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.     

Arsenic mobility in groundwater/surface water systems in carbonate-rich Pleistocene glacial drift aquifers (Michigan)

Within the Lower Peninsula of Michigan, groundwaters from the Marshall Formation (Mississippian) contain As derived from As-rich pyrites, often exceeding the World Heath Organization drinking water limit of 10 μg/L. Many Michigan watersheds, established on top of Pleistocene glacial drift derived from erosion of the underlying Marshall Formation, also have waters with elevated As. The Huron River watershed in southeastern Lower Michigan is a well characterized hydrogeochemical system of glacial drift deposits, proximate to the Marshall Fm. subcrop, which hosts carbonate-rich groundwaters, streams, and wetlands (fens), and well-developed soil profiles. Aqueous and solid phase geochemistry was determined for soils, soil waters, surface waters (streams and fens) and groundwaters from glacial drift aquifers to better understand the hydrogeologic and chemical controls on As mobility. Soil profiles established on the glacial drift exhibit enrichment in both Fe and As in the oxyhydroxide-rich zone of accumulation. The amounts of Fe and As present as oxyhydroxides are comparable to those reported from bulk Marshall Fm. core samples by previous workers. However, the As host in core samples is largely unaltered pyrite and arsenopyrite. This suggests that the transformation of Fe sulfides to Fe oxyhydroxides largely retains As and Fe at the oxidative weathering site. Groundwaters have the highest As values of all the waters sampled, and many were at or above the World Health limit. Most groundwaters are anaerobic, within the zones of Fe³⁺ and As(V) reduction. Although reduction of Fe(III) oxyhydroxides is the probable source of As, there is no correlation between As and Fe concentrations. The As/Fe mole ratios in drift groundwaters are about an order of magnitude greater than those in soil profiles, suggesting that As is more mobile than Fe. This is consistent with the dominance of As(III) in these groundwaters and with the partitioning of Fe²⁺ into carbonate cements. Soil waters have very low As and Fe contents, consistent with the stability of oxyhydroxides under oxidizing vadose conditions. When CO₂ charged groundwaters discharge in streams and fens, dissolved As is effectively removed by adsorption onto Fe-oxides or carbonate marls. Although Fe does not display conservative behavior with As in groundwaters, a strong positive correlation exists between As and Sr concentrations. As water–rock interactions proceed, the As/Fe and Sr/Ca ratios would be expected to increase because both As and Sr behave as incompatible elements. Comparisons with groundwater chemistries from other drift-hosted aquifers proximate to the Marshall sandstone are consistent with these relations. Thus, the Sr content of carbonate-rich groundwaters may provide useful constraints on the occurrence, origin and evolution of dissolved As in such systems.     

A review of the source,behaviour and distribution of arsenic in natural waters

The range of As concentrations found in natural waters is large, ranging from less than 0.5 μg l⁻¹ to more than 5000 μg l⁻¹. Typical concentrations in freshwater are less than 10 μg l⁻¹ and frequently less than 1 μg l⁻¹. Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be ‘affected’ (defined as those exceeding 50 μg l⁻¹) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing ‘excessive’ As. These large-scale ‘natural’ As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale, in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1–20 mg kg⁻¹. There appear to be two distinct ‘triggers’ that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO₄ concentrations are small (typically 1 mg l⁻¹ or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe.     

Selective chemical extraction of heavy metals in tailings and soils contaminated by mining activity: Environmental implications

