Mobilization of natural arsenic in groundwater: targeting low arsenic aquifers in high arsenic occurrence areas

研究表明饮用水中微小数量的砷会对人类健康产生不利影响.世界上居住在贫穷地区的人数超过了100万,目前他们正直接饮用来自含水层中砷离子含量(>10μg/L)非安全标准的地下水.砷有时称为毒中之王,在水环境中常常以五价氧化物形式出现.自2000年以来,许多国家开始执行更为严格的10μg/L(WHO认可的居民安全饮水标准)饮用水标准,可以确定地说,在世界范围内的饮用水中检测到砷的情况越来越多.亚洲地区砷中毒的人数比世界其他地区总和还多.最受影响的地区位于南亚和东南亚富砷带,环绕恒河一雅鲁藏布江-梅克纳河三角洲及恒河平原上游的冲洪积扇含水层、红河三角洲、湄公河和伊洛瓦底江;在中国境内包括内蒙黄河冲积盆地,山西大同盆地、新疆准噶尔盆地,其中的地下水富含砷和氟化物而引发砷中毒和氟中毒.尽管还未完全掌握其中的水文地质及生物地球化学作用的详细过程,但对大多数沉积介质含水层,在还原条件下砷离子容易从沉积介质转移到地下水中.孟加拉研究实例表明地质时期尺度的冲刷降低了沉积介质中的As和有机物含量从而形成低砷地下水.这一认识为孟加拉国的降砷策略提供了科学指导,是未来水文学,矿物学,地质学和生物地球化学方面很有意义的研究方向,并有利于地砷病区低砷地下水的可持续利用.     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part I: Comparative hydrochemical and hydrogeological characteristics

The present study demonstrates the importance of hydrogeochemical characteristics (groundwater flow and recharge) of an aquifer in the release of As to groundwater. The study area (∼20 km²) is located in Chakdaha block, Nadia district, West Bengal, which hosts groundwaters of variable As content. The spatial distribution pattern of As is patchy with areas containing groundwater that is high in As (>200 μg L⁻¹) found in close vicinity to low As (<50 μg L⁻¹) groundwaters (within 100 m). The concentration of groundwater As is found to decrease with depth. In addition, the data shows that there is no conspicuous relationship between high groundwater As concentration and high groundwater abstraction, although the central cone of depression has enlarged over 2 a and is extending towards the SE of the study area. The river Hooghly, which forms the NW boundary of the study site, shows dual behaviour (effluent and influent during pre- and post-monsoon periods, respectively), complicating the site hydrogeology. The observed groundwater flow lines tend to be deflected away from the high As portion of the aquifer, indicating that groundwater movement is very sluggish in the As-rich area. This leads to a high residence time for this groundwater package, prolonging sediment–water interaction, and hence facilitating groundwater As release.     

Groundwater pollution by arsenic concentration in sedimentary aquifers in the Indo-Gangetic Basin of Nepal

Naturally occurring arsenic （As） in groundwater of sedimentary aquifers has emerged as a global problem, and the Indo-Gangetic Basin of Nepal is no exception. In the sedimentary aquifers of the Indo-Gangetic Basin, arsenic enrichment and mobilization in groundwater by natural processes is considered an issue of major environmental health concern. About 11 million people, 47% of Nepali＇s total population living in the region and approximately 90% of them are relying on groundwater as their major source of drinking water. Moreover, 205555 ha of agricultural land in the basin are irrigated by groundwater through large numbers of deep and shallow irrigation tubewells for extensive cultivation of crops which has lead to creation of other pathways for arsenic ingestion among the people living in the region. It is estimated that around 0.5 million people in the basin are living at risk of arsenic poisoning In addition, some recent research shows the prevalence of dermatosis related to arsenicosis from 1.3% to 5.2 % and the accumulation of arsenic in biological samples like hair and nails much higher than the acceptance level. In these circumstances, the source of the natural arsenic concentration in groundwater of the Indo-Gangetic Basin is quite essential to explore. Therefore, to investigate the source of arsenic and management of arseniferious groundwater used for human consumption and irrigation, along with treatment or avoidance, is clearly an issue of research concern. Present research is carried out to understand the source of natural arsenic and its mobilization in groundwater of the Indo-Gangetic Basin. For this, we have collected bedrock, aquifer sediments and water samples from the study area. Sediment samples were taken from different lithofacies viz. clay, silt, sand and sandy gravels from groundwater aquifer where arsenic concentration is high. Water samples were taken from domestic shallow and deep tubewells in grid pattern. Bedrock samples were collected from source of the aquifer sediment, northern part of the Indo-Gangetic Basin, especially from the Siwalik and Lesser Himalaya. This paper particularly focuses on geology, mineralogy, hydro-geochemistry and sedimentary analysis of the high arsenic affected districts; Rautahat, Sarlahi, Parsa, and Kailali of Nepal.     

