Applied Geochemistry (v.23, #11)

Lydia Press (1984–2008) by D.A. Polya (2967).

Arsenic in Groundwaters of South-East Asia: With Emphasis on Cambodia and Vietnam by David A. Polya; Michael Berg; Andrew G. Gault; Yoshio Takahashi (2968-2976).

Arsenicosis in Cambodia: Case studies and policy response by M.L. Sampson; B. Bostick; Hannah Chiew; J.M. Hagan; A. Shantz (2977-2986).
Naturally occurring As found in groundwaters has been identified to be a problem in at least 10 provinces of Cambodia with Kandal being one of the most heavily impacted. Estimates, using groundwater quality and population data for Kandal Province of Cambodia, suggest that over 100,000 people are at high risk of chronic As exposure. Levels in some areas approach 3500 μg/L, against the Cambodian Standard of 50 μg/L. Considerable work remains to adequately characterize the extent of As hazard and its possible health effects in Cambodia and the region. It is likely that additional populations will develop health problems attributed to As, of particular concern is arsenicosis. The symptoms of arsenicosis have been generally assumed to develop after 8–10 years of consumption of water with elevated As levels, however, new cases discovered in Cambodia have been identified with exposure times as short as 3 years. The rapid onset of arsenicosis may be attributed to contributing risk factors related to socioeconomic status, including malnutrition. It is thus imperative to develop strategies to rapidly identify possible regions of enrichment, to minimize exposure to As-rich waters, and to educate affected populations. To date the response to the As hazard has been led by the Ministry of Rural Development in cooperation with international organizations and NGOs, to identify at risk areas, and educate communities of the risk of As-rich water. However better coordination between government bodies, NGOs and donor agencies active in the field of water supply and treatment is essential to minimize future As exposure.

The importance or otherwise of rice as an exposure pathway for As ingestion by people living in Bengal and other areas impacted by hazardous As-bearing groundwaters is currently a matter of some debate. Here this issue is addressed by determining the overall increased cancer risk due to ingestion of rice in an As-impacted district of West Bengal. Human target cancer health risks have been estimated through the intake of As-bearing rice by using combined field, laboratory and computational methods. Monte Carlo simulations were run following fitting of model probability curves to measured distributions of (i) As concentration in rice and drinking water and (ii) inorganic As content of rice and fitting distributions to published data on (i) ingestion rates and (ii) body weight and point estimates on bioconcentration factors, exposure duration and other input variables. The distribution of As in drinking water was found to be substantially lower than that reported by previous authors for As in tube wells in the same area, indicating that the use of tube well water as a proxy for drinking water is likely to result in human health risks being somewhat overestimated. The calculated median increased lifetime cancer risk due to cooked rice intake was 7.62 × 10−4, higher than the 10−4–10−6 range typically used by the USEPA as a threshold to guide determination of regulatory values and similar to the equivalent risk from water intake. The median total risk from combined rice and water intake was 1.48 × 10−3. The contributions to this median risk from drinking water, rice and cooking of rice were found to be 48%, 44% and 8%, respectively. Thus, rice is a major potential source of As exposure in the As-affected study areas in West Bengal and the most important exposure pathway for groups exposed to low or no As in drinking water.

Calculation of excess disease burden for As exposed populations is becoming increasingly important to enable quantitative estimation of the impacts of various As mitigation options. There are several methods by which such calculations may be carried out. In this study, two methods, recently applied to estimating groundwater As-related health risks in southern Asia, to estimate disease burden arising from lung, bladder and liver cancer from As exposure for an As-effected area of West Bengal have been compared. Both utilized calculated distributions of exposure of the studied population to As from groundwater. Method (I) then entailed calculating disease burden by combining published background rates for death and disability adjusted life years (DALYs) and standard mortality ratios (SMRs) for excess health impacts related to As exposure, whilst for Method (II), disease burden from As exposure was estimated using the basic DALY formula, combined with incidence rates based on the NRC multistage Weibull model. Dose–response data for both methods were based on studies in Taiwan.When the same dose–response model was used for both methods, the two methods were broadly comparable, agreeing to within a factor of 4 for both deaths and DALYs. Much larger differences, up to a factor of 40, were noted when SMRs from different previous studies were utilized by Method (I). Thus, the death and DALYs calculations are most sensitive to the choice of dose–response model and less so to the calculation method. The differences are also partly ascribed to different background (i.e. for As non-exposed populations) rates for lung, bladder and liver cancers between Chakdha block and Taiwan. However, the differences also highlight some of systematic uncertainties in the application of epidemiological studies in one part of the world to another, emphasizing that accurate health risk estimates are likely to be better obtained by large scale systematic surveys of health outcomes in the study population. Irrespective of the comparability of the results of the two methods, it is noted that the lack of detailed consideration of confounding factors such as genetic polymorphisms, smoking and dietary habits, and, in particular, exposure to As through other routes, notably ingestion of As-bearing rice, may significantly impact on the accuracy of the results obtained by either method.

