rural groundwater samples from private small-scale tubewells. In a highly affected rural area, the groundwater used directly as drinking water had an av. concn. of 430 μg/L. Anal. of raw groundwater pumped from the lower aquifer for the Hanoi water supply yielded As levels of 240-320 μg/L in 3 of 8 treatment plants and 37-82 μg/L in another 5 plants. Aeration and sand filtration that are applied in the treatment plants for Fe removal lowered the As concns. to levels of 25-91 μg/L, but 50% remained above the Vietnamese Std. of 50 μg/L. Exts. of sediment samples from 5 bore cores showed a correlation of As and Fe contents (r2 =0.700, n =64). The As in the sediments may be assocd. with Fe oxyhydroxides and released to the groundwater by reductive dissoln. of Fe. Oxidn. of sulfide phases could also release As to the groundwater, but S concns. in sediments were <1 mg/g. The high As concns. 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 As poisoning.
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  9. 9
    Buschmann, J.; Berg, M.; Stengel, C.; Winkel, L; Sampson, M. L.; Trang, P. T. K.; Viet, P. H. Contamination of drinking water resources in the Mekong delta Floodplains: Arsenic and other trace metals pose serious health risks to population Environ. Int. 2008,

    in press. (DOI: 10.1016/j.envint.2007.12.025)
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    Welch, A. H.; Oremland, R. S.; Davis, J. A.; Watkins, S. A. Arsenic in groundwater: a review of current knowledge and relation to CALFED solution area with recommendations for needed research San Francisco Estuary Watershed Sci. 2006, 4 (2) 1– 32
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    Buschmann, J.; Berg, M.; Stengel, C.; Sampson, M. L. Arsenic and Manganese Contamination of Drinking Water Resources in Cambodia: Coincidence of Risk Areas with Low Relief Topography Environ. Sci. Technol. 2007, 41, 2146– 2152
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    Arsenic and Manganese Contamination of Drinking Water Resources in Cambodia: Coincidence of Risk Areas with Low Relief Topography
    Buschmann, Johanna; Berg, Michael; Stengel, Caroline; Sampson, Mickey L.
    Environmental Science & Technology (2007), 41 (7), 2146-2152CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    As groundwater pollution has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water. A comprehensive groundwater survey was conducted in the Mekong River floodplain, comprising an area of 3700 km2 (131 samples, 30 parameters); seasonal fluctuations were also studied. As concns. were 1-1340 μg/L (av., 163 μg/L), with 48% >10 μg/L. Elevated Mn concns. (57% >0.4 mg/L) pose an addnl. health threat to the 1.2 million people living in this area. With 350 people/km2 potentially exposed to chronic As poisoning, the magnitude is similar to that of Bangladesh (200/km2). Elevated As concns. are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers in Kandal Province. As in this province averaged 233 μg/L (median, 100 μg/L); concns. to the west and east of these rivers were <10 μg/L. As release from Holocene sediment between the rivers is most likely caused by reductive dissoln. of metal oxides. Regions exhibiting low and elevated As concns. are co-incident with the existing low relief topog. featuring which gently increase in elevation to the west and east of a shallow valley, understood to be a relict of pre-Holocene topog.
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    Rowland, H. A .L.; Polya, D. A.; Lloyd, J. R.; Pancost, R. D. Characterization of organic matter in a shallow, reducing, arsenic-rich aquifer, West Bengal Org. Geochem. 2006, 37, 1101– 1114
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    Characterization of organic matter in a shallow, reducing, arsenic-rich aquifer, West Bengal
    Rowland, H. A. L.; Polya, D. A.; Lloyd, J. R.; Pancost, R. D.
    Organic Geochemistry (2006), 37 (9), 1101-1114CODEN: ORGEDE; ISSN:0146-6380. (Elsevier Ltd.)
    Elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in West Bengal and Bangladesh is causing serious impacts on human health. A key mechanism for the genesis of As in these waters is microbially mediated reductive transformation of arsenic-bearing Fe(III) hydrated oxides. The role of org. C in this process, whether from in situ org. matter (OM), i.e. OM from within the sediment, or from other sources, is widely recognized. Despite this, there is a paucity of data about the characteristics of OM in these As-rich aquifers. Extn. and anal. of the polar and apolar fractions from seven different sediments from a known groundwater As "hotspot" in West Bengal revealed OM characteristic of the original terrestrial depositional environment. However, this was overprinted by abundant hydrocarbons with thermally mature (e.g. petroleum) distributions. These hydrocarbons included abundant high mol. wt. n-alkanes and unresolved complex mixts. (UCMs), as well as thermally mature distributions of hopanes and steranes. Addnl., at certain depths (surface sands: 8 and 13 m, deeper sands: 25 m) the OM appeared to be biodegraded, with the preferential removal of petroleum-sourced n-alkanes, suggesting that indigenous microbes within the aquifer can utilize this carbon source. The presence of this previously unreported source of bioavailable org. carbon is of importance as it has the potential to promote microbial activity and subsequent As release in the aquifers.