Vertical distribution and mobilization of arsenic in shallow alluvial aquifers of Chapai-Nawabganj district, Northwestern Bangladesh

Core sediments from two boreholes and groundwater from fifty four As-contaminated well waters were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10 to 40 m of depth) range from 2.76?C315.15 mg/l (average 48.81 mg/l). Arsenic concentration in sediments ranges from 3.26?C10 mg/kg. Vertical distribution of arsenic in both groundwater and sediments shows that maximum As concentration (462 mg/l in groundwater and 10 mg/kg in sediments) occurs at a depth of 24 m. In January 2008, 2009 and 2010, maximum As concentration occurs at the same depth. Environmental scanning electron microscope (ESEM) with EDAX was used to investigate the presence of major and trace elements in the sediments. The dominant groundwater type is Ca-HCO₃ with high concentrations of As and Fe, but with low levels of NO₃ ^? and SO₃ ^?2. Statistical analysis clearly shows that As is closely associated with Fe (R² = 0.64) and Mn (R² = 0.91) in sediments while As is not correlated with Fe and Mn in groundwater samples. Comparatively low Fe and Mn concentrations in some groundwater, suggest that probably siderite and/or rhodochrosite precipitated as secondary mineral on the surface of the sediment particles. The correlations along with results of sequential leaching experiments suggest that reductive dissolution of FeOOH and MnOOH mediated by anaerobic bacteria represents mechanism for releasing arsenic into the groundwater.     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part II: Comparative geochemical profile and leaching study

Aquifer sediments from areas of low- and high-As groundwater were characterized mineralogically and geochemically at a field site in the Nadia district of West Bengal, India. Leaching experiments and selective extraction of the sediments were also carried out to understand the release mechanism of As in the sub-surface. The correlation between measured elements (major, minor and trace) from low- and high-As groundwater areas are only significant for As, Fe and Mn. The borehole lithology and percentage of silt and clay fraction demonstrates the dominance of finer sediments in the high-As aquifer. Multivariate analysis of the geochemical parameters showed the presence of four different mineral phases (heavy-mineral fraction, phyllosilicates/biotite/Fe-oxyhydroxides, carbonates and sulphides) in the sediments. Selective extraction of sediment reveals that amorphous Fe-oxyhydroxide acts as a potential sink for As in the sub-surface. The result is consistent with microbially mediated redox reactions, which are controlled in part by the presence of natural organic matter within the aquifer sediments. The occurrences of As-bearing redox traps, primarily formed of Fe- and Mn-oxides/hydroxides, are also important factors that control the release of As into groundwater at the study site.     