Modelling arsenic hazard in Cambodia: A geostatistical approach using ancillary data by Luis Rodríguez Lado; David Polya; Lenny Winkel; Michael Berg; Aimee Hegan (3010-3018).
The As concentration in shallow groundwater in Cambodia was estimated using 1329 georeferenced water samples collected during the period 1986–2004 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.

Hydrogeological survey assessing arsenic and other groundwater contaminants in the lowlands of Sumatra, Indonesia by Lenny Winkel; Michael Berg; Caroline Stengel; Thomas Rosenberg (3019-3028).
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.

Geochemistry of aquifer sediments and arsenic-rich groundwaters from Kandal Province, Cambodia by Helen A.L. Rowland; Andrew G. Gault; Paul Lythgoe; David A. Polya (3029-3046).
Elevated As is well known to be present in aquifers utilised for drinking water and irrigation in West Bengal and Bangladesh. This problem has also more recently been discovered in other parts of Asia, including Vietnam, Cambodia, Inner Mongolia and the Middle Ganges Plain. Analysis of groundwaters in Kandal Province of Cambodia found waters with comparable geochemistry to the As-rich groundwaters of the West Bengali Delta. Similarities included high but heterogeneous As distributions, predominantly in the form As(III), high Fe, moderate to high HCO 3 - , circumneutral pH, low SO 4 2 - and geochemical components indicative of reducing conditions. Good positive correlations between As, Fe, HCO 3 - and NH 4 + , and dissolved organic C is consistent with As release predominantly via microbially mediated reductive dissolution of As bearing Fe(III) oxides. Further evidence for such a process is found from correlations between As, Fe and organic matter from analysis of aquifer sediments, by the presence of goethite in the finer fractions and from the association of As with amorphous, poorly crystalline and well crystallised hydrous Fe oxides. The presence of several high As, but low Fe, wells implies that microbes could have a more direct role in mediating As release via the direct utilisation of Fe(III) or As(V) as electron acceptors. The presence of elevated As in waters with short aquifer residence times (as indicated by their geochemical signature) highlights the possible vulnerability of these aquifers to the influx of surface derived waters, providing an additional source of labile organic C that could exacerbate As release by stimulating microbial activity.

Hopane, sterane and n-alkane distributions in shallow sediments hosting high arsenic groundwaters in Cambodia by Bart E. van Dongen; Helen A.L. Rowland; Andrew G. Gault; David A. Polya; Charlotte Bryant; Richard D. Pancost (3047-3058).
The presence of elevated As in ground waters exploited for drinking water and irrigation in South-East Asia is causing serious impacts on human health. A key mechanism that causes the mobilization of As in these waters is microbially mediated reductive transformation of As-bearing Fe(III) hydrated oxides and the role of degradable organic matter (OM) in this process is widely recognized. A number of different types of OM that drive As release in these aquifers have been suggested, including petroleum derived hydrocarbons naturally seeping into shallow sediments from deeper thermally mature source rocks. However, the amount of information on the characteristics of the OM in South-East Asian aquifers is limited. Here the organic geochemical analyses of the saturated hydrocarbon fractions and radiocarbon analysis, of two additional sites in SE Asia are reported. The results show that the OM in a given sedimentary horizon likely derives from multiple sources including naturally occurring petroleum. The importance of naturally occurring petroleum as one of the sources was clearly indicated by the n-alkane CPI of approximately 1, the presence of an unresolved complex mixture, and hopane (dominated by 17α(H),21β(H) hopanes) and sterane distribution patterns. The results also indicate that the OM in these aquifers varies tremendously in content, character and potential bioavailability. Furthermore, the presence of petroleum derived OM in sediments at both sites doubles the number of locations where their presence has been observed in association with As-rich, shallow aquifers, suggesting that the role of petroleum derived OM in microbially mediated As release might occur over a wider range of geographical locations than previously thought.

Integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the Mekong delta by Benjamin D. Kocar; Matthew L. Polizzotto; Shawn G. Benner; Samantha C. Ying; Mengieng Ung; Kagna Ouch; Sopheap Samreth; Bunseang Suy; Kongkea Phan; Michael Sampson; Scott Fendorf (3059-3071).
Arsenic is contaminating the groundwater of Holocene aquifers throughout South and Southeast Asia. To examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of As within soils/sediments in the Mekong River delta, a ∼50 km2 field site was established near Phnom Penh, Cambodia, where aqueous As concentrations are dangerously high and where groundwater retrieval for irrigation is minimal. Dissolved As concentrations vary spatially, ranging up to 1300 μg/L in aquifer groundwater and up to 600 μg/L in surficial clay pore water. Groundwaters with high As concentrations are reducing with negligible dissolved O2 and high concentrations of Fe(II), NH 4 + , and dissolved organic C. Within near-surface environments, these conditions are most pronounced in sediments underlying permanent wetlands, often found within oxbow channels near the Mekong River. There, labile C, co-deposited with As-bearing Fe (hydr)oxides under reducing conditions, drives the reductive mobilization (inclusive of Fe and As reduction) of As. Here, conditions are described under which As is mobilized from these sediments, and near-surface As release is linked to aquifer contamination over long time periods (100s to 1000s of years). Site biogeochemistry is coupled with extensive hydrologic measurements, and, accordingly, a comprehensive interpretation of spatial As release and transport within a calibrated hydraulic flow-field is provided of an As-contaminated aquifer that is representative of those found throughout South and Southeast Asia.

Groundwater flow in an arsenic-contaminated aquifer, Mekong Delta, Cambodia by Shawn G. Benner; Matthew L. Polizzotto; Benjamin D. Kocar; Somenath Ganguly; Kongkea Phan; Kagna Ouch; Michael Sampson; Scott Fendorf (3072-3087).
To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km2 area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K  ≈ 10−4  ms−1) and overlying clay aquitard (K  ≈ 10−8  ms−1) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma−1 downward through the clay and 1–13 ma−1 horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer.

Enriched As in drinking water wells in south and Southeast Asia has increased the risk of cancer for nearly 100 million people. This enrichment is generally attributed to the reductive dissolution of Fe oxides; however, the complex expression of As enrichment in these areas is not yet well understood. Here, the coupled sedimentological and geochemical factors that contribute to the extent and spatial distribution of groundwater As concentrations in the Mekong River delta, Cambodia in an avulsed scroll bar sequence are examined. X-Ray absorption spectroscopy (XAS) was used to determine Fe and As speciation in redox preserved sediment collected from drilled cores. Dissolved As, Fe and S solution concentrations in existing and newly drilled wells (cores) differed considerably depending on their source sedimentology. The rapid burial of organic matter in the scroll bar sequence facilitated the development of extensive Fe-reducing conditions, and As release into the aquifer. In older features organic C levels are high enough to sustain extensive Fe reduction and provide ample SO4 which is reduced to sulfide. This S reduction impacts As levels; As is sequestered in sulfide minerals outside of the scrollbar sequence, decreasing pore water concentrations. In contrast, As is depleted in sediments from the scroll sequence, and associated with elevated pore water aqueous concentrations. The concentration and form of organic C in the scrollbar sequence is related to depositional environment, and can facilitate Fe and S mineral transformations, distinct sedimentary environments explain a portion of the inherent heterogeneity of aquifer As concentrations.

Controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the Red River flood plain, Vietnam by Flemming Larsen; Nhan Quy Pham; Nhan Duc Dang; Dieke Postma; Søren Jessen; Viet Hung Pham; Thao Bach Nguyen; Huy Duc Trieu; Luu Thi Tran; Hoan Nguyen; Julie Chambon; Hoan Van Nguyen; Dang Hoang Ha; Nguyen Thi Hue; Mai Thanh Duc; Jens Christian Refsgaard (3099-3115).
Geological and hydrogeological processes controlling recharge and the mobilization of As were investigated in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The geology was investigated using surface resistivity methods, geophysical borehole logging, drilling of boreholes and installation of more than 200 piezometers. Recharge processes and surface–groundwater interaction were studied using (i) time-series of hydraulic head distribution in surface water and aquifers, (ii) the stable isotope composition of waters and (iii) numerical groundwater modeling. The Red River and two of its distributaries run through the field site and control the groundwater flow pattern. For most of the year, there is a regional groundwater flow towards the Red River. During the monsoon the Red River water stage rises up to 6 m and stalls the regional groundwater flow. The two distributaries recharge the aquifer from perched water tables in the dry season, whilst in the flooding period surface water enters the aquifer through highly permeable bank sediments. The result is a dynamic groundwater flow pattern with rapid fluctuations in the groundwater table. A transient numerical model of the groundwater flow yields an average recharge rate of 60–100 mm/a through the confining clay, and a total recharge of approximately 200 mm/a was estimated from 3H/3He dating of the shallow groundwater. Thus in the model area, recharge of surface water from the river distributaries and recharge through a confining clay is of the same magnitude, being on average around 100 mm/a. The thickness of the confining clay varies between 2 and 10 m, and affects the recharge rate and the transport of electron acceptors (O2, NO 3 - and SO 4 2 - ) into the aquifer. Where the clay layer is thin, an up to 2 m thick oxic zone develops in the shallow aquifer. In the oxic zone the As concentration is less than 1 μg/L but increases in the reduced zone below to 550 μg/L. In the Holocene aquifer, As is mobilized at a rate of around 14 μg/L/a. An As mass balance for the field site shows that around 1100 kg of As is annually leached from the Holocene sand and discharged into the Red River, corresponding to 0.01% of the total pool of As now present in the Holocene sand.

Palaeo-hydrogeological control on groundwater As levels in Red River delta, Vietnam by Søren Jessen; Flemming Larsen; Dieke Postma; Pham Hung Viet; Nguyen Thi Ha; Pham Quy Nhan; Dang Duc Nhan; Mai Thanh Duc; Nguyen Thi Minh Hue; Trieu Duc Huy; Tran Thi Luu; Dang Hoang Ha; Rasmus Jakobsen (3116-3126).
To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta’s Quaternary geological development was reviewed. The water samples (n  = 30) were analyzed for As, major ions, Fe2+, H2S, NH4, CH4, δ18O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15–20 km wide and 50–60 m deep Holocene incised valley. The NE part of the transect consists of 60–120 m of Pleistocene yellowish alluvial deposits underneath 10–30 m of estuarine clay overlain by a 10–20 m veneer of Holocene sediments. The distribution of δ18O-values (range −12.2‰ to −6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe2+ (0.03–2.0 mM), Mn (0.7–320 μM), SO4 (<2.1 μM–0.75 mM), H2S (<0.1–7.0 μM), NH4 (0.03–4.4 mM), and CH4 (0.08–14.5 mM). Generally, higher concentrations of NH4 and CH4 and low concentrations of SO4 were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013–11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH4, CH4 and SO4. Low concentrations of As (⩽0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe2+ and H2S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19–65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO4 was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration.

Arsenic mobilisation in a new well field for drinking water production along the Red River, Nam Du, Hanoi by Jenny Norrman; Charlotte J. Sparrenbom; Michael Berg; Dang Duc Nhan; Pham Quy Nhan; Håkan Rosqvist; Gunnar Jacks; Emma Sigvardsson; David Baric; Johanna Moreskog; Peter Harms-Ringdahl; Nguyen Van Hoan (3127-3142).
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 HCO 3 - 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.

