Sulfide-driven arsenic mobilization from arsenopyrite and black shale pyrite

We examined the hypothesis that sulfide drives arsenic mobilization from pyritic black shale by a sulfide-arsenide exchange and oxidation reaction in which sulfide replaces arsenic in arsenopyrite forming pyrite, and arsenide (As−1) is concurrently oxidized to soluble arsenite (As+3). This hypothesis was tested in a series of sulfide-arsenide exchange experiments with arsenopyrite (FeAsS), homogenized black shale from the Newark Basin (Lockatong formation), and pyrite isolated from Newark Basin black shale incubated under oxic (21% O₂), hypoxic (2% O₂, 98% N₂), and anoxic (5% H₂, 95% N₂) conditions. The oxidation state of arsenic in Newark Basin black shale pyrite was determined using X-ray absorption-near edge structure spectroscopy (XANES). Incubation results show that sulfide (1 mM initial concentration) increases arsenic mobilization to the dissolved phase from all three solids under oxic and hypoxic, but not anoxic conditions. Indeed under oxic and hypoxic conditions, the presence of sulfide resulted in the mobilization in 48 h of 13-16 times more arsenic from arsenopyrite and 6-11 times more arsenic from isolated black shale pyrite than in sulfide-free controls. XANES results show that arsenic in Newark Basin black shale pyrite has the same oxidation state as that in FeAsS (−1) and thus extend the sulfide-arsenide exchange mechanism of arsenic mobilization to sedimentary rock, black shale pyrite. Biologically active incubations of whole black shale and its resident microorganisms under sulfate reducing conditions resulted in sevenfold higher mobilization of soluble arsenic than sterile controls. Taken together, our results indicate that sulfide-driven arsenic mobilization would be most important under conditions of redox disequilibrium, such as when sulfate-reducing bacteria release sulfide into oxic groundwater, and that microbial sulfide production is expected to enhance arsenic mobilization in sedimentary rock aquifers with major pyrite-bearing, black shale formations.     

Comparison of arsenic geochemical evolution in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), China

Insightful knowledge of geochemical processes controlling As mobility is fundamental to understanding the occurrence of elevated As in groundwater. A comparative study of As geochemistry was conducted in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), two strongly As-enriched areas in China. The results show that As concentrations ranged from <1–1160 μg L⁻¹ (n = 37) in the Datong Basin and <1–804 μg L⁻¹ (n = 62) in the Hetao Basin. The groundwater is of the Na-HCO₃ type in the Datong Basin and Na-Cl-HCO₃ type in the Hetao Basin. Silicate mineral weathering and cation exchange processes dominated the groundwater geochemistry in the two study areas. Principal component analysis of 99 groundwater samples using 12 geochemical parameters indicated positive correlations between concentrations of As and Fe/Mn in the Datong Basin, but no correlation of As and Fe/Mn in the Hetao Basin. Phosphate correlated well with As in the Datong Basin and Hetao Basin, suggesting phosphate competition might be another process affecting As concentrations in groundwater. High concentrations of As, Fe, and Mn occurred in the pe range −2 to −4. The results of this study further understanding of the similarities and differences of As occurrence and mobility at various locations in China.     

松辽盆地北部登娄库组二段烃源岩分布定量研究

勘探开发初期有限的油气资源评价资料限制了暗色泥岩预测方法的可靠性,松辽盆地北部登娄库组烃源岩主要为登二段灰色泥岩,其分布情况是评价松辽盆地北部深层致密砂岩气藏资源量的重要基础。本文以井震信息和基于沉积特征的多元地震属性定量方法,预测研究区泥岩厚度平面分布,综合地层沉积厚度平面信息、地震相平面信息及井点暗色泥岩和泥岩厚度比值信息,预测全区暗色泥岩和泥岩厚度比值的平面分布,预测结果显示松辽盆地北部登二段烃源岩主要有3个较大的发育,同时伴有其它零星小范围暗色泥岩发育,大部分厚度分布在70 m以下,最大暗色泥岩预测厚度约为350 m。该方法以井震信息为基础结合沉积规律定量预测暗色泥岩分布,为松辽盆地北部深层致密砂岩气藏资源量的可靠评价提供了必要的基础。     

Arsenic distribution, concentration and speciation in groundwater of the Osijek area, eastern Croatia