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    Kirk, M. F.; Holm, T. R.; Park, J.; Jin, Q. S.; Sanford, R. A.; Fouke, B. W.; Bethke, C. M. Bacterial sulfate reduction limits natural arsenic contamination in groundwater Geology 2004, 32 (11) 953– 956
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    Twarakavi, N. K. C.; Kaluarachchi, J. J. Arsenic in the shallow ground waters of conterminous United States: assessment, health risk, and costs from MCL compliance J. Am. Water Resour. Assoc. 2006, 42, 275– 294
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    Arsenic in the shallow ground waters of conterminous United States: assessment, health risks, and costs for MCL compliance
    Kumar, Navin; Twarakavi, C.; Kaluarachchi, Jagath J.
    Journal of the American Water Resources Association (2006), 42 (2), 275-294CODEN: JWRAF5; ISSN:1093-474X. (American Water Resources Association)
    A methodol. consisting of ordinal logistic regression (OLR) is used to predict the probability of occurrence of arsenic concns. in different threshold limits in shallow ground waters of the conterminous United States (CONUS) subject to a set of influencing variables. The anal. considered a no. of max. contaminant level (MCL) options as threshold values to est. the probabilities of occurrence of arsenic in ranges defined by a given MCL of 3, 5, 10, 20, and 50 μg/l and a detection limit of 1 μg/l. The fit between the obsd. and predicted probability of occurrence was around 83 percent for all MCL options. The estd. probabilities were used to est. the median background concn. of arsenic in the CONUS. The shallow ground water of the western United States is more vulnerable than the eastern United States. Arizona, Utah, Nevada, and California in particular are hotspots for arsenic contamination. The risk assessment showed that counties in southern California, Arizona, Florida, and Washington and a few others scattered throughout the CONUS face a high risk from arsenic exposure through untreated ground water consumption. A simple cost effectiveness anal. was performed to understand the household costs for MCL compliance in using arsenic contaminated ground water. The results showed that the current MCL of 10 μg/l is a good compromise based on existing treatment technologies.
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    Berg, M.; Stengel, C.; Trang, P. T. K.; Viet, P. H.; Sampson, M. L.; Leng, M.; Samreth, S.; Fredericks, D. Magnitude of arsenic pollution in the Mekong and Red River Deltas - Cambodia and Vietnam Sci. Total Environ. 2007, 372, 413– 425
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    Polya, D. A.; Gault, A. G.; Diebe, N. Arsenic hazard in shallow Cambodian groundwaters Mineral. Mag. 2005, 69 (5) 807– 823
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    Arsenic hazard in shallow Cambodian groundwaters
    Polya, D. A.; Gault, A. G.; Diebe, N.; Feldman, P.; Rosenboom, J. W.; Gilligan, E.; Fredericks, D.; Milton, A. H.; Sampson, M.; Rowland, H. A. L.; Lythgoe, P. R.; Jones, J. C.; Middleton, C.; Cooke, D. A.
    Mineralogical Magazine (2005), 69 (5), 807-823CODEN: MNLMBB; ISSN:0026-461X. (Mineralogical Society)
    Our recent discovery of hazardous concns. of arsenic in shallow sedimentary aquifers in Cambodia raises the specter of future deleterious health impacts on a population that, particularly in non-urban areas, extensively use untreated groundwater as a source of drinking water and, in some instances, as irrigation water. We present here small-scale hazard maps for arsenic in shallow Cambodian groundwaters based on >1000 groundwater samples analyzed in the Manchester Anal. Geochem. Unit and elsewhere. Key indicators for hazardous concns. of arsenic in Cambodian groundwaters include: (1) well depths greater than 16 m; (2) the Holocene host sediments; and (3) proximity to major modern channels of the Mekong (and its distributary the Bassac). However, high-arsenic well waters are also commonly found in wells not exhibiting these key characteristics, notably in some shallower Holocene wells, and in wells drilled into older Quaternary and Neogene sediments. It is emphasized that the maps and tables presented are most useful for identifying current regional trends in groundwater arsenic hazard and that their use for predicting arsenic concns. in individual wells, for example for the purposes of well switching, is not recommended, particularly because of the lack of sufficient data (esp. at depths >80 m) and because, as in Bangladesh and West Bengal, there is considerable heterogeneity of groundwater arsenic concns. on a scale of meters to hundreds of meters. We have insufficient data at this time to det. unequivocally whether or not arsenic concns. are increasing in shallow Cambodian groundwaters as a result of groundwater-abstraction activities.