澜沧江—湄公河流域跨界含水层研究

跨界含水层由位于不同国家的含水层组成,是地下水资源系统的重要组成部分。妥善管理并公平合理地利用跨界含水层,对促进可持续发展及国家之间的睦邻友好关系有着重要意义。通过分析澜沧江—湄公河流域水文地质条件,划定流域的跨界含水层系统,并对地下水资源与环境问题进行探索,提出跨界含水层综合评价指标体系,结合澜沧江—湄公河流域4个跨界含水层相关的指标进行了综合评价。本次研究,对管理澜沧江—湄公河流域跨界含水层,建设和谐的东南亚国际环境有着重要意义。     

Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin,Inner Mongolia

Observed As concentrations in groundwater from boreholes and wells in the Huhhot Basin of Inner Mongolia, northern China, range between <1 μg l⁻¹ and 1480 μg l⁻¹. The aquifers are composed of Quaternary (largely Holocene) lacustrine and fluvial sediments. High concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells (well depths ranging between <10 m and 400 m). Populations from the affected areas experience a number of As-related health problems, the most notable of which are skin lesions (keratosis, melanosis, skin cancer) but with internal cancers (lung and bladder cancer) also having been reported. In both the shallow and deep aquifers, groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low-lying central part of the basin. High As concentrations occur in anaerobic groundwaters from this low-lying area and are associated with moderately high dissolved Fe as well as high Mn, NH₄, dissolved organic C (DOC), HCO₃ and P concentrations. Many of the deep groundwaters have particularly enriched DOC concentrations (up to 30 mg l⁻¹) and are often brown as a result of the high concentrations of organic acid. In the reducing groundwaters, inorganic As(III) constitutes typically more than 60% of the total dissolved As. The highest As concentrations tend to be found in groundwater with low SO₄ concentrations and indicate that As mobilisation occurs under strongly reducing conditions, where SO₄ reduction has been an active process. High concentrations of Fe, Mn, NH₄, HCO₃ and P are a common feature of reducing high-As groundwater provinces (e.g. Bangladesh, West Bengal). High concentrations of organic acid (humic, fulvic acid) are not a universal feature of such aquifers, but have been found in groundwaters from Taiwan and Hungary for example. The observed range of total As concentrations in sediments is 3–29 mg kg⁻¹ (n=12) and the concentrations correlate positively with total Fe. Up to 30% of the As is oxalate-extractable and taken to be associated largely with Fe oxides. The release of As into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the Fe oxide minerals. The association of dissolved As with constituents such as HCO₃, DOC and P may be a coincidence related to the prevalent reducing conditions and slow groundwater flow, but they may also be directly involved because of their competition with As for binding sites on the Fe oxides. The Huhhot groundwaters also have some high concentrations of dissolved U (up to 53 μg l⁻¹) and F⁻ (up to 6.8 mg l⁻¹). In contrast to As, U occurs predominantly under the more oxidising conditions along the basin margins. Fluoride occurs dominantly in the shallow groundwaters which have Na and HCO₃ as the dominant ions. The combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved As concentrations observed as the sediments are likely to contain newly-formed and reactive minerals and have not been well flushed since burial.     

Understanding arsenic behavior in carbonate aquifers: Implications for aquifer storage and recovery (ASR)

Geochemical reactive transport modeling was coupled to bench-scale leaching experiments to investigate and verify the mobilization of geogenic arsenic (As) under a range of redox conditions from an arsenic-rich pyrite bearing limestone aquifer. Modeling and experimental observations showed similar results and confirmed the following: (1) native groundwater and aquifer matrix, including pyrite, were in chemical equilibrium, thus preventing the release of As due to pyrite dissolution under ambient conditions; (2) mixing of oxygen- and nitrate-rich surface water with oxygen-depleted native groundwater changed the redox conditions and promoted the dissolution of pyrite, and (3) the behavior of As along a flow path was controlled by a complex series of interconnected reactions. This included the oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed hydrous ferric oxides (HFO), followed by the reductive dissolution of HFO and secondary release of adsorbed As under reducing conditions. Arsenic contamination of drinking water in these systems is thus controlled by the re-equilibration of the system to more reducing conditions rather than a purely oxidative process.     

