Applied Geochemistry (v.77, #C)

Worldwide, West-Bengal is one of the areas most affected by elevated levels of arsenic in groundwater (50–3000 μg/l). This groundwater does not only endanger humans owing to its use as drinking water. More and above that, irrigation of rice paddies consumes huge quantities of arsenic contaminated groundwater. Consequently, arsenic accumulates in soil and endangers the nutrition chain via arsenic uptake by plants. Rice is one of the staple foods in this region. Lately, there is a considerable intensification of research on the fate of arsenic in affected agricultural systems with most of them resorting to bulk analytical methods. However, so far, knowledge on the μ-scale distribution of arsenic in soil and plants in such agricultural systems is rather limited.This case study combined μ-synchrotron studies on soil, rice root and rice grain from a rice paddy irrigated with groundwater containing about 519 μg/L As. The investigation of a soil aggregate has shown that As is mainly associated with Fe and is not equally distributed over the whole aggregate but occurs in local enrichments of few tens μm in size.In soil, As was mainly associated with Fe-(oxy)hydroxides. Rice root coatings consisted of a similar assemblage of arsenic bearing minerals. Furthermore the incorporation of soil matter in the coating could be shown. On μm-scale, As concentrations in rice root coatings showed an inhomogeneous, patchy distribution (100–2400 mg/kg; median 500 mg/kg) and correlated with Fe concentrations. Some small amounts of arsenic could also be detected in the interior of the root (3–60 mg/kg; median 21 mg/kg). In the rice grain, trace elements such as Zn and Cu were mainly enriched along the grain coating, while As in contrast showed the highest concentrations in the germ and some hot spots in the coating (up to 13 mg/kg). Thus, peeling of rice grain would remove some, but not all of the arsenic.
Keywords: Arsenic; West Bengal; Soil plant system; Synchrotron; As speciation;

On the scalability of hydrogeochemical factors controlling arsenic mobility in three major inland basins of P.R. China by Huaming Guo; Di Zhang; Ping Ni; Yongsheng Cao; Fulan Li; Yongfeng Jia; Hailong Li; Li Wan; Guangcai Wang (15-23).
High As groundwater has been widely found in inland basins of P.R. China, which has posed a serious health risk to the local residents. Although these inland basins experience the same arid to semiarid climate and are filled with Quaternary sediments, As concentrations show big variations. Three inlands basins have been investigated to characterize hydraulic conductivities and geochemistry of the groundwater and sediments, and evaluate their controls on dissolved As concentrations. Dissolved As concentrations ranged between <0.1 and 105 μg/L (average 27.8 μg/L), between <0.1 and 338 μg/L (average 94.0 μg/L), and between 0.33 and 857 μg/L (average 130 μg/L) in the Yinchuan basin (YC), the Songnen basin (SN), and the Hetao basin (HT), respectively. In the YC, although Fe and Mn concentrations are the highest, groundwater has the lowest As concentration. Ionically bound As fraction and strongly adsorbed As fraction of sediments are the highest in the HT and the lowest in the YC basin. Groundwater As is predominantly regulated by these mobilizable As forms in sediments. However, the predominant factors controlling groundwater As are dependent on the scale of the study area. At the average multi-basin scale, groundwater flushing evidently decreases groundwater As concentrations. At the individual scale of the sampling site, due to the similar groundwater flow rate, redox conditions are the key factor controlling groundwater As, with more As partitioned into groundwater of lower Eh values. Variations in these factors controlling groundwater As suggest that safe wells can be expected in the basins with high groundwater flow rates and at sites with higher groundwater Eh values.Display Omitted
Keywords: Redox; Multi-basin scale; Sequential extraction; Aquifer; Reductive dissolution;

