Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4 · 2H2O) and their application to arsenic behavior in buried mine tailings

Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 10^4.04 (FeH₂AsO₄²⁺), 10^9.86 (FeHAsO₄⁺), and 10^18.9 (FeAsO₄). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log K_sp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2-8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log K_sp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)₃) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2-3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1-2 mg/L with increasing age and depth. In the TMF, average apparent log K_sp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.     

Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene

Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, ⁸⁷Sr/⁸⁶Sr ratios range from 0.7080 to 0.7090, and δ³⁴S-SO₄ values range from +10 to 30‰. A few waters have elevated δ¹³C_DIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ¹⁸O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ¹⁸O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ¹⁸O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ¹³C_DIC = −14.7 to−11.1‰ and ⁸⁷Sr/⁸⁶Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.     

Characteristics of deep groundwater flow in a basin marginal setting at Sellafield,Northwest England: Cl and halide evidence

A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in ³⁶Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10⁻³ by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10⁻³ by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable ³⁶Cl/Cl ratios, which partly reflect differing in situ³⁶Cl production rates in different rock formations. Most ³⁶Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10⁻¹⁵) and western sedimentary cover (mean = 10 × 10⁻¹⁵) are at or close to equilibrium with in situ³⁶Cl production. These variations in ³⁶Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.     

Sr fluxes and Sr/Sr characterization of river waters from a karstic versus granitic watershed in the Yangtze River

The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the ⁸⁷Sr/⁸⁶Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27 μg l^− 1. Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. ⁸⁷Sr/⁸⁶Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l^− 1). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 122 t km^− 2 a^− 1 and 0.079 t km^− 2 a^− 1, respectively, which are higher than those of the SGW (56 t km^− 2 a^− 1 and 0.021 t km^− 2 a^− 1, respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.     

Hydrogeological and geochemical control of the variations of Rn concentrations in a hard rock aquifer: Insights into the possible role of fracture-matrix exchanges

To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L⁻¹. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m⁻² s⁻¹ in granite and from 0.5 to 0.9 mBq m⁻² s⁻¹ in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10⁻⁵ to 10⁻³ m² s⁻¹. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm.     

Background concentrations of elements in surface soils and their changes as affected by agriculture use in the desert-oasis ecotone in the middle of Heihe River Basin,North-west China

The concentrations of twenty four chemical elements in the surface layer of natural desert soils and the cultivated farmland soils were measured at a desert-oasis ecotone in the middle of Heihe river basin, north-west China. Background values were estimated for (a) major elements (Si 335.3 g kg^− 1, Al 49.4 g kg^− 1, Fe 19.1 g kg^− 1, Ca 29.4 g kg^− 1, Mg 8.9 g kg^− 1, K 20.1 g kg^− 1, Na 17.5 g kg^− 1 and P 0.338 g kg^− 1), (b) heavy metals and non-metals (Cr 55.8 mg kg^− 1, Mn 404.8 mg kg^− 1, Ni 17.7 mg kg^− 1, Cu 5.1 mg kg^− 1, Zn 33.7 mg kg^− 1, Pb 15.5 mg kg^− 1 and As 5.2 mg kg^− 1) and (c) other trace elements (Ti 2.0 mg kg^− 1, V 55.3 mg kg^− 1, Co 5.7 mg kg^− 1, Rb 82.4 mg kg^− 1, Sr 232.9 mg kg^− 1, Y 14.7 mg kg^− 1, Zr 194.9 mg kg^− 1, Nb 7.8 mg kg^− 1 and Ba 720.6 mg kg^− 1). After natural desert soil was cultivated for agricultural use, significant changes in element concentrations occurred under tillage, irrigation and fertilisation management. Compared to natural soil, the for the levels of Si, K, Na, Sr, Zr and Ba decreased, and no changes were observed for Rb, while the values of the other 17 elements increase in agricultural soil from 1.2 to 3.5 times. However, their absolute concentrations are still low, suggesting that the arable soil in this region remains comparatively a clean soil. The increased silt, clay and organic carbon content, under long-term irrigation, enriched the fine-grained materials, and application of fertilisers and manure contributed to the accumulation of most elements in arable soil. The accumulation of elements in agricultural soil increased with increasing cultivation years and extent of soil development.     

Quantification of vanadium adsorption by German soils

Vanadium adsorption by soils representing different soil types from Germany has been studied. For 30 soils ‘Freundlich’ type sorption isotherms have been deduced from laboratory vanadium(V) adsorption experiments. The native adsorbed vanadium quantity of a soil (S₀) and the Freundlich parameters m and log k have been determined by non linear regression of the experimental data to the Freundlich model. Pronounced differences in vanadium adsorption of different soils exist and could be quantified. The vanadium adsorption data could be generalized by grouping the soils into four classes according to their vanadium adsorption properties. For each class (sandy soils, top soils, sub soils with pH < 5.5, and sub soils with pH > 5.5) mean Freundlich parameters m and log k have been calculated to be 0.59, 0.72, 0.52, 0.57 and 2.55, 2.89, 4.29, 3.41, respectively. These parameters can be used to estimate vanadium sorption properties of soils for which no vanadium sorption experiments are available. Aqua regia soluble vanadium contents of the studied soils (range 1.7–143 mg/kg; median 32 mg/kg) and leached vanadium concentrations from experiments without vanadium addition (range 0.08–37 µg/l; median 2.1 µg/l) are also given.     