Groundwater is the main source of drinking water for the population of nearly 200,000 people in eastern Croatia. The largest town in the region is Osijek whose citizens are supplied with drinking water obtained from groundwater from the “Vinogradi” well field. This study investigated and determined As occurrence in groundwater of the Osijek area. Groundwater samples were taken from 18 water wells and 12 piezometers with a depth ranging between 21 and 200 m. Over the 10-a period to 2007, a mean As concentration of 240 μg L⁻¹ was found. There was no statistically significant secular change in concentration over that period, however small but significant seasonal variations were noted, with the highest seasonal As concentrations over the period May 2006-February 2007 being observed in summer. The predominant As species observed was As(III), constituting 85% and 93% of total As in piezometers and water wells, respectively. Higher concentrations of As tended to be found in deeper wells with the mean As concentration in shallow groundwater (<50 m) and deep groundwater (>50 m) being 27 μg L⁻¹, and 205 μg L⁻¹, respectively. Geochemically, the groundwaters show similarities to those in other parts of the Pannonian Basin. Arsenic(tot) is weakly correlated with pH and Fe, negatively correlated with Mn and has no significant correlation with any of EC, COD-Mn or alkalinity.     

The Drenchwater deposit,Alaska: An example of a natural low pH environment resulting from weathering of an undisturbed shale-hosted Zn–Pb–Ag deposit

The Drenchwater shale-hosted Zn–Pb–Ag deposit and the immediate vicinity, on the northern flank of the Brooks Range in north-central Alaska, is an ideal example of a naturally low pH system. The two drainages, Drenchwater and False Wager Creeks, which bound the deposit, differ in their acidity and metal contents. Moderately acidic waters with elevated concentrations of metals (pH ? 4.3, Zn ? 1400 μg/L) in the Drenchwater Creek drainage basin are attributed to weathering of an exposed base-metal-rich massive sulfide occurrence. Stream sediment and water chemistry data collected from False Wager Creek suggest that an unexposed base-metal sulfide occurrence may account for the lower pH (2.7–3.1) and very metal-rich waters (up to 2600 μg/L Zn, ? 260 μg/L Cu and ?89 μg/L Tl) collected at least 2 km upstream of known mineralized exposures. These more acidic conditions produce jarosite, schwertmannite and Fe-hydroxides commonly associated with acid-mine drainage. The high metal concentrations in some water samples from both streams naturally exceed Alaska state regulatory limits for freshwater aquatic life, affirming the importance of establishing base-line conditions in the event of human land development. The studies at the Drenchwater deposit demonstrate that poor water quality can be generated through entirely natural weathering of base-metal occurrences, and, possibly unmineralized black shale.     

Chemical and isotopic tracers of the contribution of microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin

In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH₄ (0.01–50.55 mol%) with low δ¹³C–CH₄ values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ¹³C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH₄ to ethane and propane ratios (3–35 mol%; C₁/C₂ + C₃) and high δ¹³C and δD values of CH₄ (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH₄ was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ¹³C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO₄ values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ¹³C–DIC values (<−22‰) and relatively high SO₄ concentrations (up to 12.31 mmole/L) in many brine samples point to SO₄ reduction, which likely limits microbial CH₄ generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH₄ in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH₄ are present at shallow depths along Lake Erie and in western NY.     

Groundwater chemistry of Al under Dutch sandy soils: Effects of land use and depth

Aluminium has received great attention in the second half of the 20th century, mainly in the context of the acid rain problem mostly in forest soils. In this research the effect of land use and depth of the groundwater on Al, pH and DOC concentration in groundwater under Dutch sandy soils has been studied. Both pH and DOC concentration play a major role in the speciation of Al in solution. Furthermore, the equilibrium with mineral phases like gibbsite, amorphous Al(OH)₃ and imogolite, has been considered. Agricultural and natural land use were expected to have different effects on the pH and DOC concentration, which in turn could influence the total Al concentration and the speciation of Al in groundwater at different depths (phreatic, shallow and deep). An extensive dataset (n = 2181) from the national and some provincial monitoring networks on soil and groundwater quality was used. Land use type and groundwater depth did influence the pH, and Al and DOC concentrations in groundwater samples. The Al concentration ranged from <0.4 μmol L⁻¹ at pH > 7 to 1941 μmol L⁻¹ at pH < 4; highest Al concentrations were found for natural-phreatic groundwater. The DOC concentration decreased and the median pH increased with depth of the groundwater. Natural-phreatic groundwater showed lower pH than the agricultural-phreatic groundwater. Highest DOC concentrations were found for the agricultural-phreatic groundwater, induced by the application of organic fertilizers. Besides inorganic complexation, the NICA-Donnan model was used to calculate Al³⁺ concentrations for complexation with DOC. Below pH 4.5 groundwater samples were mainly in disequilibrium with a mineral phase. This disequilibrium is considered to be the result of kinetic constraints or equilibrium with organic matter. Log K values were derived by linear regression and were close to theoretical values for Al(OH)₃ minerals (e.g. gibbsite or amorphous Al(OH)₃), except for natural-phreatic groundwater for which lower log K values were found. Complexation of Al with DOC is shown to be an important factor for the Al concentrations, especially at high DOC concentrations as was found for agricultural-phreatic groundwater.     