Manganese redox buffering limits arsenic release from contaminated sediments, Union Lake, New Jersey by Alison R. Keimowitz; Brian J. Mailloux; Karen Wovkulich; Jennifer S. Harkness; James M. Ross; Steven N. Chillrud (24-30).
The sediments of Union Lake in Southern New Jersey are contaminated with arsenic released from the Vineland Chemical Company Superfund site 11 km upstream. Seasonal anoxia has been shown to release arsenic from sediments to similar lakes; this process was hypothesized as a major arsenic source to Union Lake. Data indicate, however, that releases of arsenic to bottom waters from the sediments or from pore waters within the sediments are relatively minor: bottom water arsenic concentrations reached ∼30 ppb (∼12 μM) at most, representing <13% of the dissolved arsenic content of the lake. Manganese concentrations increase more quickly and to higher levels than arsenic and iron concentrations; maximum [Mn] = ∼13 ppm (∼250 μM), maximum [Fe] = ∼6 ppm (∼120 μM). Incubation experiments support the hypothesis that manganese acts as a redox buffer and prevents large arsenic releases. Under the observed conditions, little of the arsenic in the water column is from contaminated bottom sediments. This study also suggests that arsenic release from sediment to lake water may be more important in lakes that remain anoxic more continuously.Display Omitted
Keywords: Arsenic; Lake; Sediment; Redox; Manganese;

Role of sulfur redox cycling on arsenic mobilization in aquifers of Datong Basin, northern China by Kunfu Pi; Yanxin Wang; Xianjun Xie; Teng Ma; Chunli Su; Yaqing Liu (31-43).
Sulfur redox cycling potentially exerts important influences on arsenic (As) fate in shallow groundwater systems. Hydrochemical and sediment geochemical analysis combined with thermodynamic modeling study were conducted at Datong Basin to elaborate the effects of sulfur redox cycling on As speciation and mobilization under a strongly reducing environment. Dissolved As and sulfide concentration in 32 groundwater samples with depths of 19–40 m below the land surface varied from 8 to 2700 μg/L and from <5 to 490 μg/L, respectively, while dissolved Fe(II) was relatively low ranging from <20 to 280 μg/L. The apparent co-increase in dissolved sulfide and As concentration, especially for samples with As content larger than 500 μg/L, indicates that sulfidogenesis may significantly contribute to the mobilization of As via sulfide-induced reduction of both As-bearing Fe(III) oxide minerals and As(V). Thermodynamic calculations indicate that groundwater As might be also thiolated in the presence of high-level sulfide, particularly to a large extent for As(V) speciation, instead of sequestration by As-sulfide precipitates. Results of sequential extraction and scanning electron microscopy array on sediments indicate the presence of Fe(II) sulfide mineral phases and an appreciable amount of co-existent As in the sediments, suggesting the precipitation of Fe(II) sulfides can restrict the build-up of dissolved Fe(II) and sequester As from groundwater, but not strongly enough, thereby lowering down As to a moderate level of about 500 μg/L. Thus, redox processes involving As, S and Fe species under sulfidic conditions as observed in Datong Basin not only facilitate the enrichment of As(III) species and As(V) potentially existing as thiolated species, but also the depletion of Fe(II) concentration in groundwater due to Fe(II) sulfide formation.
Keywords: Arsenic; Sulfidogenesis; Thioarsenic; Iron mineral transformation; Enrichment mechanism;

Migration and transformation of arsenic: Contamination control and remediation in realgar mining areas by Yang Wu; Xiao-yong Zhou; Mei Lei; Jun Yang; Jie Ma; Peng-wei Qiao; Tong-bin Chen (44-51).
Realgar is a mineral that contains high concentrations of arsenic (As). During the course of realgar mining and smelting, significant levels of As are released. Following a series of migrations and transformations, As can become the major soil, atmospheric, and water pollutant in realgar mining areas. In this study, the transformation principles and processes by physical, chemical, and biological reactions are reviewed. The causes of increasing As toxicity in realgar mining areas are thus clarified. The results of this study show that the migration processes, operating from source to atmosphere, water and soil, lead to contamination situations. On the basis of this finding, pollution control and remediation options are recommended.
Keywords: Realgar; Arsenic migration; Arsenic transformation; Contamination;