Sulfur speciation in natural hydrothermal waters, Iceland

The speciation of aqueous dissolved sulfur was determined in hydrothermal waters in Iceland. The waters sampled included hot springs, acid-sulfate pools and mud pots, sub-boiling well discharges and two-phase wells. The water temperatures ranged from 4 to 210 °C, the pH_T was between 2.20 and 9.30 at the discharge temperature and the SO₄ and Cl concentrations were 0.020-52.7 and <0.01-10.0 mmol kg⁻¹, respectively. The analyses were carried out on-site within ∼10 min of sampling using ion chromatography (IC) for sulfate (SO₄²⁻), thiosulfate (S₂O₃²⁻) and polythionates (S_xO₆²⁻) and titration and/or colorimetry for total dissolved sulfide (S²⁻). Sulfite (SO₃²⁻) could also be determined in a few cases using IC. Alternatively, for few samples in remote locations the sulfur oxyanions were stabilized on a resin on site following elution and analysis by IC in the laboratory. Dissolved sulfate and with few exceptions also S²⁻ were detected in all samples with concentrations of 0.02-52.7 mmol kg⁻¹ and <1-4100 μmol kg⁻¹, respectively. Thiosulfate was detected in 49 samples of the 73 analyzed with concentrations in the range of <1-394 μmol kg⁻¹ (S-equivalents). Sulfite was detected in few samples with concentrations in the range of <1-3 μmol kg⁻¹. Thiosulfate and SO₃²⁻ were not detected in <100 °C well waters and S₂O₃²⁻ was observed only at low concentrations (<1-8 μmol kg⁻¹) in ∼200 °C well waters. In alkaline and neutral pH hot springs, S₂O₃²⁻ was present in significant concentrations sometimes corresponding to up to 23% of total dissolved sulfur (S_TOT). In steam-heated acid-sulfate waters, S₂O₃²⁻ was not a significant sulfur species. The results demonstrate that S₂O₃²⁻ and SO₃²⁻ do not occur in the deeper parts of <150 °C hydrothermal systems and only in trace concentrations in ∼200-300 °C systems. Upon ascent to the surface and mixing with oxygenated ground and surface waters and/or dissolution of atmospheric O₂, S²⁻ is degassed and oxidized to SO₃²⁻ and S₂O₃²⁻ and eventually to SO₄²⁻ at pH >8. In near-neutral hydrothermal waters the oxidation of S²⁻ and the interaction of S²⁻ and S⁰ resulting in the formation of S_x²⁻ are considered important. At lower pH values the reactions seemed to proceed relatively rapidly to SO₄²⁻ and the sulfur chemistry of acid-sulfate pools was dominated by SO₄²⁻, which corresponded to >99% of S_TOT. The results suggest that the aqueous speciation of sulfur in natural hydrothermal waters is dynamic and both kinetically and source-controlled and cannot be estimated from thermodynamic speciation calculations.     

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO₂-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO₂ discharge through soils of 1.8 × 10⁹ mol a⁻¹ shows that large amounts of CO₂ move from the deep reservoir to the surface. A large range in δ¹³C_TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10⁹ mol a⁻¹ of deeply sourced CO₂ in spring waters. The total natural CO₂ discharge (2.1 × 10⁹ mol a⁻¹) is slightly less than minimum CO₂ output estimated by an indirect method (2.8 × 10⁹ mol a⁻¹), but present-day release of 5.8 × 10⁹ mol a⁻¹ CO₂ from deep geothermal wells may have reduced natural CO₂ discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m⁻²) and reaches extremely high values (up to 2700 mW m⁻²) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO₂ flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO₂ degassing and heat transfer patterns.     