Spatial relationship of groundwater arsenic distribution with regional topography and water-table fluctuations in the shallow aquifers in Bangladesh

The present study has examined the relationship of groundwater arsenic (As) levels in alluvial aquifers with topographic elevation, slope, and groundwater level on a large basinal-scale using high-resolution (90 m × 90 m) Shuttle Radar Topography Mission (SRTM) digital elevation model and water-table data in Bangladesh. Results show that high As (>50 μg/l) tubewells are located in low-lying areas, where mean surface elevation is approximately 10 m. Similarly, high As concentrations are found within extremely low slopes (<0.7°) in the country. Groundwater elevation (weekly measured by Bangladesh Water Development Board) was mapped using water-table data from 950 shallow (depth <100 m) piezometers distributed over the entire country. The minimum, maximum and mean groundwater elevation maps for 2003 were generated using Universal Kriging interpolation method. High As tubewells are located mainly in the Ganges–Brahmaputra–Meghna delta, Sylhet Trough, and recent floodplains, where groundwater elevation in shallow aquifers is low with a mean value of 4.5 m above the Public Works Datum (PWD) level. Extremely low groundwater gradients (0.01–0.001 m/km) within the GBM delta complex hinder groundwater flow and cause slow flushing of aquifers. Low elevation and gentle slope favor accumulation of finer sediments, As-carrying iron-oxyhydroxide minerals, and abundant organic matter within floodplains and alluvial deposits. At low horizontal hydraulic gradients and under reducing conditions, As is released in groundwater by microbial activity, causing widespread contamination in the low-lying deltaic and floodplain areas, where As is being recycled with time due to complex biogeochemical processes.     

Electrical resistivity investigation of the arsenic affected alluvial aquifers in West Bengal,India: usefulness in identifying the areas of low and high groundwater arsenic

This study aims at finding out possible relation between lithology and spatial pattern of dissolved arsenic (As) in groundwater around Chakdaha municipality, West Bengal, India. Satellite image, coupled with electrical resistivity survey and borehole drilling helps to delineate surface and sub-surface lithological framework of the As affected alluvial aquifers. The satellite imagery demonstrate that the high As area are presumably under active flood plain environment (low-lying areas), that constantly receive organics due to periodic flooding. Thick low resistive (fine-grained) layer was observed at the top around the high As areas, which, however, not found in low As areas. The result suggests that hydraulic properties of the surface/sub-surface soil/sediment have an important control on the fate and transport of As in the aquifer. This study demonstrates that electrical resistivity tools can be effectively used for the reconnaissance survey in characterizing the plausible lithological framework of an alluvial aquifer containing As.     

Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh

Background  

The origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh.     

Seasonal disparity in the co-occurrence of arsenic and fluoride in the aquifers of the Brahmaputra flood plains,Northeast India

Arsenic (As) and fluoride (F^?) in groundwater are increasing global water quality and public health concerns. The present study provides a deeper understanding of the impact of seasonal change on the co-occurrence of As and F^?, as both contaminants vary with climatic patterns. Groundwater samples were collected in pre- and post-monsoon seasons (n = 40 in each season) from the Brahmaputra flood plains (BFP) in northeast India to study the effect of season on As and F^? levels. Weathering is a key hydrogeochemical process in the BFP and both silicate and carbonate weathering are enhanced in the post-monsoon season. The increase in carbonate weathering is linked to an elevation in pH during the post-monsoon season. A Piper diagram revealed that bicarbonate-type water, with Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations, is common in both seasons. Correlation between Cl^? and NO₃ ^? (r = 0.74, p = 0.01) in the post-monsoon indicates mobilization of anthropogenic deposits during the rainy season. As was within the 10 µg L^?1 WHO limit for drinking water and F^? was under the 1.5 mg L^?1 limit. A negative correlation between oxidation reduction potential and groundwater As in both seasons (r = ?0.26 and ?0.49, respectively, for pre-monsoon and post-monsoon, p = 0.05) indicates enhanced As levels due to prevailing reducing conditions. Reductive hydrolysis of Fe (hydr)oxides appears to be the predominant process of As release, consistent with a positive correlation between As and Fe in both seasons (r = 0.75 and 0.73 for pre- and post-monsoon seasons, respectively, at p = 0.01). Principal component analysis and hierarchical cluster analysis revealed grouping of Fe and As in both seasons. F^? and sulfate were also clustered during the pre-monsoon season, which could be due to their similar interactions with Fe (hydr)oxides. Higher As levels in the post-monsoon appears driven by the influx of water into the aquifer, which drives out oxygen and creates a more reducing condition suitable for reductive dissolution of Fe (hydr)oxides. An increase in pH promotes desorption of As oxyanions AsO₄ ^3? (arsenate) and AsO₃ ^3? (arsenite) from Fe (hydr)oxide surfaces. Fluoride appears mainly released from F^?-bearing minerals, but Fe (hydr)oxides can be a secondary source of F^?, as suggested by the positive correlation between As and F^? in the pre-monsoon season.     