Mobility of arsenic and trace element inventories in sediment cores from Masuda City,southwestern Japan

The vertical distribution of arsenic and other trace and major elements has been studied in four sediment cores from Masuda City, Nagashima and Okite in the Shimane Prefecture of southwestern Japan. The sediment cores were also subjected to leaching techniques and ¹⁴C dating. The stratigraphic sequences in the cores consist of silt and sandy silt at top, passing downward into gray to black clays. Elevated values of As, Pb, Zn, Cu, Ni, Cr, and V are present in several horizons while abundances of these elements tend to be higher in the black and gray clays, probably due to adsorption onto clay sediments. Higher concentrations of Fe and total sulfur (TS) occur in black clays. The correlations of the trace metals with iron suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur in some cores indicate that they were precipitated as Fe-sulfides. Age determinations suggest that clay horizons at ∼5 m depth were deposited at around 5,000 and 6,000 years BP (¹⁴C ages) during the transgressive phase of sea level change. The results of the leaching techniques in the core samples show that higher amounts of As were extracted with deionized water. Even at neutral pH, As can be released from sediments to groundwater, and therefore groundwater pollution is a concern in Masuda City and the surrounding area.     

Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin,Inner Mongolia

Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 μg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 μg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F⁻ concentrations. Greater As concentrations were found with increasing well depth. However, F⁻ concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 μg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O₂ into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca²⁺ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F⁻ adsorption–desorption are likely controlling the concentration of F⁻ in groundwater.     

Comparison of fluid geochemistry and microbiology of multiple organic-rich reservoirs in the Illinois Basin, USA: Evidence for controls on methanogenesis and microbial transport

Microbial methane in sedimentary basins comprises approximately 20% of global natural gas resources, yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities. The Illinois Basin, located in the midcontinent of the United States, is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur: (1) in three organic-rich reservoirs of different geologic ages and organic matter types - Upper Devonian New Albany Shale (up to 900 m depth), Pennsylvanian coals (up to 600 m depth), and Quaternary glacial sediments (shallow aquifers); (2) across steep salinity gradients; and (3) with variable concentrations of . For all three organic-rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis, with near neutral pH, concentrations <2 mM, and Cl⁻ concentrations <3 M. Also, carbon isotopic fractionation of CH₄, CO₂, and DIC is consistent with microbial methanogenesis, and increased carbon isotopic fractionation with average reservoir depth corresponds to a decrease of groundwater flushing rates with average depth of reservoir. Plots of stable isotopes of water and Cl⁻ show mixing between a brine endmember and freshwater, suggesting that meteoric groundwater recharge has affected all microbial methanogenic systems. Additionally, similar methanogenic communities are present in all three reservoirs with comparable cell counts (8.69E3-2.58E6 cells/mL). TRFLP results show low numbers of archaea species with only two dominant groups of base pairs in coals, shale, and limestone aquifers. These results compare favorably with other methanogen-containing deep subsurface environments. Individual hydrogeochemical parameters that have a Spearman correlation coefficient greater than 0.3 to variations in methanogenic species include stable isotopes of water (δ¹⁸O and δD), type of substrate (i.e. coals versus shale), pH, and Cl⁻ concentration. The matching of variations between methanogenic TRFLP data and conservative tracers suggests that deep circulation of meteoric waters influenced archaeal communities in the Illinois Basin. In addition, coalification and burial estimates suggest that in the study area, coals and shale reservoirs were previously sterilized (>80 °C in nutrient poor environments), necessitating the re-introduction of microbes into the subsurface via groundwater transport.     