Geochemical processes underlying a sharp contrast in groundwater arsenic concentrations in a village on the Red River delta, Vietnam by Elisabeth Eiche; Thomas Neumann; Michael Berg; Beth Weinman; Alexander van Geen; Stefan Norra; Zsolt Berner; Pham Thi Kim Trang; Pham Hung Viet; Doris Stüben (3143-3154).
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 NH 4 + (∼10 mg/L), HCO 3 - (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 NH 4 + , HCO 3 - , P and DOC in the portion of the aquifer where groundwater As concentrations are also elevated.

Hydrological and geochemical constraints on the mechanism of formation of arsenic contaminated groundwater in Sonargaon, Bangladesh by Takaaki Itai; Harue Masuda; Ashraf A. Seddique; Muneki Mitamura; Teruyuki Maruoka; Xiaodong Li; Minoru Kusakabe; Biswas K. Dipak; Abida Farooqi; Toshiro Yamanaka; Shinji Nakaya; Jun-ichi Matsuda; Kazi Matin Ahmed (3155-3176).
The geochemical characteristics and hydrological constraints of high As groundwater in Sonargaon, in mid-eastern Bangladesh were investigated in order to ascertain the mechanism of As release into the groundwaters from host sediments in the Ganges–Brahmaputra delta. Samples of groundwater were collected from ca. 230 tube wells in both the rainy and dry seasons. Similar to previous studies, high As groundwater was found in the Holocene unconfined aquifer but not in the Pleistocene aquifer. Groundwaters in the Holocene aquifer were of the Ca–Mg–HCO3 type with major solutes derived from chemical weathering of detrital minerals such as plagioclase and biotite. Groundwater with high As was generally characterized by high NH 4 + , possibly derived from the agricultural application of fertilizer as suggested by the small variation of δ 15 N NH 4 (mostly 2–4‰). Concentrations of Fe changed between the rainy and dry seasons by precipitation/dissolution of Fe oxyhydroxide and siderite, whilst there was not an apparent concomitant change in As. Inhomogeneous spatial distribution of δ 18O in the Holocene unconfined aquifer indicates poor mixing of groundwater in the horizontal direction. Spatial variation of redox conditions is associated with localized variations in subsurface permeability and the recharge/discharge cycle of groundwater. Hydrogeochemical data presented in this paper suggest that reduction of Fe oxyhydroxide is not the only mechanism of As mobilization, and chemical weathering of biotite and/or other basic minerals in the Holocene aquifer could also be important as a primary cause of As mobilization.

Biogeochemical characterization of bacterial assemblages in relation to release of arsenic from South East Asia (Bangladesh) sediments by J. Akai; A. Kanekiyo; N. Hishida; M. Ogawa; T. Naganuma; H. Fukuhara; H.N. Anawar (3177-3186).
Arsenic release experiments using natural indigenous microbial assemblages and natural sediment samples in Bangladesh have been performed. The As release appears to be facilitated by moderate organic input. Addition of some nutrients caused reducing conditions, which may generate the appropriate environment for Fe-reducing bacteria to become active. Detailed cellular phospholipid fatty acid (PLFA) analysis suggests the presence of SO4-reducing and Fe-reducing bacteria in the sediments. These Fe-reducing bacteria may serve as the agents catalyzing As release in the organic-rich sediments. 16S rDNA analysis of one cultured sample suggests the presence of clostridia, some of which are known to mediate Fe reduction. Based on new PLFA analyses, it is proposed that combined microbial processes of SO4 reduction to generate anaerobic conditions and Fe reduction to co-reduce As are important biogeochemical factors for As release in the Bangladesh sediments.