Exploring sustainability of aquifers based on predictive modeling of sorption characteristics of arsenic enriched Holocene sediments in Bangladesh

The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (R_r) of 1.42 × 10⁻¹⁶ mol/m²/s was derived for the shallower aquifer from the inverse mass-balance model. The high R_r may enhance As release processes in the upper sediment. The field-based reaction rate (K_r) was extrapolated to be roughly 1.23 × 10⁻¹³ s⁻¹ and 6.24 × 10⁻¹⁴ s⁻¹ for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment-water interface (K_d-As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for K_d-As (R² = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating K_d and K_r showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.     

The sources of geogenic arsenic in aquifers at Datong basin,northern China: Constraints from isotopic and geochemical data

Mineralogical, geochemical and zircon U–Pb dating studies were carried out to identify the sources of arsenic in the shallow aquifers of Datong Basin in northern China. A sediment sample from 18 m depth containing 10.3 mg/kg arsenic showed a Zircon U–Pb concordant age of 2528 ± 20 to 271 ± 4 Ma that can be divided into two groups (2528 ± 20 to 1628 ± 21 Ma and 327 ± 4 to 271 ± 4 Ma) and is comparable to that of the sedimentary rocks of Taiyuan (upper Carboniferous) and Shanxi Formation (lower Permian) outcropping to the west of Datong Basin. In contrast, a sediment sample from 22.5 m depth containing 5.7 mg/kg arsenic displayed a Zircon U–Pb concordant age ranging from 2561 ± 21 to 1824 ± 26 Ma that is comparable to that of the Hengshan Complex (Ne-Archaean Precambrian) outcropping to the east of .     

A geologically-based approach to map arsenic risk in crystalline aquifers:Analysis of the Tampere region,Finland

The study illustrates the critical role of accurate geological structural mapping to delineate crystalline aquifer zones more prone to high health risk due to elevated dissolved As in drinking wells.The analysis revisits the results from more than 1200 groundwater samples collected over ten years from domestic wells across the Tampere region(Finland).It is demonstrated that the highest dissolved As concentrations in the region(up to 2230 μg/L)are exclusively found near major faults and deformation zones(FDZs)detected via geophysical and geological surveys,and that a clear correlation exists between dissolved concentrations and the distance from the FDZs(r).Almost all values exceeding the drinking water limit(10 μg/L)occur at r8 km,while concentrations above 100 μg/L occur at r4 km.Solidphase As concentrations in bedrock show less dependency on FDZ than aqueous concentrations.This behavior is explained considering different mechanisms,which include enhanced sulfide oxidation and fracture connectivity,promoting preferential transport of dissolved As to FDZs and mixing of waters from different redox zones,mobilizing preferentially As(Ⅲ)or As(V).Fe hydro-oxides may also precipitate/dissolve preferentially because of FDZs,while residence time may influence the contact time between water and As-bearing minerals.It is concluded that the accurate mapping of FDZs,and in general of structural geology,provides an important preliminary information to identify where localized,sitespecific characterization of hydrogeology and geochemistry is more urgent to reduce As-related health risk from groundwater intake.     