Total concentrations of chemical elements in soils may not be enough to understand the mobility and bioavailability of the elements. It is important to characterise the degree of association of chemical elements in different physical and chemical phases of soil. Another geochemical characterisation methodology is to apply sequential selective chemical extraction techniques. A seven-step sequential extraction procedure was used to investigate the mobility and retention behaviour of Al, Fe, Mn, Cu, Zn, Pb, Cr, Co, Ni, Mo, Cd, Bi, Sn, W, Ag, As and U in specific physical–chemical and mineral phases in mine tailings and soils in the surroundings of the abandoned Ervedosa mine. The soil geochemical data show anomalies associated with mineralised veins or influenced by mining. Beyond the tailings, the highest recorded concentrations for most elements are in soils situated in mineralised areas or under the influence of tailings. The application of principal components analysis allowed recognition of (a) element associations according to their geochemical behaviour and (b) distinction between samples representing local geochemical background and samples representing contamination. Some metal cations (Mn, Cd, Cu, Zn, Co, Cr, Ni) showed important enrichment in the most mobilisable and bioavailable (i.e., water-soluble and exchangeable) fractions due likely to the acidic conditions in the area. In contrast, oxy-anions such as Mo and As showed lower mobility because of adsorption to Fe oxy-hydroxides. The residual fraction comprised largest proportions of Sn and Al and to a lesser extent Zn, Pb, Ni, Cr, Bi, W, and Ag, which are also present at low concentrations in the bioavailable fractions. The elements in secondary mineral phases (mainly Fe, Mn, Cu, Zn, Cd, Pb, W, Bi, Mo, Cr, Ni, Co, As and U) as well as in organic matter and sulphides are temporarily withheld, suggesting that they may be released to the environment by changes in physico-chemical conditions.     

Redox chemistry of uranium and iron,radium geochemistry,and uranium isotopes in the groundwaters of the Lode`ve Basin,Massif Central,France

This paper discusses the geochemistry, origin and evolution of groundwaters in the Lode`ve Basin Massif Central, France, including major and trace elements (U, Ra, Ba and Fe) and the significance of the redox potential. These groundwaters originate from thermal waters by CO₂ outgassing and mixing with meteoric waters. The measured redox potential (Eh > −50 mV) is generally controlled by the ferric oxide-Fe²⁺ equilibrium; below this potential the groundwaters are saturated with respect to FeS.Redox systems other than Fe are not necessarily at equilibrium, and this is particularly true for U. In the relatively reduced waters (Eh< 100mV), U concentrations may be controlled by coffinite or uraninite. Above 100 mV, U concentrations range from 10 to 200 ppb; they correspond either to mineral dissolution, or could be controlled by an oxidized mineral.Radium mobility is strongly dependent on SO₄ concentration, and it coprecipitates with barite. Radium probably has its origin in calcium-sodium bicarbonate type waters and is not related to U mineralization.     

Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa,Argentina

Groundwaters from Quaternary loess aquifers in northern La Pampa Province of central Argentina have significant quality problems due to high concentrations of potentially harmful elements such as As, F, NO₃-N, B, Mo, Se and U and high salinity. The extent of the problems is not well-defined, but is believed to cover large parts of the Argentine Chaco-Pampean Plain, over an area of perhaps 10⁶ km². Groundwaters from La Pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km. Dissolved As spans over 4 orders of magnitude (<4–5300 μg l⁻¹) and concentrations of F have a range of 0.03–29 mg l⁻¹, B of 0.5–14 mg l^−l, V of 0.02–5.4 mg l⁻¹, NO₃–N of <0.2–140 mg l⁻¹, Mo of 2.7–990 μg l⁻¹ and U of 6.2–250 μg l⁻¹. Of the groundwaters investigated, 95% exceed 10 μg As l⁻¹ (the WHO guideline value) and 73% exceed 50 μg As l⁻¹ (the Argentine national standard). In addition, 83% exceed the WHO guideline value for F (1.5 mg l⁻¹), 99% for B (0.5 mg l⁻¹), 47% for NO₃-N (11.3 mg l⁻¹), 39% for Mo (70 μg l⁻¹), 32% for Se (10 μg l⁻¹) and 100% for U (2 μg l⁻¹). Total dissolved solids range between 730 and 11400 mg l⁻¹, the high values resulting mainly from evaporation under ambient semi-arid climatic conditions. The groundwaters are universally oxidising with high dissolved-O₂ concentrations. Groundwater pHs are neutral to alkaline (7.0–8.7). Arsenic is present in solution predominantly as As(V). Groundwater As correlates positively with pH, alkalinity (HCO₃), F and V. Weaker correlations are also observed with B, Mo, U and Be. Desorption of these elements from metal oxides, especially Fe and Mn oxides under the high-pH conditions is considered an important control on their mobilisation. Mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility. Weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved As and other trace elements. Concentrations of As and other anions and oxyanions appear to be particularly high in groundwaters close to low-lying depressions which act as localised groundwater-discharge zones. Concentrations up to 7500 μg l⁻¹ were found in saturated-zone porewaters extracted from a cored borehole adjacent to one such depression. Concentrations are also relatively high where groundwater is abstracted from close to the water table, presumably because this zone is a location of more active weathering reactions. The development of groundwaters with high pH and alkalinity results from silicate and carbonate reactions, facilitated by the arid climatic conditions. These factors, together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of As and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal.     