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    Lin, Y. B.; Lin, Y. P.; Liu, C. W.; Tan, Y. C. Mapping of spatial multi-scale sources of Arsenic variation in groundwater of ChiaNan floodplain of Taiwan Sci. Total Environ. 2006, 370, 168– 181
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    Neku, A.; Tandukar, N. An overview of arsenic contamination in groundwater of Nepal and its removal at household level J. Phys. IV 2003, 107, 941– 944
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    Lindberg, A. L.; Goessler, W.; Gurzau, E. Arsenic exposure in Hungary, Romania and Slovakia J. Environ. Monit. 2006, 8, 203– 208
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    Smedley, P. L.; Kinniburgh, D. G.; Macdonald, D. M. J. Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina Appl. Geochem. 2005, 20 (5) 989– 1016
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    Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina
    Smedley, P. L.; Kinniburgh, D. G.; Macdonald, D. M. J.; Nicolli, H. B.; Barros, A. J.; Tullio, J. O.; Pearce, J. M.; Alonso, M. S.
    Applied Geochemistry (2005), 20 (5), 989-1016CODEN: APPGEY; ISSN:0883-2927. (Elsevier Ltd.)
    Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concns. of As (≤5300 μg/L) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concns. in 45 loess samples collected from the aquifer have a range of 3-18 mg/Kg with a mean of 8 mg/Kg. These values are comparable to world-av. sediment As concns. Five samples of rhyolitic ash from the area have As concns. of 7-12 mg/Kg. Chem. anal. included loess sediments and extd. porewaters from 2 specially cored boreholes. Results reveal a large range of porewater As concns., being generally higher in the horizons with highest sediment As concns. The displaced porewaters have As concns. ranging ≤7500 μg/L as well as exceptionally high concns. of some other oxyanion species, including V ≤12 mg/L. The highest concns. are found in a borehole located in a topog. depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective ext. data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is assocd. with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Ext. compns. also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidizing, with dissolved O, NO3- and SO42- normally present and As(V) usually the dominant dissolved As species. Under such conditions, the soly. of Fe and Mn oxides is low and As mobilization is strongly controlled by sorption-desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0-8.8) and high concns. of potential competitors (e.g. V, P, HCO3-). PHREEQC modeling suggests that the presence of V at the concns. obsd. in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by ≤1 order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate ext. concns. have been used to provide an upper est. of the amt. of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes studied has been used to est. an approx. Kd value for the sediments of 0.94 L/Kg. This low value indicates that the sediments have an unusually low affinity for As.
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    Geochemical behaviour of major and trace elements in suspended particulate material of the Irtysh river, the main tributary of the Ob river, Siberia
    Gordeev, V. V.; Rachold, V.; Vlasova, I. E.
    Applied Geochemistry (2004), 19 (4), 593-610CODEN: APPGEY; ISSN:0883-2927. (Elsevier Science Ltd.)
    In July 2001, samples of surface suspended particulate material (SPM) of the Irtysh river in its middle and lower reaches (from Omsk City to the confluence with the Ob river) and its main tributaries were collected (18 stations along 1834 km). The SPM samples were analyzed for major and trace element compn. The results show that the geochem. of Irtysh river SPM is related to landscape and geochem. peculiarities of the river basin on one hand and to industrial activities within the drainage area on the other hand. In the upper basin polymetallic and cinnabar deposits and phosphorite deposits with high As content are widespread. The open-cut mining and developed oil-refining, power plants and other industries lead to the contamination of the environment by heavy metals and other contaminants. The territory of the West Siberian lowland, esp. the Ob-Irtysh interfluve, is characterized by the occurrence of swamps and peat-bogs. Tributaries of the Irtysh river originating in this region, have a brown color and the chem. compn. of the SPM is specific for stagnant water. In the first 500-700 km downstream from Omsk City the Irtysh river has the typical Al-Si-rich suspended matter compn. After the inflow of the tributaries with brown water the SPM compn. is significantly changed: an increase of POC, Fe, P, Ca, Sr, Ba and As concns. and a strong decrease of the lithogenic elements Al, Mg, K, Na, Ti, Zr can be obsd. The data show that Fe-org. components (Fe-humic amorphous compds., which contribute ca. 75-85% to the total Fe) play a very important role in SPM of the tributaries with brown water and in the Irtysh river in its lower reaches. Among the trace metals significant enrichments relative to the av. for global river SPM could only be obsd. for As and Cd (coeff. of enrichment up to 16 for As and 3-3.5 for Cd). It can be shown that the enrichment of As in the SPM is related to natural processes, i.e. the weathering of phosphate contg. deposits with high As concns. in the upper Irtysh basin and the high As-P affinity in the swamp peaty soil. Dissolved P and As are absorbed by amorphous org. C/Fe oxyhydroxide components which act as carriers during the transport to the main stream of the Irtysh river. The role of anthropogenic factors is probably insignificant for As. In contrast, the enrichment of Cd is mainly related to anthropogenic input. The threefold enrichment of Cd in the SPM just below Omsk City and its continuous decrease down to background level at a distance of 500-700 km downstream points quite definitely to the municipal and industrial sewage of Omsk City as the main source of Cd in the SPM of the Irtysh river.