Mine waste acidic potential and distribution of antimony and arsenic in waters of the Xikuangshan mine, China by Jianwei Zhou; Mathews T. Nyirenda; Lina Xie; Yi Li; Baolong Zhou; Yue Zhu; Huilin Liu (52-61).
The Xikuangshan (XKS) mine in China has vast quantities of waste material and reported antimony (Sb) and arsenic (As) contamination of water in the mine area. This study estimated the potential of acid mine drainage (AMD) generation by waste material at XKS mine by using paste pH, acid base accounting and net acid generation geochemical static tests. Distribution of Sb and As in surface and groundwater in relation to mine waste AMD producing potential was also investigated. Thirty four (34) water samples and representative samples of three mine wastes from different periods (fresh, 10 and 50 years) were collected for this study: waste rock, smelting slag and tailings. The AMD prediction shows that waste rock (from 10 year period) is acid producing while the fresh mine waste had alkaline paste pH indicating the presence of reactive carbonates. Hence AMD generation may have occurred after a long time due to dissolution of carbonates. Water analysis found Sb with higher concentration than As with means of 3.74 mg/L and 0.19 mg/L respectively. Highest Sb and As concentrations were observed in the North mine along the water flow path from waste heaps and tailing pond; Mine water in the South mine also had elevated Sb and As concentrations. Mining activities at the XKS mine have accelerated Sb and As releases because of the disturbed natural equilibrium. Proper mine waste management and collection and treatment of outflow from the waste rock heaps and tailing ponds seem to be a promising mitigation options.
Keywords: Acid base accounting; Net acid generation; Paste pH; Arsenic; Antimony;

Comparative study on the hydrogeochemical environment at the major drinking water based arsenism areas by Jie Tang; Jianmin Bian; Zhaoyang Li; Yanmei Li; Wei Yang; Shuang Liang (62-67).
In major drinking water based arsenism areas of China, the hydrogeochemical environment is controlled by the geology, structure and depositional environments. The arsenism prevalence and degree are closely linked to local hydro-chemical environments and the form and valence of arsenic in the groundwater. In this paper, field trips, samples monitoring and comprehensive comparison methods, and hydrogeochemical environment are comparatively analyzed at five drinking water based arsenism areas of China comprising Taiwan, Xinjiang Uygur Autonomous Region, Inner Mongolia Autonomous Region, Shanxi and Jilin provinces. The arsenism areas, except Taiwan, are located in arid and semi-arid areas indicating that the climate condition is not the main factor that affects the groundwater arsenic enrichment. All arsenism areas are located in the center of sedimentary basins or relatively lower zones within the plains, and the groundwater for drinking is taken from the Cenozoic layer. The groundwater in Taiwan, Inner Mongolia and Shanxi arsenism areas are located in a complex organic-rich reductive environment, where not only high As (III) contents occur but also detectable levels of organic matter, humic acid and methyl arsine. In the groundwater samples of the Inner Mongolia arsenism area, many gases are detected, such as H2S, CH4, and NH3. Groundwater environments in Xinjiang Uygur Autonomous Region and Jilin province show oxidic conditions with As (V), and organic arsine is rarely or never detected.
Keywords: Drinking water; Arsenism; Hydrogeochemistry; As (III); Organic arsenic; Humic acid;

Geogenic As and Mo groundwater contamination caused by an abundance of domestic supply wells by Thomas Pichler; Carl E. Renshaw; Jürgen Sültenfuß (68-79).
Lacking a connection to a municipal water supply, each household in the municipality of Lithia, approximately 30 km southeast of Tampa, Florida (USA), is responsible for its own supply of drinking water, causing a high-density of private domestic supply wells (DSW) in this area. There, a multilayered aquifer system exists, which can be subdivided into three distinct hydro stratigraphic units, which are, from the top down: the Surficial Aquifer System (SAS), the Intermediate Aquifer System (IAS), and the Upper Floridan Aquifer System (UFA). Despite the relatively small area, the geochemical and hydrogeological setting in Lithia is complex, consisting of: i) extensive cyclical pumping in a municipal well field to the west, ii) large seasonal changes in hydraulic head, ii) multiple aquifers with different hydraulic heads, and iv) a large density of domestic supply wells (DSW). Within the zone of highest As concentrations, there are approximately 100 wells in an area of 2.5 km × 1.5 km. Most of these wells have large open screened intervals, often open to all three aquifers, allowing the downward flow of oxygenated and upward flow of anoxic groundwater. A survey of groundwater quality found that As and Mo concentrations in the DSW were up to 371 μg/L and 4740 μg/L, respectively. To obtain information about the individual aquifers, 5 well clusters with 4 monitoring intervals (approximately 50 m, 65 m, 80 m and 95 m below surface) and 8 push core wells (approximately 9 m below surface) were installed and sampled. In those wells, As and Mo were only elevated in a permeable layer within the IAS at a depth of 50 m. Values were up to 195 μg/L for As and up to 5050 μg/L for Mo. Using the tritium-helium (3H–3He) method, the ages of those samples high in As and Mo were determined to be 40, 30 and 30 years, respectively, while all other samples had ages older than 50 years. This indicated that mixing between young and old groundwater could be responsible for the high As and Mo concentrations. A good negative correlation for the whole data set was also observed between the concentration 3H and δ18O values, which together with hydrogeological modeling confirmed that the increased permeability created by the high density of DSW resulted in flow paths that permitted the perpetual mixing of shallow and deep groundwater. The release of the As and Mo appeared to be a consequence of changes to the physicochemical conditions in the aquifer, either via the introduction of oxygen-rich fluids into the IAS or the mixing of different fluids in the IAS or the introduction of oxygen-depleted fluids into the IAS. While the mobilization of geogenic trace metals is often associated with pumping-induced hydraulic gradient changes, we found that a certain density of multi-aquifer wells can be sufficient to alter hydrologic flow paths and induce the mobilization of geogenic trace metals even in the absence of significant pumping. In Lithia, the DSW effectively increased the local scale permeability of the aquifer, causing the mixing of oxygen-rich surface and deeper anoxic groundwater across a confining unit. Because the alteration to the hydrologic flow paths was a consequence of changes to the physical structure of the aquifer system rather than due to pumping, the alteration is not easily reversible, thus significantly complicating site remediation. Our results provide a cautionary warning of the risks of undue private DSW development in rapidly growing communities.Display Omitted
Keywords: Molybdenum; Floridan Aquifer; Arsenic; Groundwater; Domestic supply wells;