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.     

Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn–Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*]_NASC as low as 0.08), produced by oxidation of Ce³⁺ to Ce⁴⁺ and preferential removal of Ce⁴⁺ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.     

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca₅(PO₄)₃OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO_2(g) and O_2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe^III[(OH)₃(PO₄)(As^VO₄)]·nH₂O or Fe^III[(OH)₃( PO₄)(As^VO₄)(As^IIIO₃)_minor]·nH₂O, where As^IIIO₃ is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe^III(OH)₃ and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe^III[(OH)₃, PO₄]·nH₂O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO₄·2H₂O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.     

Trace metal and As solid-phase speciation in sulphide mine tailings – Indicators of spatial distribution of sulphide oxidation in active tailings impoundments

Redistribution of potentially harmful metals and As was studied based on selective extractions in two active sulphide mine tailings impoundments in Finland. The Hitura tailings area contains residue from Ni ore processing, while the Luikonlahti site includes tailings from the processing of Cu–Co–Zn–Ni and talc ores. To characterize the element solid-phase speciation with respect to sulphide oxidation intensity and the water saturation level of the tailings, drill cores were collected from border zones and mid-impoundment locations. The mobility and solid-phase fractionation of Ni, Cu, Co, Zn, Cr, Fe, Ca, Al, As, and S were analysed using a 5-step non-sequential (parallel) selective extraction procedure. The results indicated that metal redistribution and sulphide oxidation intensity were largely controlled by the disposal history and strategy of the tailings (sorting, exposure of sulphides due to delayed burial), impoundment structure and water table, and reactivity of the tailings. Metal redistribution suggested sulphide weathering in the tailings surface, but also in unsaturated proximal areas beside the earthen dams, and in water-saturated bottom layers, where O₂-rich infiltration is possible. Sulphide oxidation released trace metals from sulphide minerals at both locations. In the Hitura tailings, with sufficient buffering capacity, pH remained neutral and the mobilized metals were retained by secondary Fe precipitates deeper in the oxidized zone. In contrast, sulphide oxidation-induced acidity and rise in the water table after oxidation apparently remobilized the previously retained metals in Luikonlahti. In general, continuous disposal of tailings decreased the sulphide oxidation intensity in active tailings, unless there was a delay in burial and the reactive tailings were unsaturated after deposition.     

Trace elements contamination of agricultural soils affected by sulphide exploitation (Iberian Pyrite Belt,Sw Spain)