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  38. 38
    Allen-Gil, S. M.; Ford, J.; Lasorsa, B. K.; Monettie, M.; Vlasova, T.; Landers, D. H. Heavy metal contamination in Taimyr Peninsula, Siberian Arctic Sci. Total Environ. 2003, 301, 119– 138
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  • Figure 1

    Figure 1. Scatter plot of measured versus predicted arsenic concentration in (a) reducing and (b) high-pH/oxidizing process regions for training and testing of the neuro-fuzzy model.

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    Figure 2

    Figure 2. Modeled global probability of geogenic arsenic contamination in groundwater for (a) reducing groundwater conditions, and (b) high-pH/oxidizing conditions where arsenic is soluble in its oxidized state.

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    Figure 3

    Figure 3. Probability map of arsenic concentration exceeding guideline value of 10 µg L−1 in the United States. Black dots indicate a pixel (10 km × 10 km) with mean measured arsenic concentration greater than 10 µg L−1

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    Figure 4

    Figure 4. Probability map of arsenic concentration exceeding the guideline value of 10 µg L−1 in Bangladesh. Black dots indicate a pixel (10 km × 10 km) with mean measured arsenic concentration greater than 10 µg L−1.

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    This article references 38 other publications.

    1. 1
      Kapaj, S.; Peterson, H.; Liber, K.; Bhattacharya, P. Human health effects from chronic arsenic poisoning- a review J. Environ. Sci. Health, Part A 2006, 41, 2399– 2428
      There is no corresponding record for this reference.
    2. 2
      Smith, A. H.; Lingas, E. O.; Rahman, M. Contamination of drinking-water by arsenic in Bangladesh: a public health emergency Bull. World Health Organisation 2000, 78, 1093– 1103
      2
      Contamination of drinking-water by arsenic in Bangladesh: a public health emergency
      Smith A H; Lingas E O; Rahman M
      Bulletin of the World Health Organization (2000), 78 (9), 1093-103 ISSN:0042-9686.
      The contamination of groundwater by arsenic in Bangladesh is the largest poisoning of a population in history, with millions of people exposed. This paper describes the history of the discovery of arsenic in drinking-water in Bangladesh and recommends intervention strategies. Tube-wells were installed to provide "pure water" to prevent morbidity and mortality from gastrointestinal disease. The water from the millions of tube-wells that were installed was not tested for arsenic contamination. Studies in other countries where the population has had long-term exposure to arsenic in groundwater indicate that 1 in 10 people who drink water containing 500 micrograms of arsenic per litre may ultimately die from cancers caused by arsenic, including lung, bladder and skin cancers. The rapid allocation of funding and prompt expansion of current interventions to address this contamination should be facilitated. The fundamental intervention is the identification and provision of arsenic-free drinking water. Arsenic is rapidly excreted in urine, and for early or mild cases, no specific treatment is required. Community education and participation are essential to ensure that interventions are successful; these should be coupled with follow-up monitoring to confirm that exposure has ended. Taken together with the discovery of arsenic in groundwater in other countries, the experience in Bangladesh shows that groundwater sources throughout the world that are used for drinking-water should be tested for arsenic.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BD3cvmsFSqtA%253D%253D&md5=32fa551e2e2d2648d6bccf11d9e3e99d
    3. 3
      Rich, C. H.; Biggs, M. L.; Smith, A. H. Lung and kidney cancer mortality associated with arsenic in drinking water in Cordoba, Argentina Int. J. Epidemiol. 1998, 27, 561– 569
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      Xia, Y.; Liu, J. An overview on chronic arsenism via drinking water in PR China Toxicology 2004, 198, 25– 29
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      Saha, K. C. Review of Arsenicosis in West Bengal, India- A clinical perspective. Critical Reviews Environ. Sci. Technol. 2003, 30 (2) 127– 163
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      Smedley, P. L.; Kinniburgh, D. G. A review of the source, behaviour and distribution of arsenic in natural waters Appl. Geochem. 2002, 17, 517– 568
      6
      A review of the source, behaviour and distribution of arsenic in natural waters
      Smedley, P. L.; Kinniburgh, D. G.
      Applied Geochemistry (2002), 17 (5), 517-568CODEN: APPGEY; ISSN:0883-2927. (Elsevier Science Ltd.)
      A review. The range of As concns. found in natural waters is large, ranging from less than 0.5 μg l-1 to more than 5000 μg l-1. Typical concns. in freshwater are less than 10 μg l-1 and frequently less than 1 μg l-1. Rarely, much higher concns. are found, particularly in groundwater. In such areas, more than 10% of wells may be 'affected' (defined as those exceeding 50 μg l-1) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concns. is greatest in the Bengal Basin with more than 40 million people drinking water contg. 'excessive' As. These large-scale 'natural' As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geol. young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localized scale, in areas of mining activity and where oxidn. of sulfide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1-20 mg kg-1. There appear to be two distinct 'triggers' that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evapn. rates. This pH change leads either to the desorption of adsorbed As (esp. As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, esp. Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissoln. of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO4 concns. are small (typically 1 mg l-1 or less). Large concns. of phosphate, bicarbonate, silicate and possibly org. matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concns. in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concn. of As in a particular well from the results of neighboring wells and means that there is little alternative but to analyze each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concns. of dissolved Fe.