As a metalloid element, Arsenic (As) is widely distributed in the natural environment. Its ingestion can cause cancer, deformities, and mutations. Therefore, it has become an important environmental problem in recent years. There are large amounts of arsenic ore and coal with high arsenic content in Southwest China, a karst area. There, arsenic occurs in wastewater from mines contaminate soil, vegetation, and surface water. Karst underground aquifers are also contaminated through pipes, cracks, scuttles, and sinkholes, leading to a more serious arsenic pollution problem than in non-karst areas due to the unique karst hydrogeological conditions. To prevent and curb karst underground water contamination and guarantee water resource security and public health, a review on the arsenic contamination in the karst area is necessary. This paper discusses the progress of geochemical studies on arsenic. Through an analysis of the hydrogeology of karst areas, this paper proposes that studies on arsenic pollution in karst regions should be combined with the spatial distribution and redox characteristics of groundwater. More attention should be paid to chemical compositions of water, soil, and rocks as well as adsorption–desorption processes between water and sediment when conducting arsenic geochemical research in karst groundwater.
Keywords: Arsenic pollution; Geochemistry; Karst; Underground water;

Enrichment of arsenic (As) in sediment (12–227 mg/kg) in the upstream tributaries of the Indus and Brahmaputra Rivers originating from the Tibetan Plateau where hot springs were abundant has been found. Sandy sediment samples from the Nu-Salween River, Lantsang-Mekong River and Jinsha-Yangtze River also originating from the Tibetan Plateau were collected in July, 2012 and were found to contain 6.8 mg/kg to 30.5 mg/kg As (average17.3 ± 6.5 mg/kg, n = 12). Deposits collected within 1 m of two hot springs in Changdu, Tibet displayed significantly higher As levels: 263.7 mg/kg in LD-2 with 65% quartz and 101.8 mg/kg in NuD-1 with 82% calcite. To evaluate the valence states of As and also the phases responsible for sorption, X-Ray Absorption Spectroscopy (XAS) was employed to analyze the two hot spring deposits and three river sediment samples: coarse sand NuD-6 (As 14.6 mg/kg), fine sand NuD-4 (As 17.8 mg/kg), and silty sand LD-1 (As 30.5 mg/kg). The X-ray absorption near edge spectrum (XANES) data indicate that 70% of As from 3 samples in the Nu-Salween River drainage (NuD-1, NuD-4 and NuD-6) is As(V) or arsenate, with the rest being As(0) or As–Fe sulfides. The proportion of As(V) is 90% for 2 samples in the Lantsang-Mekong River drainage (LD-1 and LD-2). Linear combination fit of the iron extended X-ray fine structure spectrum (EXAFS) show that 3 samples from the Nu-Salween River contain 20% ferrihydrite and 10% goethite without any hematite being detected but 2 samples from the Lantsang-Mekong River contain <10% ferrihydrite, 20% goethite and with 30–60% of hematite. Concentrations of reductively leached As and Fe are correlated (Pearson correlation coefficient 0.673), with an average value of extracted As being 1.7 ± 0.6 mg/kg (n = 8) and 4.3 ± 2.0 mg/kg (n = 3) for the Nu-Salween and Lantsang-Mekong river, respectively. Parameters from the Langmuir isotherm fit to sorption experiments of As(III) and As(V) onto three river sediment samples were used to estimate “sorbed” As concentrations in river sediment in equilibrium with the average river water As concentrations. The “sorbed” As concentrations were 0.8 mg/kg and 2.8 mg/kg for the Nu-Salween and Lantsang-Mekong drainage, respectively. Taken together, the data suggest that this pool of “sorbed” As in river sand, likely to have a geothermal As component, remains largely particle-bound in the oxic and circumneutral riverine environment during transport; it is subject to mobilization once buried in the floodplain areas down gradient.
Keywords: X-ray Absorption Spectroscopy; Arsenic; Hot spring; Salween; Yangtze; Mekong;

