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.     

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.     

Irrigation produces elevated arsenic in the underlying groundwater of a semi-arid basin in Southwestern Idaho

The shallow aquifer beneath the Western Snake River Plain (Idaho, USA) exhibits widespread elevated arsenic concentrations (up to 120 μg L⁻¹). While semi-arid, crop irrigation has increased annual recharge to the aquifer from approximately 1 cm prior to a current rate of >50 cm year⁻¹. The highest aqueous arsenic concentrations are found in proximity to the water table (all values >50 μg L⁻¹ within 50 m) and concentrations decline with depth. Despite strong vertical redox stratification within the aquifer, spatial distribution of aqueous species indicates that redox processes are not primary drivers of arsenic mobilization. Arsenic release and transport occur under oxidizing conditions; groundwater wells containing dissolved arsenic at >50 μg L⁻¹ exhibit elevated concentrations of O₂ (average 4 mg L⁻¹) and NO₃ (average 8 mg L⁻¹) and low concentrations of dissolved Fe (<20 μg L⁻¹). Sequential extractions and spectroscopic analysis of surficial soils and sediments indicate solid phase arsenic is primarily arsenate and is present at elevated concentrations (4–45 mg kg⁻¹, average: 17 mg kg⁻¹) relative to global sedimentary abundances. The highest concentrations of easily mobilized arsenic (up to 7 mg kg⁻¹) are associated with surficial soils and sediments visibly stained with iron oxides. Batch leaching experiments on these materials using irrigation waters produce pore water arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L⁻¹). While As:Cl aqueous phase relationships suggest minor evaporative enrichment, this appears to be a relic of the pre-irrigation environment. Collectively, these data indicate that infiltrating irrigation waters leach arsenic from surficial sediments to the underlying aquifer.     

Uranium and arsenic dynamics in volcano-sedimentary basins – An exemplary study in North-Central Mexico

Uranium and As in deep groundwater of the volcano-sedimentary Villa de Reyes Graben around the city of San Luis Potosí in semi-arid North-Central Mexico (mean U: 7.6 μg L⁻¹, max. 138 μg L⁻¹; mean As: 11.4 μg L⁻¹, max. 25.8 μg L⁻¹) partly exhibit concentrations in excess of the WHO guideline values and thus endanger the quality of the most important drinking water source. To unravel the mechanisms for their enrichment in groundwater, the potential trace element sources, volcanic rocks and basin fill sediments, were characterized. A total of 131 solid and liquid samples were analyzed for major and trace element composition. The As/U hydrogeochemical signatures, their behavior during rock alteration and evidence from other major and trace element distributions, especially rare earth elements, strongly argue for dissolution of acid volcanic glass to be the dominating process of U and As release into groundwater. This natural baseline quality representing water–acid volcanic rock interaction is modified by additional trace element (preferentially As) mobilization from the sedimentary basin fill, representing a secondary source, in the course of decarbonatization of playa lake sediments and desorption from Fe-(hydr)oxide coated clastic material. The common behavior of both elements during magmatic differentiation and growing drift apart in sedimentary environments are important findings of this work. Comparison with recent findings in a similar environment suggests a common primary trace element source identification but significant differences in the evolution of As and U distribution. Geological and climatic similarity to numerous volcano-sedimentary basins makes the findings useful for water management purposes and transferable to other semi-arid regions facing challenges of geogenically impacted drinking water quality.     

Geochemical comparison of waters and stream sediments close to abandoned Sb-Au and As-Au mining areas,northern Portugal

Waters from abandoned Sb-Au mining areas have higher Sb (up to 2138 μg L⁻¹), As (up to 1252 μg L⁻¹) and lower Al, Zn, Li, Ni and Co concentrations than those of waters from the As-Au mining area of Banjas, which only contain up to 64 μg L⁻¹ As. In general, Sb occurs mainly as SbO₃⁻ and As H₂AsO₄⁻. In general, waters from old Sb-Au mining areas are contaminated in Sb, As, Al, Fe, Cd, Mn, Ni and NO₂⁻, whereas those from the abandoned As-Au mining area are contaminated in Al, Fe, Mn, Ni, Cd and rarely in NO₂⁻. Waters from the latter area, immediately downstream of mine dumps are also contaminated in As. In stream sediments from Sb-Au and As-Au mining areas, Sb (up to 5488 mg kg⁻¹) and As (up to 235 mg kg⁻¹) show a similar behaviour and are mainly associated with the residual fraction. In most stream sediments, the As and Sb are not associated with the oxidizable fraction, while Fe is associated with organic matter, indicating that sulphides (mainly arsenopyrite and pyrite) and sulphosalts containing those metalloids and metal are weathered. Arsenic and Sb are mainly associated with clay minerals (chlorite and mica; vermiculite in stream sediments from old Sb-Au mining areas) and probably also with insoluble Sb phases of stream sediments. In the most contaminated stream sediments, metalloids are also associated with Fe phases (hematite and goethite, and also lepidocrocite in stream sediments from Banjas). Moreover, the most contaminated stream sediments correspond to the most contaminated waters, reflecting the limited capacity of stream sediments to retain metals and metalloids.     