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    7. 7
      Welch, A. H.; Westjohn, D. B.; Helsel, D. R.; Wanty, R. B. Arsenic in ground water of the United States: occurrence and geochemistry Ground Water 2000, 38, 589– 604
      7
      Arsenic in ground water of the United States: occurrence and geochemistry
      Welch, Alan H.; Westjohn, D. B.; Helsel, Dennis R.; Wanty, Richard B.
      Ground Water (2000), 38 (4), 589-604CODEN: GRWAAP; ISSN:0017-467X. (National Ground Water Association)
      Concns. of naturally occurring As in groundwater vary regionally due to a combination of climate and geol. Although slightly less than half of 30,000 As analyses of groundwater in the US were ≤1 μg/L, ∼10% exceeded 10 μg/L. At a broad regional scale, As concns. >10 μg/L appear to be more frequently obsd. in the western US than in the eastern half. Arsenic concns. in groundwater of the Appalachian Highlands and the Atlantic Plain generally are very low (≤1 μg/L). Concns. are somewhat greater in the Interior Plains and the Rocky Mountain System. Studies of groundwater in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that As concns. >10 μg/L are more widespread and common than previously recognized. Arsenic release from Fe oxide appears to be the most common cause of widespread As concns. >10 μg/L in groundwater. This can occur in response to different geochem. conditions, including release of As to groundwater through reaction of Fe oxide with either natural or anthropogenic (i.e., petroleum products) org. C. Fe oxide also can release As to alk. groundwater, such as that found in some felsic volcanic rocks and alk. aquifers of the western US. Sulfide minerals are both a source and sink for As. Geothermal water and high evapn. rates also are assocd. with As concns. ≥10 g/L in groundwater and surface water, particularly in the west.
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    8. 8
      Berg, M.; Tran, H. C.; Nguyen, T. C.; Pham, H. V.; Schertenleib, R.; Giger, W. Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat Environ. Sci. Technol. 2001, 35 (13) 2621– 2626
      8
      Arsenic Contamination of Groundwater and Drinking Water in Vietnam: A Human Health Threat
      Berg, Michael; Tran, Hong Con; Nguyen, Thi Chuyen; Pham, Hung Viet; Schertenleib, Roland; Giger, Walter
      Environmental Science and Technology (2001), 35 (13), 2621-2626CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      This paper deals with As contamination of the Red River alluvial tract in the city of Hanoi and in the surrounding rural districts. Due to naturally occurring org. matter in the sediments, the groundwaters are anoxic and rich in Fe. With an av. As concn. of 159 μg/L, the contamination levels were 1-3050 μg/L in rural groundwater samples from private small-scale tubewells. In a highly affected rural area, the groundwater used directly as drinking water had an av. concn. of 430 μg/L. Anal. of raw groundwater pumped from the lower aquifer for the Hanoi water supply yielded As levels of 240-320 μg/L in 3 of 8 treatment plants and 37-82 μg/L in another 5 plants. Aeration and sand filtration that are applied in the treatment plants for Fe removal lowered the As concns. to levels of 25-91 μg/L, but 50% remained above the Vietnamese Std. of 50 μg/L. Exts. of sediment samples from 5 bore cores showed a correlation of As and Fe contents (r2 =0.700, n =64). The As in the sediments may be assocd. with Fe oxyhydroxides and released to the groundwater by reductive dissoln. of Fe. Oxidn. of sulfide phases could also release As to the groundwater, but S concns. in sediments were <1 mg/g. The high As concns. 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 As poisoning.
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      Buschmann, J.; Berg, M.; Stengel, C.; Winkel, L; Sampson, M. L.; Trang, P. T. K.; Viet, P. H. Contamination of drinking water resources in the Mekong delta Floodplains: Arsenic and other trace metals pose serious health risks to population Environ. Int. 2008,

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    10. 10
      Welch, A. H.; Oremland, R. S.; Davis, J. A.; Watkins, S. A. Arsenic in groundwater: a review of current knowledge and relation to CALFED solution area with recommendations for needed research San Francisco Estuary Watershed Sci. 2006, 4 (2) 1– 32
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      Nordstrom, D. K. Worldwide occurrences of arsenic in groundwater Science 2002, 296, 2143– 2144
      There is no corresponding record for this reference.
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      Buschmann, J.; Berg, M.; Stengel, C.; Sampson, M. L. Arsenic and Manganese Contamination of Drinking Water Resources in Cambodia: Coincidence of Risk Areas with Low Relief Topography Environ. Sci. Technol. 2007, 41, 2146– 2152
      12
      Arsenic and Manganese Contamination of Drinking Water Resources in Cambodia: Coincidence of Risk Areas with Low Relief Topography
      Buschmann, Johanna; Berg, Michael; Stengel, Caroline; Sampson, Mickey L.