Elevated arsenic in deeper groundwater of the western Bengal basin, India: Extent and controls from regional to local scale

The deeper groundwater (depending on definition) of the Bengal basin (Ganges-Brahmaputra delta) has long been considered as an alternate, safe drinking-water source in areas with As-enrichment in near-surface groundwater. The present study provides the first collective discussion on extent and controls of elevated As in deeper groundwater of a regional study area in the western part of the Bengal basin. Deeper groundwater is defined here as non-brackish, potable (Cl⁻ ? 250 mg/L) groundwater available at the maximum accessed depth (∼80-300 m). The extent of elevated As in deeper groundwater in the study area seems to be largely controlled by the aquifer-aquitard framework. Arsenic-enriched deeper groundwater is mostly encountered north of 22.75°N latitude, where an unconfined to semi-confined aquifer consisting of Holocene- to early Neogene-age gray sand dominates the hydrostratigraphy to 300 m depth below land surface. Aquifer sediments are not abnormally enriched in As at any depth, but sediment and water chemistry are conducive to As mobilization in both shallow and deeper parts of the aquifer(s). The biogeochemical triggers are influenced by complex redox disequilibria. Results of numerical modeling and profiles of environmental tracers at a local-scale study site suggest that deeper groundwater abstraction can draw As-enriched water to 150 m depth within a few decades, synchronous with the advent of wide-scale irrigational pumping in West Bengal (India).     

Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh,India, Vietnam,and Nepal

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.     

鄂尔多斯盆地北部山西组页岩沉积相及其对页岩储层的控制作用

鄂尔多斯盆地北部山西组页岩气储层与煤层共生,页岩有机质含量较高,具有较大的资源勘探潜力。为精细表征控制山西组页岩储层分布的沉积微相,根据测井数据、录井数据、岩芯观察描述、薄片分析、粒度分析、X射线衍射全岩分析以及总有机碳含量(TOC),研究了山西组页岩储层岩相与沉积微相类型,在山西组识别出(灰)黑色碳质页岩、(灰)黑色含碳质页岩、(灰)黑色含碳质(粉)砂质页岩、(深)灰色含碳质页岩、(深)灰色页岩、(深)灰色(粉)砂质页岩和含钙质页岩7种岩相。这些岩相沉积在富植沼泽、贫植沼泽、三角洲平原分流间洼地、曲流河河间洼地、天然堤与决口扇远端微相环境,其中富植沼泽、贫植沼泽与成煤的泥炭沼泽、泥炭坪环境比邻共生,且在山二段时期更发育。富植沼泽和贫植沼泽微相控制了具有较高TOC的(灰)黑色碳质页岩、(灰)黑色含碳质页岩、(灰)黑色含碳质(粉)砂质页岩和(深)灰色含碳质页岩的分布,是控制山西组页岩储层分布的有利微相。     

Geochemical processes and mobilization of toxic metals and metalloids in an As-rich base metal waste pile in Zimapán,Central Mexico

The geochemistry and mineralogy of samples collected along depth profiles from an As-rich tailing deposit with abundant calcite was studied to determine the processes that influence the mobility of Fe, Zn, Cu, Ni, Cd, As, Sb, Cr and Tl. In spite of their near neutral pH, almost all of them are acid potential generators. Total concentrations decreased as: Fe > As > Zn > Pb > Cu > Sb > Cd > Cr > Ni > Tl. Soluble contents were lower and followed a slightly different order. Mobility decreased as: Tl > Cd, Zn, Cu, Sb, Ni, As > Fe, Pb > Cr. Higher soluble concentrations of Fe, Cu, Zn, As, Pb, and Ni were found in low-pH samples and of Sb and Tl in near-neutral samples. Sulfide oxidation processes are developing in the tailing’s dam. These processes do not have a trend with depth but occur mainly in acid layers. Near neutral layers formed by primary sulfides and calcite probably correspond to wastes produced from the processing of ore coming mainly from pods within the skarn, and acid layers with abundant secondary minerals from material mined from chimneys and mantos. The presence of calcite influences speciation, neutralizes acid mine drainage (AMD), and decreases the mobility of most toxic metals and metalloids (TMMs). However, a hard-pan layer was not observed in the studied profiles. Retention of TMM within tailings probably occurs through the formation of low solubility metal carbonates and from elevation of pH that promotes Fe hydroxides precipitation that may retain As, Sb and metals. Calcite occurrence promotes As, Cd, Cu, Fe, Zn, Pb, Cd and Cr retention, does not play a role on Tl and Ni mobilization, and increases Sb release.     

Arsenic and antimony contamination of waters,stream sediments and soils in the vicinity of abandoned antimony mines in the Western Carpathians,Slovakia