Hydrothermal alteration of arsenopyrite by acidic solutions by Xiaoxiao Hu; Yuanfeng Cai; Yang Zhang (102-115).
In an effort to understand what occurred at the mineral-solution interface during the arsenopyrite (Apy) oxidation process, one arsenopyrite block was cut into 15 cubes (side length of approximately 2.5 mm), which were reacted with sulphuric acid at 100-300 °C in a reaction autoclave sealed by a metal screw-like vessel. The pH levels of the acid solutions are 0, 1, and 3. After the reaction, various techniques were used to investigate both the chemical, morphological and mineralogical changes. Among these techniques, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the remaining solutions after the reaction. Scanning electron microscopy (SEM) and Transmission Electron Microscopy (TEM) were used for the morphological analysis and mineral analysis of the products. The products at the residual solid surface were examined using X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA), the X-ray powder diffractometry (XRD) and micro-XRD test. The ICP-AES results indicate that a large amount of As moves into solution and that the total level of As ions in the solution significantly increased as the temperature and acid concentration increased. The secondary electron image (SEI) coupled with the backscattered electron image (BEI) of the morphology shows that the dissolution of arsenopyrite starts from the edges of the cube and fracture. Then, the action interface gradually migrates into the inside of the mineral cube. The XPS narrow scan of the residual solid surface indicates that Fe3+, S8, SO32-, SO42-, As3+, As5+ and a small amount of As1+ are generated at the surface, among which As3+ dominates. The XPS profile analysis of As 2p3/2 indicates that with an increase in the erosion depth from inner to the surface, the concentration of oxidized As increases; the reaction prompts arsenopyrite to oxidize and generates the oxidized state of As. And multiple-layer concentric banded structure may be reached according to the disappearance of every oxidized state of As. Due to the transfer of a large amount of oxidized As to the hydrothermal fluid from the As-containing minerals and rocks, the oxidation of arsenopyrite means that As would be released into hydrothermal fluid as well as to the superficial water and groundwater circulation.Display Omitted
Keywords: Arsenopyrite; Interfacial-reaction; Hydrotheral alteration; XPS;

Distribution of groundwater arsenic in Xinjiang, P.R. China by Yinzhu Zhou; Yanyan Zeng; Jinlong Zhou; Huaming Guo; Qiao Li; Ruiliang Jia; Yunfei Chen; Jiangtao Zhao (116-125).
Although potential contamination of groundwater As is expected to occur in Xinjiang, P.R. China, few data are available for the regional distribution of groundwater As. In this study, the spatial distribution of groundwater As was investigated in the Chepaizi (CPZ-N) and Shihezi (SHZ-N) areas of northern Xinjiang, the Balikun-Yiwu Basin (BY-E) in eastern Xinjiang, and the Tarim (TRM-S) and Yanqi (YQ-S) basins in southern Xinjiang. Arsenic concentrations greater than 10 μg/L were found in 12% of analyzed groundwaters. All groundwater samples collected in CPZ-N had As concentrations greater than 10 μg/L (25–185 μg/L), 30% in SHZ-N (<0.25–49 μg/L), 2.7% in BY-E, and 6.1% in TRM-S and YQ-S. No high As groundwater (As >10 μg/L) was found in the eastern and southern TRM-S and YQ-S. Distribution of groundwater As showed a tremendous spatial variability, which greatly varied over a short distance horizontally. Arsenic concentration generally increased with increasing sampling depth. The spatial distribution of groundwater As would be regulated by As source and hydrogeochemical processes. Higher pH and/or lower ORP values were generally observed in high As groundwater (>10 μg/L) in comparison with low As groundwater (<10 μg/L). Arsenic mobility in SHZ-N and CPZ-N may result from As desorption under relatively high pH conditions, and more tentatively from reductive dissolution of Fe(III) oxides in BY-E and TRM-S. However, detailed mechanisms of As mobilization in these regions need further investigation.
Keywords: High As groundwater; Aquifer; Xinjiang; Inland basin; Arsenic mobilization;