Dynamics of arsenic adsorption in the targeted arsenic-safe aquifers in Matlab, south-eastern Bangladesh: Insight from experimental studies

Targeting shallow low-As aquifers based on sediment colour may be a viable solution for supplying As-safe drinking water to rural communities in some regions of Bangladesh and West Bengal in India. The sustainability of this solution with regard to the long-term risk of As-safe oxidized aquifers becoming enriched with As needs to be assessed. This study focuses on the adsorption behaviour of shallow oxidized sediments from Matlab Region, Bangladesh, and their capacity to attenuate As if cross-contamination of the oxidized aquifers occurs. Water quality analyses of samples collected from 20 tube-wells in the region indicate that while there may be some seasonal variability, the groundwater chemistry in the reduced and oxidized aquifers was relatively stable from 2004 to 2009. Although sediment extractions indicate a relatively low amount of As in the oxidized sediments, below 2.5 mg kg⁻¹, batch isotherm experiments show that the sediments have a high capacity to adsorb As. Simulations using a surface complexation model that considers adsorption to amorphous Fe(III) oxide minerals only, under-predict the experimental isotherms. This suggests that a large proportion of the adsorption sites in the oxidized sediments may be associated with crystalline Fe(III) oxides, Mn(IV) and Al(III) oxides, and clay minerals. Replicate breakthrough column experiments conducted with lactose added to the influent solution demonstrate that the high adsorption capacity of the oxidized sediments may be reduced if water drawn down into the oxidized aquifers contains high levels of electron donors such as reactive dissolved organic C.     

Manganese-reducing Pseudomonas fluorescens-group bacteria control arsenic mobility in gold mining-contaminated groundwater

Previous studies show the importance of iron- and arsenate-reducing bacteria in mobilizing arsenic in groundwater. Here the authors present experimental evidence of arsenic mobilization in connection with bacterially mediated manganese reduction in groundwater affected by mining activities. Manganese-reducing Pseudomonas species were enriched, isolated and identified by 16S rRNA gene phylogeny from groundwater containing high co-dissolved arsenic (as As^III) and manganese. Enrichment cultures dissolved synthetic birnessite and hausmannite efficiently, but Mn reduction by isolates was reduced at the upper range of environmental levels of dissolved As^III. Results suggest either a self-limiting release of arsenic coupled to bacterial manganese reduction, in the absence of other electron donors like sulfide, or increased arsenic resistance conferred to Mn-reducing bacteria in consortia.     

Rhizosphere and flooding regime as key factors for the mobilisation of arsenic and potentially harmful metals in basic,mining-polluted salt marsh soils

The occurrence of mining areas in the vicinities of salt marshes may affect their ecological functions and facilitate the transfer of pollutants into the food chain. The mobilisation of metals in salt marsh soils is controlled by abiotic (pH, redox potential) and biotic (influence of rhizosphere) factors. The effect of the rhizosphere of two plant species (Sarcocornia fruticosa and Phragmites australis) and different flooding regimes on potentially harmful metals and As mobilisation from salt marsh soil polluted by mining activities were investigated (total concentrations: 536 mg kg⁻¹ As, 37 mg kg⁻¹ Cd, 6746 mg kg⁻¹ Pb, 15,320 mg kg⁻¹ Zn). The results show that the changes in redox conditions (from 300 mV to −100 mV) and pH after flooding and rewetting periods may mobilise the contaminant elements into soil solution (e.g., 100 μg L⁻¹ Cd, 30 μg L⁻¹ Pb, 7 mg L⁻¹ Zn), where they are available for plants or may be leached from the soil. Drying periods generated peaks of concentrations in the soil solution (up to 120 μg L⁻¹ Cd and 50 μg L⁻¹ Pb). The risk assessment of As and metal-polluted salt marshes should take into account flood dynamics in order to prevent metal(loid) mobilisation.     

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5 m) and piezometer wells (depth 12.2–79.2 m) demonstrate that the groundwater is mostly the Ca-HCO₃ type and anoxic in nature (mean Eh_SHE = 34 mV). Arsenic concentrations ranged from <10–538 μg/L, with high concentrations only present in the shallow to medium depth (30–50 m) of the aquifer along with high Fe (0.07–9.8 mg/L) and relatively low Mn (0.15–3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO₃, SO₄ and NO₂ along with higher values of DOC and HCO₃, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.