Environmental contamination with As and Sb caused by past mining activities at Sb mines is a significant problem in Slovakia. This study is focused on the environmental effects of the 5 abandoned Sb mines on water, stream sediment and soil since the mines are situated in the close vicinity of residential areas. Samples of mine wastes, various types of waters, stream sediments, soils, and leachates of the mine wastes, stream sediments and selected soils were analyzed for As and Sb to evaluate their geochemical dispersion from the mines. Mine wastes collected at the mine sites contained up to 5166 mg/kg As and 9861 mg/kg Sb. Arsenic in mine wastes was associated mostly with Fe oxides, whereas Sb was present frequently in the form of individual Sb, Sb(Fe) and Fe(Sb) oxides. Waters of different types such as groundwater, surface waters and mine waters, all contained elevated concentrations of As and Sb, reaching up to 2150 μg/L As and 9300 μg/L Sb, and had circum-neutral pH values because of the buffering capacity of abundant Ca- and Mg-carbonates. The concentrations of Sb in several household wells are a cause for concern, exceeding the Sb drinking water limit of 5 μg/L by as much as 25 times. Some attenuation of the As and Sb concentrations in mine and impoundment waters was expected because of the deposition of metalloids onto hydrous ferric oxides built up below adit entrances and impoundment discharges. These HFOs contained >20 wt.% As and 1.5 wt.% Sb. Stream sediments and soils have also been contaminated by As and Sb with the peak concentrations generally found near open adits and mine wastes. In addition to the discharged waters from open adits, the significant source of As and Sb contamination are waste-rock dumps and tailings impoundments. Leachates from mine wastes contained as much as 8400 μg/L As and 4060 μg/L Sb, suggesting that the mine wastes would have a great potential to contaminate the downstream environment. Moreover, the results of water leaching tests showed that Sb was released from the solids more efficiently than As under oxidizing conditions. This might partly explain the predominance of Sb over As in most water samples.     

Mineralogical profiling of alluvial sediments from arsenic-affected Ganges-Brahmaputra floodplain in central Bangladesh

Mineral assemblages (heavy and light fractions) and sedimentological characteristics of the Quaternary alluvial aquifers were examined in the central Bengal Basin where As concentrations in groundwater are highly variable in space but generally decrease downward. Chemical compositions of sediment samples from two vertical core profiles (2-150 m below ground level, bgl) were analyzed along with groundwater in moderately As-enriched aquifers in central Bangladesh (Manikganj district), and the As mobilization process in the alluvial aquifer is described. Heavy minerals such as biotite, magnetite, amphibole, apatite and authigenic goethite are abundant at shallow (<100 m below ground level (mbgl)) depths but less abundant at greater depths. It is interpreted that principal As-bearing minerals were derived from multiple sources, primarily from ophiolitic belts in the Indus-Tsangpo suture in the northeastern Himalayan and Indo-Burman Mountain ranges. Authigenic and amorphous Fe-(oxy)hydroxide minerals that are generally formed in river channels in the aerobic environment are the major secondary As-carriers in alluvial sediments. Reductive dissolution (mediated by Fe-reducing bacteria) of Fe-(oxy)hydroxide minerals under anoxic chemical conditions is the primary mechanism responsible for releasing As into groundwater. Authigenic siderite that precipitates under reducing environment at greater depths decreases Fe and possibly As concentrations in groundwater. Presence of Fe(III) minerals in aquifers shows that reduction of these minerals is incomplete and this can release more As if further Fe-reduction takes place with increased supplies of organic matter (reactive C). Absence of authigenic pyrite suggests that SO₄ reduction (mediated by SO₄-reducing bacteria) in Manikganj groundwater is limited in contrast to the southeastern Bengal Basin where precipitation of arsenian pyrite is thought to sequester As from groundwater.     

Heavy metals and rare earth elements (REEs) in soil from the Nam Co Basin,Tibetan Plateau

Twenty-eight soil samples collected from the Nam Co Basin, Tibetan Plateau, have been analyzed for heavy metals (Cd, Cr, Ni, Cu, Zn, Pb and Mn), arsenic (As) and rare earth elements (REEs). In addition, for establishing the basic physio-chemical characteristics of the soil, pH, total organic concentration, electrical conductivity, and effective cation exchange capacity were measured. The results indicate that soil in the Nam Co Basin is still at an early alkaline weathering stage (pH = 7.94). Mean concentrations of heavy metals and As decreased as follows: Mn > Cr > Zn > Pb > Ni > Cu > As > Cd. The values of Cd, Cr, As, Pb and REEs in soil from the Nam Co Basin are higher relative to averaged background values over China. The chondrite-normalized diagrams of REEs suggest high REE(ΣLa → Eu)—enrichment, HREE(ΣGd → Lu)—depletion and Eu—depletion. Vertical profiles indicate that both heavy metals (except As) and REEs primarily exist in the surface soil. However, heavy metals and REEs vary in the surface soil between the southern and the northern bank of Nam Co. These differences are controlled mainly by parent materials.     

Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso

This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L⁻¹, they have a large range from <0.5 to 1630 μg L⁻¹. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L⁻¹. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O₂ concentrations <2 mg L⁻¹. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.