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.     

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.     

Environmental geochemistry of an active Hg mine in Xunyang,Shaanxi Province,China

The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m⁻³. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg⁻¹ and 1.2 to 11 μg kg⁻¹, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg⁻¹ and 1.0 to 39 μg kg⁻¹, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L⁻¹, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L⁻¹. Rice samples exhibited high concentrations of 50–200 μg kg⁻¹ in HgT and of 8.2–80 μg kg⁻¹ in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.     

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.     

Dissolved metals and associated constituents in abandoned coal-mine discharges,Pennsylvania, USA. Part 1: Constituent quantities and correlations

Complete hydrochemical data are rarely reported for coal-mine discharges (CMD). This report summarizes major and trace-element concentrations and loadings for CMD at 140 abandoned mines in the Anthracite and Bituminous Coalfields of Pennsylvania. Clean-sampling and low-level analytical methods were used in 1999 to collect data that could be useful to determine potential environmental effects, remediation strategies, and quantities of valuable constituents. A subset of 10 sites was resampled in 2003 to analyze both the CMD and associated ochreous precipitates; the hydrochemical data were similar in 2003 and 1999. In 1999, the flow at the 140 CMD sites ranged from 0.028 to 2210 L s⁻¹, with a median of 18.4 L s⁻¹. The pH ranged from 2.7 to 7.3; concentrations (range in mg/L) of dissolved (0.45-μm pore-size filter) SO₄ (34–2000), Fe (0.046–512), Mn (0.019–74), and Al (0.007–108) varied widely. Predominant metalloid elements were Si (2.7–31.3 mg L⁻¹), B (<1–260 μg L⁻¹), Ge (<0.01–0.57 μg L⁻¹), and As (<0.03–64 μg L⁻¹). The most abundant trace metals, in order of median concentrations (range in μg/L), were Zn (0.6–10,000), Ni (2.6–3200), Co (0.27–3100), Ti (0.65–28), Cu (0.4–190), Cr (<0.5–72), Pb (<0.05–11) and Cd (<0.01–16). Gold was detected at concentrations greater than 0.0005 μg L⁻¹ in 97% of the samples, with a maximum of 0.0175 μg L⁻¹. No samples had detectable concentrations of Hg, Os or Pt, and less than half of the samples had detectable Pd, Ag, Ru, Ta, Nb, Re or Sn. Predominant rare-earth elements, in order of median concentrations (range in μg/L), were Y (0.11–530), Ce (0.01–370), Sc (1.0–36), Nd (0.006–260), La (0.005–140), Gd (0.005–110), Dy (0.002–99) and Sm (<0.005–79). Although dissolved Fe was not correlated with pH, concentrations of Al, Mn, most trace metals, and rare earths were negatively correlated with pH, consistent with solubility or sorption controls. In contrast, As was positively correlated with pH.     

Formation,fate and leaching of chloroform in coniferous forest soils

Chloroform is a common groundwater pollutant but also a natural compound in forest ecosystems. Leaching of natural chloroform from forest soil to groundwater was followed by regular analysis of soil air and groundwater from multilevel wells at four different sites in Denmark for a period of up to 4 a. Significant seasonal variation in chloroform was observed in soil air 0.5 m below surface ranging at one site from 120 ppb by volume in summer to 20 ppb during winter. With depth, the seasonal variation diminished gradually, ranging from 30 ppb in summer to 20 ppb during winter, near the groundwater table. Chloroform in the shallowest groundwater ranged from 0.5–1.5 μg L⁻¹ at one site to 2–5 μg L⁻¹ at another site showing no clear correlation with season. Comparing changes in chloroform in soil air versus depth with on-site recorded meteorological data indicated that a clear relationship appears between rain events and leaching of chloroform. Chloroform in top soil air co-varied with CO₂ given a delay of 3–4 weeks providing evidence for its biological origin. This was confirmed during laboratory incubation experiments which further located the fermentation layer as the most chloroform producing soil horizon. Sorption of chloroform to soils, examined using ¹⁴C–CHCl₃, correlated with organic matter content, being high in the upper organic rich soils and low in the deeper more minerogenic soils. The marked decrease in chloroform in soil with depth may in part be due to microbial degradation which was shown to occur at all depths by laboratory tests using ¹⁴C–CHCl₃.     