Mineralogy and geochemistry affecting arsenic solubility in sediment profiles from the shallow basin-fill aquifer of Cache Valley Basin, Utah by Xianyu Meng; R. Ryan Dupont; Darwin L. Sorensen; Astrid R. Jacobson; Joan E. McLean (126-141).
Elevated arsenic concentrations have been reported in groundwater samples collected from the semi-arid Western U.S., including the Cache Valley Basin, Utah. The volcanic rock in the basin-fill aquifers underlying portions of the West is considered the primary source of arsenic, but there is debate about the mechanisms that control arsenic solubilization in these semi-arid and arid climates. Sediment cores were collected from a shallow basin fill aquifer in the Cache Valley Basin to systematically determine arsenic mineralogy and solubilization mechanisms in relation to non-redox and redox induced soil processes. Soluble arsenic was present throughout the two studied profiles in varying abundance and oxidation state, with the highest concentration of soluble As(V) and As(III) at the depth of the water table. Sequential chemical extractions of arsenic, with oxidation preservation strategies, revealed mineral sources and sinks of arsenic vulnerable to altering redox conditions down the profile. Weathering of primary arsenic-bearing minerals resulted in soluble arsenic in the vadose zone. Once soluble arsenic was leached to the deeper profile, arsenic solubility was controlled by carbonate minerals that concentrate at the water table. In the zone with alternating oxidizing and reducing conditions, iron oxides became the controlling mineral phase. The association between arsenic and sulfides limited arsenic solubility at depths under permanent water saturation. Arsenic solubility was revealed to be controlled by a sequence of processes that prevail under different geochemical regimes down the profile.
Keywords: Arsenic; Carbonates; Semi-arid; Geochemistry; Sediment profile; Redox stratification;

Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh by Kathleen A. Radloff; Yan Zheng; Martin Stute; Beth Weinman; Benjamin Bostick; Ivan Mihajlov; Margaret Bounds; M.Moshiur Rahman; M.Rezaul Huq; Kazi M. Ahmed; Peter Schlosser; Alexander van Geen (142-157).
The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10–20 m depth range, groundwater ages along the transect determined by the 3H–3He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 μg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push–pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, Kd, of 0.15–1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100–1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources.Display Omitted
Keywords: Arsenic; Bangladesh; Adsorption; Hydrology; Groundwater age dating; Push–pull tests;

A study on soil-environmental quality criteria and standards of arsenic by Qixing Zhou; Yong Teng; Yang Liu (158-166).
Arsenic (As) originating from natural and anthropogenic sources is an ubiquitous metalloid in soils, and can pose potential risks on human health and ecosystems. It is of great practical significance to carry out this study to prompt development or revision of soil-environmental quality standards, further to efficiently control its risks and hazards. Firstly, we briefly introduced the researching background of As soil-environmental quality criteria and standards. Secondly, we summarized the methods of deriving As soil-environmental quality criteria, respectively, based on its soil-environmental background values, ecotoxicological effects, accumulation, exposure models and soil-water partition equations. Thirdly, we reckoned and assessed the quantitative or qualitative relationships of As soil-environmental quality standards in various countries and regions according to land uses, tiers and extractants. In addition, the variation on soil-environmental quality standards was generally analyzed. Finally, we prospected the future researches on soil-environmental quality criteria and standards of As.
Keywords: Arsenic; Soil; Environmental quality criteria; Environmental standard;