Agricultural soils of the Riotinto mining area (Iberian Pyrite Belt) have been studied to assess the degree of pollution by trace elements as a consequence of the extraction and treatment of sulphides. Fifteen soil samples were collected and analysed by ICP-OES and INAA for 51 elements. Chemical analyses showed an As–Cu–Pb–Zn association related with the mineralisation of the Iberian Pyrite Belt. Concentrations were 19–994 mg kg⁻¹ for As, 41–4,890 mg kg⁻¹ for Pb, 95–897 mg kg⁻¹ for Zn and of 27–1,160 mg kg⁻¹ for Cu. Most of the samples displayed concentrations of these elements higher than the 90th percentile of the corresponding geological dominium, which suggests an anthropogenic input besides the bedrock influence. Samples collected from sediments were more contaminated than leptosols because they were polluted by leachates or by mining spills coming from the waste rock piles. The weathering of the bedrock is responsible for high concentrations in Co, Cr and Ni, but an anthropogenic input, such as wind-blown dust, seems to be indicative of the high content of As, Cu, Pb and Zn in leptosols. The metal partitioning patterns show that most trace elements are associated with Fe amorphous oxy-hydroxides, or take part of the residual fraction. According to the results obtained, the following mobility sequence is proposed for major and minor elements: Mn, Pb, Cd, > Zn, Cu > Ni > As > Fe > Cr. The high mobility of Pb, Cu and Zn involve an environmental risk in this area, even in soils where the concentrations are not so high.     

Control of organic and iron colloids on arsenic partition and transport in high arsenic groundwaters in the Hetao basin, Inner Mongolia

Due to the importance of colloids in regulating element transport and mobility in aquifers, As distribution in the colloidal fraction needs to be identified in high As groundwaters. Groundwater samples were filtered in the field through a progressively decreasing pore size (0.45 μm, 100, 30, 10, 5 kDa) using a filtration technique under a N₂ atmosphere. Major and trace elements and organic C (OC) were measured in (ultra)filtrates. The studied groundwater samples have typical physio-chemical characteristics of the basin waters. Declines in concentrations of alkali (Na, K), alkaline-earth (Mg, Ca, Sr, Ba) elements, Mo, Si and Se during ultrafiltration are smaller relative to other elements. Arsenic, Cu, Cr, U and V are generally about 30% lower in 5 kDa ultrafiltrates in comparison with 0.45 μm filtrates. Around 50% of Fe, OC and Al are bound to colloids with grain size between 5 kDa and 0.45 μm. Two types of colloids, including large-size Fe colloids and small-size organic colloids, have been identified. Results indicate that As would be more likely to be associated with small-size organic colloids than Fe colloids. SEM images and EDS analysis and synchrotron XRF analyses confirm the association of As with NOM with molecular weights of 5-10 kDa. The better correlation between As(V) and OC in the 5-10 kDa fraction indicates that the small-size organic colloids have a greater affinity for As(V) than As(III). Arsenic associated with organic complexes may not be immobilized by adsorption, and, therefore, easily transported in the aquifer. Thus, the presence of As-containing colloidal complexes in high As groundwaters must be considered in the modeling of As transport in the aquifers.     

Arsenate and arsenite adsorption onto Al-containing ferrihydrites. Implications for arsenic immobilization after neutralization of acid mine drainage

Natural ferrihydrites (Fh) often contain impurities such as aluminum, especially in acid mine drainage, and these impurities can potentially impact the chemical reactivity of Fh with respect to metal (loid) adsorption. In the present study, we have investigated the influence of aluminum on the sorption properties of ferrihydrite with respect to environmentally relevant aqueous arsenic species, arsenite and arsenate. We have conducted sorption experiments by reacting aqueous As(III) and As(V) with synthetic Al-free and Al-bearing ferrihydrite at pH 6.5. Our results reveal that, when increasing the Al:Fe molar ratio in Fh, the sorption density dramatically decreased for As(III), whereas it increased for As(V). Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy analysis at the As K-edge indicated that the As^IIIO₃ pyramid binds to FeO₆ octahedra on both Al-free Fh and Al-bearing Fh, by forming bidentate mononuclear edge-sharing (²E) and bidentate binuclear corner-sharing (²C) surface complexes characterized by As–Fe distances of 2.9 Å and 3.4 Å, respectively. The decrease in As(III) sorption density with increasing Al:Fe ratio in Fh could thus be explained by a low affinity of the As(OH)₃ molecule for Al surface sites compared to Fe ones. In contrast, on the basis of available literature on As(V) adsorption mechanisms, we suggest that, in addition to inner-sphere ²C arsenate surface complexes, outer-sphere arsenate surface complexes forming hydrogen bonds with both Al–OH and Fe–OH surface sites could explain the enhancement of As(V) sorption onto aluminous Fh relative to Al-free Fh, as observed in the present study. The presence of aluminum in Fh may thus enhance the mobility of arsenite with respect to arsenate in Acid Mine Drainage impacted systems, while mixed Al:Fe systems could present an alternative for arsenic removal from impacted waters, provided that As(III) would be oxidized to As(V).     