A combined experimental study of vivianite and As (V) reactivity in the pH range 2–11 by V. Thinnappan; C.M. Merrifield; F.S. Islam; D.A. Polya; P. Wincott; R.A. Wogelius (3187-3204).
Four different sets of experiments were completed in order to constrain vivianite [Fe3(PO4)2  · 8H2O] reactivity under conditions pertinent to As(V)-bearing groundwater systems. Firstly, titration experiments were undertaken in the pH range 4-9 to determine the zero point of charge (ZPC) of vivianite; showing that the ZPC lies at a pH of approximately 5.3. Secondly, the steady state dissolution rates of vivianite far from equilibrium were measured in aqueous solutions in the pH range 2–10 at 18.5  °C (±3 °C) using a fluidized bed reactor. The rate of vivianite dissolution, R, is given by (1) R ( moles s - 1 cm - 2 ) = 1.18 × 10 - 10 a H + 0.77 + 1 × 10 - 15 + 6.92 × 10 - 24 a H + - 1 The dissolution rate exhibits an exponential increase with increase in the activity of the H+ ion (a H+) in solution at 2 < pH < 5, is apparently pH independent at 5 < pH < 8, and increases with increasing pH at pH > 8. Thirdly, the sorption of arsenate [As(V)] onto natural well-crystallized vivianite in the pH range 3–11 under static flow conditions was determined. 25–40% of As(V) from a starting concentration (C 0) of 100 μM was adsorbed onto vivianite. Static adsorption experiments were also completed at two lower As(V) concentrations (C 0  = 10 and 1 μM). Sorption was determined to be only weakly dependent on pH. Fourthly, the final part of this study investigated the sorption of As(V) onto vivianite at pH 9 under dynamic flow conditions. An input solution of 4 mM As(V) was applied to water saturated columns, followed by leaching with deionised water (DIW). Breakthrough curves show that the retention and exchangeability of As within the column is enhanced with vivianite present, consistent with solid phase analysis of unreacted and reacted solid materials. A simple calculation based on a model shallow Bengal sediment having about 0.2 wt% of vivianite and total initial dissolved As concentrations of 100 μM showed that under such conditions 88% of dissolved As(V) could potentially be adsorbed onto vivianite. These results will help to better understand As mobility in the presence of the Fe(II) mineral phase vivianite, thus providing better prediction of As mobility in partially reduced environments.

Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters by J.A. Saunders; M.-K. Lee; M. Shamsudduha; P. Dhakal; A. Uddin; M.T. Chowdury; K.M. Ahmed (3205-3214).
Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO4 to stimulate metabolism of indigenous SO4-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO4, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS2. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO4 reduction commenced but increased to pre-experiment level once SO4 reduction ended. In the ongoing experiment, both SO4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.In summary, when anaerobic bacterial metabolism is optimized by providing both electron donors and acceptors, As is mobile under Fe-reducing conditions, immobile under SO4-reducing conditions, and arsenian pyrite is the likely stable mineral phase formed under SO4-reducing conditions, instead of pure As–S phases such as realgar or orpiment.

Molecular and cultivation-dependent analysis of metal-reducing bacteria implicated in arsenic mobilisation in south-east asian aquifers by Marina Héry; Andrew G. Gault; Helen A.L. Rowland; Gavin Lear; David A. Polya; Jonathan R. Lloyd (3215-3223).
The reduction of sorbed As(V) to the potentially more mobile As(III) by As-respiring anaerobic bacteria has been implicated in the mobilisation of the toxic metalloid in aquifer sediments in SE Asia. However, there is currently only a limited amount of information on the identity of the organisms that can respire As(V) in these sediment systems. Here experiments are described that have targeted As(V)-respiring bacteria using cultivation-independent molecular techniques, and also more traditional microbiological approaches that have used growth media highly selective for organisms that can grow using arsenate as the sole electron acceptor supplied for anaerobic growth. The molecular techniques used have initially targeted DNA from microcosms displaying maximal rates of arsenate reduction, both with and without added electron donor. More recent studies from the authors’ laboratory have used stable isotope probing techniques, targeting DNA from the active microbial fraction in microcosms labelled with [13C]acetate supplied as an electron donor for arsenate reduction. Phylogenetic analyses using a highly conserved genetic marker (the 16S rRNA gene) have suggested the involvement of Sulfurospirillum and Geobacter species in arsenate-respiration, and this has been supported further by complimentary experiments using more traditional microbiological techniques. Additional research required to clarify the role of these organisms in the mobilisation of As in situ are discussed.