Evidence of decoupling between arsenic and phosphate in shallow groundwater of Bangladesh and potential implications by Z. Aziz; B.C. Bostick; Y. Zheng; M.R. Huq; M.M. Rahman; K.M. Ahmed; A. van Geen (167-177).
Reductive dissolution of iron oxyhydroxides and reduction of arsenic are often invoked as leading causes of high dissolved As levels in shallow groundwater of Bangladesh. The second of these assumptions is questioned here by comparing the behavior As and phosphate (P), a structural analog for As (V) which also adsorbs strongly to Fe oxyhydroxides but is not subject to reduction. The first line of evidence is provided by a detailed groundwater time-series spanning two years for three wells in the 6–9 m depth range showing removal of As(III) from shallow groundwater during the monsoon without of loss of P. The data indicate a loss of ∼90% of the dissolved As from groundwater in the intermediate well relative to a level of 3 μmol/L As predicted by conservative mixing between groundwater sampled from the shallower and the deeper well. In contrast, P concentrations of ∼30 μmol/L in the intermediate well closely match the prediction from conservative mixing. Reduction therefore appears to inhibit the release of As to groundwater at this site relative to P instead of enhancing it. A re-analysis of existing groundwater As and P data from across the country provides a broader context for this finding and confirms that, without reduction, elevated concentrations of As would probably be even more widespread in shallow aquifers of Bangladesh. Without providing definite proof, X-ray absorption spectroscopy of sediment from the time-series site and elsewhere suggests that the loss of As from groundwater may be coupled to precipitation of As sulfide. Further study is needed to assess the implications of these observations for shallow aquifers that have been subjected to increased withdrawals for irrigation in recent decades.
Keywords: Groundwater; Arsenic; Bangladesh;

Recently, a statistical risk model that classifies safe and unsafe areas with respect to geogenic groundwater arsenic contamination in China has been developed. Modeling results indicate that most areas in the Huai River Basin (HRB) are associated with a considerable probability of human exposure to elevated levels of arsenic (>10 μg/L) in groundwater. The high mortality and incidence rates of cancer in the HRB have been widely reported. The predicted high probability of arsenic hazards in this region might suggest a new hypothesis that groundwater arsenic contamination contributes to this cancer cluster. Therefore, we analyzed an arsenic concentration dataset, including 69,702 groundwater samples from 2357 villages in 70 counties, in the HRB to determine whether the region was affected by arsenic. Our findings indicate that groundwater consumed in 408 (17%) villages and 41 (59%) counties contains high levels of arsenic (>10 μg/L). Coincidentally, in some counties with severe groundwater contamination people have been found to exhibit high cancer mortality rates. Furthermore, the model correctly classified 57% of the villages as high- and low-risk areas (0.4 probability cut-off threshold). The model sensitivity (ability to correctly classify villages with arsenic concentrations of >10 μg/L) and specificity (ability to correctly classify villages with arsenic concentrations of ≤10 μg/L) at the village-level were 61% and 56%, respectively. Collectively, our findings confirmed that the HRB was affected by arsenic and suggested, at least to a certain extent, that groundwater arsenic contamination is likely associated with the HRB cancer cluster. Additionally, the data indicated that the model was well-suited to predicting high-risk arsenic exposure areas at the village-level and can thus be used as a tool to prioritize sampling in villages showing high risk.
Keywords: Huai River Basin; Cancer; Arsenic; Predictive risk model; Model accuracy;

Origin and availability of organic matter leading to arsenic mobilisation in aquifers of the Red River Delta, Vietnam by Elisabeth Eiche; Michael Berg; Sarah-Madeleine Hönig; Thomas Neumann; Vi Mai Lan; Thi Kim Trang Pham; Hung Viet Pham (184-193).
Groundwater arsenic (As) concentrations in the Red River Delta (Vietnam) are often patchy and related to the microbially induced reduction of Fe oxy-hydroxides. In this study, we explored the influence of the origin, composition and availability of natural organic matter on the hydrochemical variability in the aquifers of Van Phuc. Carbon isotope signatures (δ13Corg) and C/N ratios were assessed in combination with lithology, geochemistry, hydrochemistry, hydrology and the distribution of specific biomarkers.The elationship of C/N ratios and δ13Corg distinguished four groups of sediment types that differ in their organic carbon sources. This includes organic carbon originating predominantly from vascular C3 plants (C/N: 15.4–21.0, δ13Corg: −28.6 to −26.7‰), C4 plants (C/N: 10.6; δ13Corg: −14.8‰), freshwater derived particulate organic carbon (C/N: ≤8; δ13Corg:≤−24‰) as well as mixtures incorporating both sources. At the high As sites, we found particulate organic carbon (POC) being 1–2‰ less depleted in δ13Corg than at low As sites. More importantly, however, our assessment shows that, the availability of organic matter has to be considered decisive with regard to groundwater As contamination. Fine-grained clayey sediments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer. However, at the sites that are high in dissolved As in Van Phuc, sediment layers rich in organic matter are hydraulically connected to the underlying aquifer. Here, soluble organic matter seeping into the aquifer can induce and/or enhance reducing conditions, thereby mobilising As from Fe oxy-hydroxides. Our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments.
Keywords: Arsenic; Vietnam; Organic matter; δ13Corg; C/N-ratio;