Hydrochemical Groundwater Evolution in the Bunter Sandstone Sequence of the Odenwald Mountain Range,Germany: A Laboratory and Field Study

Field and laboratory investigations were performed to identify the principal mechanisms of the hydrochemical groundwater evolution among low mineralised groundwater in the Triassic Bunter sandstone aquifer of the Odenwald low mountain range, central Germany. Hydrochemical composition comprises low pH, SO₄-rich shallow groundwaters issued by springs (Ca-Mg-SO₄-type) grading to SO₄-poor deep groundwaters with near-neutral pH (Ca-HCO₃-type). Batch experiments of the original rock were run to determine primary mineral alteration reactions and the origin of dissolved ions. Principal experimental reactions comprise the decomposition of anorthite, K-feldspar, biotite and jarosite as mineral components of the original sandstone rock and the formation of clay minerals of the smectite group (Ca-montmorillonite, beidellite), and iron hydroxides as secondary minerals. Mobilisation of fluid inclusion in quartz grains contributes to Na and Cl concentrations in the leachates. The evolution of deep groundwater circulation proceeds by mineral alteration reactions calculated by the inverse modelling of both primary and secondary minerals to produce low-T mineral phases. The dissolution of K-feldspar converts Ca-montmorillonite to illite (illitisation). The formation of Na-beidellite correlates with decreasing concentration of Na in solution. Mineral reactions further proceed to the formation of kaolinite as stable mineral phase. As indicated by modelled adsorption curves, the decrease of SO₄ concentrations during groundwater evolution relates to the adsorption of SO₄ on iron hydroxides. The leaching of calcite indicated for individual groundwaters relates to the distribution of loess in the appropriate catchment areas.     

Fe and Mn levels regulated by agricultural activities in alluvial groundwaters underneath a flooded paddy field

Iron and Mn concentrations in fresh groundwaters of alluvial aquifers are generally high in reducing conditions reflecting low SO₄ concentrations. The mass balance and isotopic approaches of this study demonstrate that reduction of SO₄, supplied from agricultural activities such as fertilization and irrigation, is important in lowering Fe and Mn levels in alluvial groundwaters underneath a paddy field. This study was performed to investigate the processes regulating Fe and Mn levels in groundwaters of a point bar area, which has been intensively used for flood cultivation. Four multilevel-groundwater samplers were installed to examine the relationship between geology and the vertical changes in water chemistry. The results show that Fe and Mn levels are regulated by the presence of NO₃ at shallow depths and by SO₄ reduction at the greater depths. Isotopic and mass balance analyses revealed that NO₃ and SO₄ in groundwater are mostly supplied from the paddy field, suggesting that the Fe-and Mn-rich zone of the study area is confined by the agricultural activities. For this reason, the geologic conditions controlling the infiltration of agrochemicals are also important for the occurrence of Fe/Mn-rich groundwaters in the paddy field area.     