      Environmental Science & Technology (2007), 41 (7), 2146-2152CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      As groundwater pollution has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water. A comprehensive groundwater survey was conducted in the Mekong River floodplain, comprising an area of 3700 km2 (131 samples, 30 parameters); seasonal fluctuations were also studied. As concns. were 1-1340 μg/L (av., 163 μg/L), with 48% >10 μg/L. Elevated Mn concns. (57% >0.4 mg/L) pose an addnl. health threat to the 1.2 million people living in this area. With 350 people/km2 potentially exposed to chronic As poisoning, the magnitude is similar to that of Bangladesh (200/km2). Elevated As concns. are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers in Kandal Province. As in this province averaged 233 μg/L (median, 100 μg/L); concns. to the west and east of these rivers were <10 μg/L. As release from Holocene sediment between the rivers is most likely caused by reductive dissoln. of metal oxides. Regions exhibiting low and elevated As concns. are co-incident with the existing low relief topog. featuring which gently increase in elevation to the west and east of a shallow valley, understood to be a relict of pre-Holocene topog.
      >> More from SciFinder ®
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      13
      Characterization of organic matter in a shallow, reducing, arsenic-rich aquifer, West Bengal
      Rowland, H. A. L.; Polya, D. A.; Lloyd, J. R.; Pancost, R. D.
      Organic Geochemistry (2006), 37 (9), 1101-1114CODEN: ORGEDE; ISSN:0146-6380. (Elsevier Ltd.)
      Elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in West Bengal and Bangladesh is causing serious impacts on human health. A key mechanism for the genesis of As in these waters is microbially mediated reductive transformation of arsenic-bearing Fe(III) hydrated oxides. The role of org. C in this process, whether from in situ org. matter (OM), i.e. OM from within the sediment, or from other sources, is widely recognized. Despite this, there is a paucity of data about the characteristics of OM in these As-rich aquifers. Extn. and anal. of the polar and apolar fractions from seven different sediments from a known groundwater As "hotspot" in West Bengal revealed OM characteristic of the original terrestrial depositional environment. However, this was overprinted by abundant hydrocarbons with thermally mature (e.g. petroleum) distributions. These hydrocarbons included abundant high mol. wt. n-alkanes and unresolved complex mixts. (UCMs), as well as thermally mature distributions of hopanes and steranes. Addnl., at certain depths (surface sands: 8 and 13 m, deeper sands: 25 m) the OM appeared to be biodegraded, with the preferential removal of petroleum-sourced n-alkanes, suggesting that indigenous microbes within the aquifer can utilize this carbon source. The presence of this previously unreported source of bioavailable org. carbon is of importance as it has the potential to promote microbial activity and subsequent As release in the aquifers.
      >> More from SciFinder ®
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      Kirk, M. F.; Holm, T. R.; Park, J.; Jin, Q. S.; Sanford, R. A.; Fouke, B. W.; Bethke, C. M. Bacterial sulfate reduction limits natural arsenic contamination in groundwater Geology 2004, 32 (11) 953– 956
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      Arsenic in the shallow ground waters of conterminous United States: assessment, health risks, and costs for MCL compliance
      Kumar, Navin; Twarakavi, C.; Kaluarachchi, Jagath J.
      Journal of the American Water Resources Association (2006), 42 (2), 275-294CODEN: JWRAF5; ISSN:1093-474X. (American Water Resources Association)
      A methodol. consisting of ordinal logistic regression (OLR) is used to predict the probability of occurrence of arsenic concns. in different threshold limits in shallow ground waters of the conterminous United States (CONUS) subject to a set of influencing variables. The anal. considered a no. of max. contaminant level (MCL) options as threshold values to est. the probabilities of occurrence of arsenic in ranges defined by a given MCL of 3, 5, 10, 20, and 50 μg/l and a detection limit of 1 μg/l. The fit between the obsd. and predicted probability of occurrence was around 83 percent for all MCL options. The estd. probabilities were used to est. the median background concn. of arsenic in the CONUS. The shallow ground water of the western United States is more vulnerable than the eastern United States. Arizona, Utah, Nevada, and California in particular are hotspots for arsenic contamination. The risk assessment showed that counties in southern California, Arizona, Florida, and Washington and a few others scattered throughout the CONUS face a high risk from arsenic exposure through untreated ground water consumption. A simple cost effectiveness anal. was performed to understand the household costs for MCL compliance in using arsenic contaminated ground water. The results showed that the current MCL of 10 μg/l is a good compromise based on existing treatment technologies.
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      Arsenic hazard in shallow Cambodian groundwaters
      Polya, D. A.; Gault, A. G.; Diebe, N.; Feldman, P.; Rosenboom, J. W.; Gilligan, E.; Fredericks, D.; Milton, A. H.; Sampson, M.; Rowland, H. A. L.; Lythgoe, P. R.; Jones, J. C.; Middleton, C.; Cooke, D. A.