Evidence of microbially mediated arsenic mobilization from sediments of the Aquia aquifer, Maryland, USA

Sediments from the Aquia aquifer in coastal Maryland were collected as part of a larger study of As in the Aquia groundwater flow system where As concentration are reported to reach levels as high as 1072 nmol kg⁻¹, (i.e., ∼80 μg/L). To test whether As release is microbially mediated by reductive dissolution of Fe(III) oxides/oxyhydroxides within the aquifer sediments, the Aquia aquifer sediment samples were employed in a series of microcosm experiments. The microcosm experiments consisted of sterilized serum bottles prepared with aquifer sediments and sterilized (i.e., autoclaved), artificial groundwater using four experimental conditions and one control condition. The four experimental conditions included the following scenarios: (1) aerobic; (2) anaerobic; (3) anaerobic + acetate; and (4) anaerobic + acetate + AQDS (anthraquinone-2,6-disulfonic acid). AQDS acts as an electron shuttle. The control condition contained sterilized aquifer sediments kept under anaerobic conditions with an addition of AQDS. Over the course of the 27 day microcosm experiments, dissolved As in the unamended (aerobic and anaerobic) microcosms remained constant at around ∼28 nmol kg⁻¹ (2 μg/L). With the addition of acetate, the amount of As released to the solution approximately doubled reaching ∼51 nmol kg⁻¹ (3.8 μg/L). For microcosm experiments amended with acetate and AQDS, the dissolved As concentrations exceeded 75 nmol kg⁻¹ (5.6 μg/L). The As concentrations in the acetate and acetate + AQDS amended microcosms are of similar orders of magnitude to As concentrations in groundwaters from the aquifer sediment sampling site (127-170 nmol kg⁻¹). Arsenic concentrations in the sterilized control experiments were generally less than 15 nmol kg⁻¹ (1.1 μg/L), which is interpreted to be the amount of As released from Aquia aquifer sediments owing to abiotic, surface exchange processes. Iron concentrations released to solution in each of the microcosm experiments were higher and more variable than the As concentrations, but generally exhibited similar trends to the As concentrations. Specifically, the acetate and acetate + AQDS amended microcosm typically exhibited the highest Fe concentrations (up to 1725 and 6566 nmol kg⁻¹, respectively). The increase in both As and Fe in the artificial groundwater solutions in these amended microcosm experiments strongly suggests that microbes within the Aquia aquifer sediments mobilize As from the sediment substrate to the groundwaters via Fe(III) reduction.     

Origin and mobility of hexavalent chromium in North-Eastern Attica,Greece

An integrated framework that is comprised of field surveys of groundwater, surface water and soils, laboratory process experiments and hydrologic and geochemical modeling is used to identify the origin (anthropogenic versus geogenic sources), fate and transport of hexavalent Cr in Tertiary and Quaternary deposits of Oropos plain in Greece. Groundwater and soils were analyzed in May 2008 and exhibited considerable Cr concentrations. Mineralogical analysis and micro-XRF analysis of the heavy soil fractions (metallic components) showed Cr bearing phases like chromites, Cr-silicate phases with positive correlation between Si, Al, Fe and Cr soil concentrations. Column experiments showed the Cr(VI) desorption ability of soils, e.g. concentration of 20 μg L⁻¹ was detected after the application of 50 mm of rain. The groundwater model simulated the variability of Cr concentrations emanating from both anthropogenic and geogenic sources, successfully using rate constants obtained from the laboratory experiments, e.g. 4.24 nM h⁻¹ for serpentine soil and 0.77 nM h⁻¹ for soil in alluvial deposits. The mineralogical and geochemical results support a geogenic origin for Cr in soils and groundwater of Oropos plain while modeling results suggest that contaminants transported by Asopos River have affected only the upper layers of the subsurface in the vicinity of the river. The framework can be used to establish background concentrations or clean up levels of Cr-contaminated soils and groundwater.     

Landscape scale controls on the vascular plant component of dissolved organic carbon across a freshwater delta