Considerations for conducting incubations to study the mechanisms of As release in reducing groundwater aquifers by Kathleen A. Radloff; Anya R. Manning; Brian Mailloux; Yan Zheng; M. Moshiur Rahman; M. Rezaul Huq; Kazi M. Ahmed; Alexander van Geen (3224-3235).
Microbial Fe reduction is widely believed to be the primary mechanism of As release from aquifer sands in Bangladesh, but alternative explanations have been proposed. Long-term incubation studies using natural aquifer material are one way to address such divergent views. This study addresses two issues related to this approach: (1) the need for suitable abiotic controls and (2) the spatial variability of the composition of aquifer sands. Four sterilization techniques were examined using orange-colored Pleistocene sediment from Bangladesh and artificial groundwater over 8 months. Acetate (10 mM) was added to sacrificial vials before sterilization using either (1) 25 kGy of gamma irradiation, (2) three 1-h autoclave cycles, (3) a single addition of an antibiotic mixture at 1× or (4) 10× the typical dose, and (5) a 10 mM addition of azide. The effectiveness of sterilization was evaluated using two indicators of microbial Fe reduction, changes in diffuse spectral reflectance and leachable Fe(II)/Fe ratios, as well as changes in P-extractable As concentrations in the solid phase. A low dose of antibiotics was ineffective after 70 days, whereas autoclaving significantly altered groundwater composition. Gamma irradiation, a high dose of antibiotics, and azide were effective for the duration of the experiment.Using gamma irradiation as an abiotic control, shallow grey sediment and groundwater from 3 closely spaced locations along a gradient of dissolved As concentrations (60–130–210 μg/L) in Bangladesh were incubated for 8 months with and without organic C addition (0.9 and 0.6 mM of acetate and lactate). Unexpectedly, levels of dissolved As (64 ± 68, 92 ± 70, 217 ± 68 μg/L) and P-extractable As (0.7 ± 0.2, 2.1 ± 0.5 and 2.0 ± 0.3 mg/kg) at each location were highly variable over the duration of the experiment and prevented the detection of the relatively small levels of As release that were anticipated. Maintenance of an adsorptive equilibrium with the P-extractable As concentrations seems to govern dissolved As variability. The sediment variability is attributed to natural patchiness in the distribution of aquifer properties rather than a sampling artifact. Sub-sampling a single batch of groundwater and aquifer solids over time can alleviate this problem to some extent, but the issue of the representativeness of particular samples remains.

Characterization of Fe(III) (hydr)oxides in soils near the Ichinokawa mine was conducted using X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies, and the structural changes were correlated with the release of As into pore-water. The Eh values decreased monotonically with depth. Iron is mainly present as poorly-ordered Fe(III) (hydr)oxides, such as ferrihydrite, over a wide redox range (from Eh = 360 to −140 mV). Structural details of the short-range order of these Fe(III) (hydr)oxides were examined using Mössbauer spectroscopy by comparing the soil phases with synthesized ferrihydrite samples having varying crystallinities. The crystallinity of the soil Fe (hydr)oxides decreased slightly with depth and Eh. Thus, within the redox range of this soil profile, ferrihydrite dominated, even under very reducing conditions, but the crystalline domain size, and, potentially, particle size, changed with the variation in Eh. In the soil–water system examined here, where As concentration and the As(III)/As(V) ratio in soil water increased with depth, ferrihydrite persisted and maintained or even enhanced its capacity for As retention with increased reducing conditions. Therefore, it is concluded that As release from these soils largely depends on the transformation of As(V) to As(III) rather than reductive dissolution of Fe(III) (hydr)oxide.

Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh, India, Vietnam, and Nepal by A. van Geen; K. Radloff; Z. Aziz; Z. Cheng; M.R. Huq; K.M. Ahmed; B. Weinman; S. Goodbred; H.B. Jung; Y. Zheng; M. Berg; P.T.K. Trang; L. Charlet; J. Metral; D. Tisserand; S. Guillot; S. Chakraborty; A.P. Gajurel; B.N. Upreti (3244-3251).
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.