      Mineralogical Magazine (2005), 69 (5), 807-823CODEN: MNLMBB; ISSN:0026-461X. (Mineralogical Society)
      Our recent discovery of hazardous concns. of arsenic in shallow sedimentary aquifers in Cambodia raises the specter of future deleterious health impacts on a population that, particularly in non-urban areas, extensively use untreated groundwater as a source of drinking water and, in some instances, as irrigation water. We present here small-scale hazard maps for arsenic in shallow Cambodian groundwaters based on >1000 groundwater samples analyzed in the Manchester Anal. Geochem. Unit and elsewhere. Key indicators for hazardous concns. of arsenic in Cambodian groundwaters include: (1) well depths greater than 16 m; (2) the Holocene host sediments; and (3) proximity to major modern channels of the Mekong (and its distributary the Bassac). However, high-arsenic well waters are also commonly found in wells not exhibiting these key characteristics, notably in some shallower Holocene wells, and in wells drilled into older Quaternary and Neogene sediments. It is emphasized that the maps and tables presented are most useful for identifying current regional trends in groundwater arsenic hazard and that their use for predicting arsenic concns. in individual wells, for example for the purposes of well switching, is not recommended, particularly because of the lack of sufficient data (esp. at depths >80 m) and because, as in Bangladesh and West Bengal, there is considerable heterogeneity of groundwater arsenic concns. on a scale of meters to hundreds of meters. We have insufficient data at this time to det. unequivocally whether or not arsenic concns. are increasing in shallow Cambodian groundwaters as a result of groundwater-abstraction activities.
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    27. 27
      Lin, Y. B.; Lin, Y. P.; Liu, C. W.; Tan, Y. C. Mapping of spatial multi-scale sources of Arsenic variation in groundwater of ChiaNan floodplain of Taiwan Sci. Total Environ. 2006, 370, 168– 181
      There is no corresponding record for this reference.
    28. 28
      Neku, A.; Tandukar, N. An overview of arsenic contamination in groundwater of Nepal and its removal at household level J. Phys. IV 2003, 107, 941– 944
      There is no corresponding record for this reference.
    29. 29
      Lindberg, A. L.; Goessler, W.; Gurzau, E. Arsenic exposure in Hungary, Romania and Slovakia J. Environ. Monit. 2006, 8, 203– 208
      There is no corresponding record for this reference.
    30. 30
      Smedley, P. L.; Kinniburgh, D. G.; Macdonald, D. M. J. Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina Appl. Geochem. 2005, 20 (5) 989– 1016
      30
      Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina
      Smedley, P. L.; Kinniburgh, D. G.; Macdonald, D. M. J.; Nicolli, H. B.; Barros, A. J.; Tullio, J. O.; Pearce, J. M.; Alonso, M. S.
      Applied Geochemistry (2005), 20 (5), 989-1016CODEN: APPGEY; ISSN:0883-2927. (Elsevier Ltd.)
      Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concns. of As (≤5300 μg/L) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concns. in 45 loess samples collected from the aquifer have a range of 3-18 mg/Kg with a mean of 8 mg/Kg. These values are comparable to world-av. sediment As concns. Five samples of rhyolitic ash from the area have As concns. of 7-12 mg/Kg. Chem. anal. included loess sediments and extd. porewaters from 2 specially cored boreholes. Results reveal a large range of porewater As concns., being generally higher in the horizons with highest sediment As concns. The displaced porewaters have As concns. ranging ≤7500 μg/L as well as exceptionally high concns. of some other oxyanion species, including V ≤12 mg/L. The highest concns. are found in a borehole located in a topog. depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective ext. data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is assocd. with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Ext. compns. also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidizing, with dissolved O, NO3- and SO42- normally present and As(V) usually the dominant dissolved As species. Under such conditions, the soly. of Fe and Mn oxides is low and As mobilization is strongly controlled by sorption-desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0-8.8) and high concns. of potential competitors (e.g. V, P, HCO3-). PHREEQC modeling suggests that the presence of V at the concns. obsd. in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by ≤1 order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate ext. concns. have been used to provide an upper est. of the amt. of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes studied has been used to est. an approx. Kd value for the sediments of 0.94 L/Kg. This low value indicates that the sediments have an unusually low affinity for As.
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    31. 31
      Chakraborti, D.; Rahman, M. M.; Paul, K. Arsenic calamity in the Indian subcontinent, What lessons have been learned Talanta 2002, 58, 3– 22
      There is no corresponding record for this reference.
    32. 32
      Yu, G.; Sun, D.; Zheng, Y. Health Effects of Exposure to Natural Arsenic in Groundwater and Coal in China: An Overview of Occurrence Environ. Health Perspect. 2007, 115 (4) 636– 642
      There is no corresponding record for this reference.