Lignin phenol concentrations and compositions were determined on dissolved organic carbon (DOC) extracts (XAD resins) within the Sacramento-San Joaquin River Delta (the Delta), the tidal freshwater portion of the San Francisco Bay Estuary, located in central California, USA. Fourteen stations were sampled, including the following habitats and land-use types: wetland, riverine, channelized waterway, open water, and island drains. Stations were sampled approximately seasonally from December, 1999 through May, 2001. DOC concentrations ranged from 1.3 mg L⁻¹ within the Sacramento River to 39.9 mg L⁻¹ at the outfall from an island drain (median 3.0 mg L⁻¹), while lignin concentrations ranged from 3.0 μg L⁻¹ within the Sacramento River to 111 μg L⁻¹ at the outfall from an island drain (median 11.6 μg L⁻¹). Both DOC and lignin concentrations varied significantly among habitat/land-use types and among sampling stations. Carbon-normalized lignin yields ranged from 0.07 mg (100 mg OC)⁻¹ at an island drain to 0.84 mg (100 mg OC)⁻¹ for a wetland (median 0.36 mg (100 mg OC)⁻¹), and also varied significantly among habitat/land-use types. A simple mass balance model indicated that the Delta acted as a source of lignin during late autumn through spring (10-83% increase) and a sink for lignin during summer and autumn (13-39% decrease). Endmember mixing models using S:V and C:V signatures of landscape scale features indicated strong temporal variation in sources of DOC export from the Delta, with riverine source signatures responsible for 50% of DOC in summer and winter, wetland signatures responsible for 40% of DOC in summer, winter, and late autumn, and island drains responsible for 40% of exported DOC in late autumn. A significant negative correlation was observed between carbon-normalized lignin yields and DOC bioavailability in two of the 14 sampling stations. This study is, to our knowledge, the first to describe organic vascular plant DOC sources at the level of localized landscape features, and is also the first to indicate a significant negative correlation between lignin and DOC bioavailability within environmental samples. Based upon observed trends: (1) Delta features exhibit significant spatial variability in organic chemical composition, and (2) localized Delta features appear to exert strong controls on terrigenous DOC as it passes through the Delta and is exported into the Pacific Ocean.     

Potentially harmful elements in rice paddy fields in mercury hot spots in Guizhou, China

Levels of the chalcophile metals Hg, Pb, Cd, Cu and Zn and the metalloid As in soils from rice paddy fields were assessed in two regions impacted by different industrial activities in Guizhou province, China. The two study areas (Wanshan and Qingzhen) have previously received attention in the international literature due to heavy Hg pollution, though levels of other potentially harmful elements have not previously been reported. The regions were selected as representing two important, but categorically different metal(loid) contaminated areas in China: Wanshan has been a major region for Hg production while Qingzhen is an important region for a large range of various heavy industries based on coal as an energy source.Within a limited distance of the Hg mine tailings in Wanshan the rice paddy fields are heavily contaminated by Hg (median concentration 25 μg g⁻¹, maximum 119 μg g⁻¹) and moderately contaminated by Zn and Cd (median concentrations of 86 and 0.9 μg g⁻¹, respectively). Zinc and Cd levels correlate well with Hg contamination, which indicates a similar source and mechanism of transport and accumulation. Other studies have concluded that the main sources of Hg contamination in Wanshan are the numerous Hg mine tailings. This is likely as these metals are all geochemically associated with the mineral cinnabar (HgS). The other chalcophile elements (Pb, Cu and As) are nevertheless found at background levels only. In Qingzhen the soils were found to contain elevated levels of As and Hg (median concentrations of 38 and 0.3 μg g⁻¹, respectively). These are elements that are known to be more associated with coal and released during coal combustion.     

Mineral control of arsenic content in thermal waters from volcano-hosted hydrothermal systems: Insights from island of Ischia and Phlegrean Fields (Campanian Volcanic Province, Italy)

This paper documents arsenic concentrations in 157 groundwater samples from the island of Ischia and the Phlegrean Fields, two of the most active volcano-hosted hydrothermal systems from the Campanian Volcanic Province (Southern Italy), in an attempt to identify the environmental conditions and mineral-solution reactions governing arsenic aqueous cycling. On Ischia and in the Phlegrean Fields, groundwaters range in composition from NaCl brines, which we interpret as the surface discharge of deep reservoir fluids, to shallow-depth circulating fluids, the latter ranging from acid-sulphate steam-heated to hypothermal, cold, bicarbonate groundwaters. Arsenic concentrations range from 1.6 to 6900 μg·l^− 1 and from 2.6 to 3800 μg·l^− 1 in the Phlegrean Fields and on Ischia, respectively. They increase with increasing water temperature and chlorine contents, and in the sequence bicarbonate groundwaters < steam-heated groundwaters < NaCl brines. According to thermochemical modeling, we propose that high As concentrations in NaCl brines form after prolonged water-rock interactions at reservoir T, fO₂ and fH₂S conditions, and under the buffering action of an arsenopyrite + pyrite + pyrrhotite rock assemblage. On their ascent toward the surface, NaCl brines become diluted by As-depleted meteoric-derived bicarbonate groundwaters, giving rise to hybrid water types with intermediate to low As contents. Steam-heated groundwaters give their intermediate to high As concentrations to extensive rock leaching promoted by interaction with As-bearing hydrothermal steam.     

Gold solubility in oxidized and reduced, water-saturated mafic melt

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g⁻¹ to 0.64 μg g⁻¹ at FMQ + 4, and 0.54 μg g⁻¹ to 0.1 μg g⁻¹ at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g⁻¹). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO₂ values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.