Source,transport, and fate of rhenium,selenium, molybdenum,arsenic, and copper in groundwater associated with porphyry–Cu deposits,Atacama Desert,Chile

We collected groundwaters in and around a large (313 Mt at 1.08% Cu and 0.3% cutoff) undisturbed porphyry copper deposit (Spence) in the hyperarid Atacama Desert of northern Chile, which is buried beneath 30–180 m of Miocene piedmont gravels. Groundwaters within and down-flow of the Spence deposit have elevated Se (up to 800 μg/l), Re (up to 31 μg/l), Mo (up to 475 μg/l) and As (up to 278 μg/l) concentrations compared to up-flow waters (interpreted to represent regional groundwater flow). In contrast, Cu is only elevated (up to 2036 μg/l) in groundwaters recovered from within the deposit; Cu concentrations are low down gradient of the deposit. The differential behavior of the metals/metalloids occurs because the former group dissolves as anions, enhancing their mobility, whereas the base metals dissolve as cations and are lost from solution most likely through adsorption to clay surface exchange sites and through formation of secondary copper chlorides, carbonates, and oxides. Most groundwaters within and down-flow of the deposit have Eh–pH values around the Fe^II/Fe^III phase boundary, limiting the impact of Fe-oxyhydroxides on oxyanions mobility. Se, Re, Mo, and As are all mobile (with filtered/unfiltered samples ～ 1) to the limit of sampling 2 km down gradient from the deposit. The increase in ore-related metals, metalloids, and sulfate and decrease in sulfate–S isotope ratios (from values similar to regional salars, + 4 to + 8‰ δ³⁴S_CDT to lower values closer to hypogene sulfides, + 1 to + 4‰ δ³⁴S_CDT) is consistent with active water–rock reactions between saline groundwaters and the Spence deposit. It is likely that hypogene and/or supergene sulfides are being oxidized under the present groundwater conditions and mineral saturation calculations suggest that secondary copper minerals (antlerite, atacamite, malachite) may also be actively forming, suggesting that supergene and exotic copper mineralization is possible even under the present hyperarid climate of the Atacama Desert.     

Arsenic associations in sediments from the loess aquifer of La Pampa,Argentina

Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations of As (up to 5300 μg L⁻¹) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concentrations in 45 loess samples collected from the aquifer have a range of 3–18 mg kg⁻¹ with a mean of 8 mg kg⁻¹. These values are comparable to world-average sediment As concentrations. Five samples of rhyolitic ash from the area have As concentrations of 7–12 mg kg⁻¹. Chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes. Results reveal a large range of porewater As concentrations, being generally higher in the horizons with highest sediment As concentrations. The displaced porewaters have As concentrations ranging up to 7500 μg L⁻¹ as well as exceptionally high concentrations of some other oxyanion species, including V up to 12 mg L⁻¹. The highest concentrations are found in a borehole located in a topographic depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective extract data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is associated with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Extract compositions also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidising, with dissolved O₂, NO₃ and SO₄ normally present and As(V) usually the dominant dissolved As species. Under such conditions, the solubility of Fe and Mn oxides is low and As mobilisation is strongly controlled by sorption–desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0–8.8) and high concentrations of potential competitors (e.g. V, P, HCO₃). PHREEQC modelling suggests that the presence of V at the concentrations observed in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by up to an order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate extract concentrations have been used to provide an upper estimate of the amount of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes investigated has been used to estimate an approximate K_d value for the sediments of 0.94 L kg⁻¹. This low value indicates that the sediments have an unusually low affinity for As.     

Arsenic,iron and organic matter in sediments and groundwater in the Pannonian Basin,Hungary