    33. 33
      Kurttio, P.; Pukkala, E.; Kahelin, H.; Auvinen, A.; Pekkanen, J. Arsenic concentrations in well water and risk of bladder and kidney cancer in Finland Environ. Health Perspect. 1999, 107, 705– 710
      There is no corresponding record for this reference.
    34. 34
      Kelepertsis, A.; Alexakis, D.; Skordas, K. Arsenic, antimony and other toxic elements in drinking water of Eastern Thessaly in Greece and its possible effects on human health Environ. Geol. 2006, 50, 76– 84
      There is no corresponding record for this reference.
    35. 35
      Bundschuh, J.; Armienta, M. A.; Bhattacharya, P.; Matschullat, J.; Birkle, P.; Rodriguez, R. Natural Arsenic in Groundwaters of Latin America; 2006.

      Available at

      http://www.lwr.kth.se/Personal/personer/bhattacharya_prosun/As-2006.htm.
      There is no corresponding record for this reference.
    36. 36
      Salminen, R.; Chekushin, V.; Tenhola, M. Geochemical atlas of eastern Barents region J. Geochem. Exploration 2004, 83, 1– 530
      36
      Special Issue: geochemical atlas of the eastern Barents region
      Salminen, R.; Chekushin, V.; Tenhola, M.; Bogatyrev, I.; Glavatskikh, S. P.; Fedotova, E.; Gregorauskiene, V.; Kashulina, G.; Niskavaara, H.; Polischuok, A.; Rissanen, K.; Selenok, L.; Tomilina, O.; Zhdanova, L.
      Journal of Geochemical Exploration (2004), 83 (1-3), 1-530CODEN: JGCEAT; ISSN:0375-6742. (Elsevier B.V.)
      There is no expanded citation for this reference.
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    37. 37
      Gordeev, V. V.; Rachold, V.; Vlasova, I. E. Geochemical behaviour of major and trace elements in suspended particulate material of the Irtysh river, the main tributary of the Ob river, Siberia Appl. Geochem. 2004, 19, 593– 610
      37
      Geochemical behaviour of major and trace elements in suspended particulate material of the Irtysh river, the main tributary of the Ob river, Siberia
      Gordeev, V. V.; Rachold, V.; Vlasova, I. E.
      Applied Geochemistry (2004), 19 (4), 593-610CODEN: APPGEY; ISSN:0883-2927. (Elsevier Science Ltd.)
      In July 2001, samples of surface suspended particulate material (SPM) of the Irtysh river in its middle and lower reaches (from Omsk City to the confluence with the Ob river) and its main tributaries were collected (18 stations along 1834 km). The SPM samples were analyzed for major and trace element compn. The results show that the geochem. of Irtysh river SPM is related to landscape and geochem. peculiarities of the river basin on one hand and to industrial activities within the drainage area on the other hand. In the upper basin polymetallic and cinnabar deposits and phosphorite deposits with high As content are widespread. The open-cut mining and developed oil-refining, power plants and other industries lead to the contamination of the environment by heavy metals and other contaminants. The territory of the West Siberian lowland, esp. the Ob-Irtysh interfluve, is characterized by the occurrence of swamps and peat-bogs. Tributaries of the Irtysh river originating in this region, have a brown color and the chem. compn. of the SPM is specific for stagnant water. In the first 500-700 km downstream from Omsk City the Irtysh river has the typical Al-Si-rich suspended matter compn. After the inflow of the tributaries with brown water the SPM compn. is significantly changed: an increase of POC, Fe, P, Ca, Sr, Ba and As concns. and a strong decrease of the lithogenic elements Al, Mg, K, Na, Ti, Zr can be obsd. The data show that Fe-org. components (Fe-humic amorphous compds., which contribute ca. 75-85% to the total Fe) play a very important role in SPM of the tributaries with brown water and in the Irtysh river in its lower reaches. Among the trace metals significant enrichments relative to the av. for global river SPM could only be obsd. for As and Cd (coeff. of enrichment up to 16 for As and 3-3.5 for Cd). It can be shown that the enrichment of As in the SPM is related to natural processes, i.e. the weathering of phosphate contg. deposits with high As concns. in the upper Irtysh basin and the high As-P affinity in the swamp peaty soil. Dissolved P and As are absorbed by amorphous org. C/Fe oxyhydroxide components which act as carriers during the transport to the main stream of the Irtysh river. The role of anthropogenic factors is probably insignificant for As. In contrast, the enrichment of Cd is mainly related to anthropogenic input. The threefold enrichment of Cd in the SPM just below Omsk City and its continuous decrease down to background level at a distance of 500-700 km downstream points quite definitely to the municipal and industrial sewage of Omsk City as the main source of Cd in the SPM of the Irtysh river.
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    38. 38
      Allen-Gil, S. M.; Ford, J.; Lasorsa, B. K.; Monettie, M.; Vlasova, T.; Landers, D. H. Heavy metal contamination in Taimyr Peninsula, Siberian Arctic Sci. Total Environ. 2003, 301, 119– 138
      There is no corresponding record for this reference.

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