The discontinuous nature of elevated arsenic (As) in drinking water wells of West Bengal and other regions in the Bengal Basin has led to increased interest in the role that sediment-derived organic matter may play in enhancing reductive dissolution and As mobilization. Higher As concentrations have been observed in groundwater in reduced Holocene (grey) aquifers when compared to oxidized Pleistocene (orange) aquifers. In order to evaluate if the differences in the chemical character of dissolved organic matter (DOM) are present in groundwater in the Holocene and the Pleistocene aquifers that may influence dissolved As concentrations, shallow groundwater and surface water samples were collected from four study sites in Murshidabad district, West Bengal, India, and analyzed for water chemistry parameters and characteristics of DOM. For wells known to typically contain high As concentrations (in Holocene sediments) in Beldanga (10–4622 μg/L, at 35–45 m depth) and Hariharpara (5–695 μg/L, at 6–37, depth) sites, as well as wells characterized by low As concentrations (Pleistocene sediiments) in Nabagram (0–16 μg/L, at 20–45 m depth) and Kandi (5–50 μg/L, at 20–55 m depth), detailed DOM characterization was carried out using fluorescence spectroscopy and parallel factor analysis (PARAFAC). Results from statistical analysis of a variety of optical (absorbance and fluorescence) DOM properties revealed that the DOM in groundwater in the Holocene aquifer had high humification index (HIX) and low freshness index (β:α) values, whereas groundwater in the Pleistocene aquifer comprised more labile and microbial DOM sources. Consistent with the more labile nature of DOM in groundwater in the Pleistocene aquifer, two ratios 1) humic-like to protein-like components (humic:protein) and 2) terrestrially-derived to microbially-derived components (terr:microb) obtained from a four-component PARAFAC model were 1.9 and 2.9 times greater, respectively, in groundwater in the Holocene aquifer than in that of the Pleistocene aquifer, which suggests that the absence of humic-like DOM may be an important limitation to As mobility.Contrasting DOM quality in groundwater in the Holocene and the Pleistocene aquifers.Display Omitted
Keywords: Dissolved organic matter characterization; Fluorescence spectroscopy; Parallel factor analysis (PARAFAC); Groundwater arsenic;

Effective treatment of arsenic-bearing water by a layered double metal hydroxide: Iowaite by Qinghai Guo; Yaowu Cao; Yaqin Zhuang; Yijun Yang; Mindai Wang; Yanxin Wang (206-212).
Iowaite with designed Mg/Fe ratios of 2.5, 3.0, 3.5, 4.0 and 5.0 was synthesized for treating both arsenate- and arsenite-spiked waters in this study. The experimental results show that there is a positive correlation between the molar ratio of Fe to Mg of synthetic iowaite and its arsenic uptake capacity. The dearsenication mechanisms were investigated based on the XRD, SEM and EDX analyses of solid samples before and after reaction with arsenic-bearing solutions. When the initial solution concentration of arsenic is not higher than 7500 μg/L, anion exchange between arsenate or arsenite in solution and chloride in the interlayer regions of iowaite is the primary mechanism for arsenic removal. However, at a very high initial arsenic concentration in solution (750 mg/L), the dissolution of iowaite and subsequent formation of new minerals, such as scorodite and karibibite, are responsible for the substantial removal of arsenic from water. Iowaite is superior to other types of layered double metal hydroxides (LDHs) such as hydrotalcite and hydrocalumite in terms of water dearsenication, and therefore promising for treatment of arsenic-rich waters, either naturally occurring waters or industrial wastewaters.
Keywords: Arsenic; LDHs; Iowaite; Anion exchange; Dissolution-reprecipitation;