In the central part of the Pannonian Basin, factors controlling the distribution of As in sediments and groundwater of the upper 500 m were studied. In core samples, the amounts of As, Fe and Mn extractable with hydroxylamine hydrochloride (NH₂OH · HCl) in 25% acetic acid, the proportion of the <0.063 mm size fraction, and the sediment organic C (C_org) contents were measured. In the groundwaters concentrations of As, humic substances, and selected major chemical components were determined. In most core samples extractable Fe, as FeOOH, and C_org are correlated, but some samples have excess Fe, or organic matter. In cases where either excess Fe or excess organic matter is found, the amount of As is also elevated. The spatial distribution of As in the groundwater and the lack of a consistent correlation of As with a single component indicate that there is no single factor controlling the concentration of dissolved As over the entire study area. The only consistent feature is enrichment of As relative to Fe in the groundwater, compared to the sediments. This suggests that the dissolution of Fe minerals, which primarily adsorb As, is not congruent. In reducing conditions Fe(III) oxyhydroxides together with adsorbed As dissolve, and siderite with little or no As precipitates. When sub-regions are separated and studied individually, it can be shown that hydrogeological features of the sediments, the proportions of Fe minerals and sedimentary organic matter, and the concentration of dissolved humic materials, all influence the accumulation and mobilization of As. The significance of the different mobilizing processes, however, and the mean concentration of As, is different in the recharge, through-flow and discharge areas.     

Behavior of arsenic and geochemical modeling of arsenic enrichment in aqueous environments

Arsenic is present in aqueous environments in +III and +V oxidation states. In oxidizing environments, the principle attenuation mechanism of As migration is its adsorption on Fe(III) oxide and hydroxides. The adsorption affinity is higher for As(V) under lower pH conditions and for As(III) under higher pH conditions. Ferric oxide and hydroxides can dissolve under low Eh and pH conditions releasing adsorbed As. Oxidation-reduction processes often involve high organic matter content in sediments and also contamination by organics such as BTEX. Arsenic may desorb under high pH conditions. Changes of pH can be related to some redox reactions, cation exchange reactions driving dissolution of carbonates, and dissolution of silicates. In very reducing environments, where SO₄ reduction takes place, secondary sulfide minerals like As-bearing pyrite and orpiment, As₂S₃, can incorporate As. Geochemical modeling can be divided into two principal categories: (a) forward modeling and (b) inverse modeling. Forward modeling is used to predict water chemistry after completion of predetermined reactions. Inverse modeling is used to suggest which processes take place along a flowpath. Complex coupled transport and geochemistry programs, which allow for simulation of As adsorption, are becoming available. A common modeling approach is based on forward modeling with surface complexation modeling (SCM) of As adsorption, which can incorporate the effect of different adsorbent/As ratios, adsorption sites density, area available for adsorption, pH changes and competition of As for adsorption sites with other dissolved species such as phosphate. The adsorption modeling can be performed in both batch and transport modes in codes such as PHREEQC. Inverse modeling is generally used to verify hypotheses on the origin of As. Basic prerequisites of inverse modeling are the knowledge of flow pattern (sampling points used in model have to be hydraulically connected) and information about mineralogy including As mineral phases. Case studies of geochemical modeling including modeling of As adsorption are presented.     

Arsenic enrichment in groundwater of West Bengal,India: geochemical evidence for mobilization of As under reducing conditions

The mechanism of As release and source(s) of As has been investigated in a small part of a watershed in the Murshidabad district of West Bengal. Analyses include major ion and trace element concentrations, as well as O, H and S isotope ratios of groundwater, surface water and a thermal spring. The results indicate that all water samples belong to the Ca–HCO₃ type, except for the thermal spring which is of the Na–HCO₃ type. Shallow and deeper groundwaters have distinct hydrochemical features. High As contents were registered only in the deeper groundwater horizon. Factor analysis and the distribution pattern of major and trace elements indicate that As is present in the aquifer as a scavenged phase by Fe(III) and to a lesser extent by Mn(IV) phases. The release of As into the groundwater occurs gradually in successive stages, corresponding to the actual redox state in the aquifer. The main stage of As release is related to the bacterial reduction of Fe(III) to Fe(II) (i.e. to the simultaneous dissolution of Fe oxyhydroxides). Low redox conditions in highly polluted areas are indicated by low SO₄ concentration and high δ³⁴S values. During bacterial SO₄ reduction, residual SO₄ in groundwater is depleted in the lighter S isotope (³²S). However, the cause of the gradual decrease of the redox state in the groundwater is still not well understood.