Monomethylmercury sources in a tropical artificial reservoir

The distribution and speciation of mercury (Hg) in the water column, the inputs (wet deposition and tributaries) and the outputs (atmospheric evasion and outlet) of an artificial partially anoxic tropical lake (Petit-Saut reservoir, French Guiana) were investigated on a seasonal basis in order to appraise the cycling and transformations of this metal. The total mercury (HgT) concentrations in the oxygenated epilimnetic waters averaged 5 ± 3 pmol L⁻¹ in the unfiltered samples (HgT_UNF) and 4 ± 2 pmol L⁻¹ in the dissolved (HgT_D) phase (<0.45 μm). On average, the monomethylmercury (MMHg) constituted 8%, 40% and 18% of the HgT in the dissolved phase, the particulate suspended matter and in the unfiltered samples, respectively. Covariant elevated concentrations of particulate MMHg and chlorophyll a in the epilimnion suggest that phytoplankton is an active component for the MMHg transfer in the lake. In the anoxic hypolimnion the HgT_UNF averages 13 ± 6 pmol L⁻¹ and the HgT_D 8 ± 4 pmol L⁻¹. The averages of MMHg_P and MMHg_D in hypolimnetic waters were two and three times the corresponding values of the epilimnion, 170 ± 90 pmol g⁻¹ and 0.9 ± 0.5 pmol L⁻¹, respectively. In the long dry and wet seasons, at the flooded forest and upstream dam sampling stations, the vertical profiles of MMHg_D concentrations accounted for two distinct maxima: one just below the oxycline and the other near the benthic interface. Direct wet atmospheric deposition accounted for 14 moles yr⁻¹ HgT_UNF, with 0.7 moles yr⁻¹ as MMHg_UNF, while circa 76 moles yr⁻¹ of HgT_UNF, with 4.7 moles yr⁻¹ as MMHg_UNF, coming from tributaries. Circa 78 moles (∼17% as MMHg) are annually exported through the dam, while 23 moles yr⁻¹ of Hg⁰ evolve in the atmosphere. A mass balance calculation suggests that the endogenic production of MMHg_UNF attained 8.1 moles yr⁻¹, corresponding to a methylation rate of 0.06% d⁻¹. As a result, the Petit-Saut reservoir is a large man-made reactor that has extensively altered mercury speciation in favor of methylated species.     

Complexation of cadmium to sulfur and oxygen functional groups in an organic soil

Cadmium (Cd) is a toxic trace element and due to human activities soils and waters are contaminated by Cd both on a local and global scale. It is widely accepted that chemical interactions with functional groups of natural organic matter (NOM) is vital for the bioavailability and mobility of trace elements. In this study the binding strength of cadmium (Cd) to soil organic matter (SOM) was determined in an organic (49% organic C) soil as a function of reaction time, pH and Cd concentration. In experiments conducted at native Cd concentrations in soil (0.23 μg g⁻¹ dry soil), halides (Cl, Br) were used as competing ligands to functional groups in SOM. The concentration of Cd in the aqueous phase was determined by isotope-dilution (ID) inductively-coupled-plasma-mass-spectrometry (ICP-MS), and the activity of Cd²⁺ was calculated from the well-established Cd-halide constants. At higher Cd loading (500-54,000 μg g⁻¹), the Cd²⁺ activity was directly determined by an ion-selective electrode (ISE). On the basis of results from extended X-ray absorption fine structure (EXAFS) spectroscopy, a model with one thiolate group (RS⁻) was used to describe the complexation (Cd²⁺ + RS⁻ ? CdSR⁺; log K_CdSR) at native Cd concentrations. The concentration of thiols (RSH; 0.047 mol kg⁻¹ C) was independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. Log K_CdSR values of 11.2-11.6 (pK_a for RSH = 9.96), determined in the pH range 3.1-4.6, compare favorably with stability constants for the association between Cd and well-defined thiolates like glutathione. In the concentration range 500-54,000 μg Cd g⁻¹, a model consisting of one thiolate and one carboxylate (RCOO⁻) gave the best fit to data, indicating an increasing role for RCOOH groups as RSH groups become saturated. The determined log K_CdOOCR of 3.2 (Cd²⁺ +  RCOO⁻ ? CdOOCR⁺; log K_CdOOCR; pK_a for RCOOH = 4.5) is in accordance with stability constants determined for the association between Cd and well-defined carboxylates. Given a concentration of reduced sulfur groups of 0.2% or higher in NOM, we conclude that the complexation to organic RSH groups may control the speciation of Cd in soils, and most likely also in surface waters, with a total concentration less than 5 mg Cd g⁻¹ organic C.     

Groundwater flow in an arsenic-contaminated aquifer,Mekong Delta,Cambodia

To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km² area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K ≈ 10⁻⁴ ms⁻¹) and overlying clay aquitard (K ≈ 10⁻⁸ ms⁻¹) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma⁻¹ downward through the clay and 1–13 ma⁻¹ horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer.     

Size and composition of inorganic colloids in a peri-alpine, glacial flour-rich lake

From June 2004 to October 2005, particle size distributions (PSD) and the mineralogy of inorganic colloids in Lake Brienz, a glacial flour-rich Swiss lake, and in its tributaries (the Aare and Lütschine rivers), were quantified by single particle counting based on light scattering (0.1-2 μm) combined with transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. The two lake tributaries differ in terms of the geology of their catchment area and in their hydrology: the River Lütschine’s glacial regime is unperturbed, while the River Aare has been significantly modified by hydroelectric installations. The dependence of the colloid concentration numbers on colloid diameters can be described by a power-law distribution in all cases. The power-law slopes in the River Aare were consistently smaller, and the total colloid number concentrations consistently higher, (mean values: −3.89 and 6.2 × 10⁸ mL⁻¹, respectively) than in the River Lütschine (−3.61 and 1.7 × 10⁸ mL⁻¹). The measured PSD in the lake were dependent on season and depth: homogenous in the whole epilimnion in winter and modified by river intrusions in summer. In this season, the presence of a river turbid plume above the thermocline is a significant source of colloids in the epilimnion, where their concentration was high (mean value: 5.1 × 10⁷ mL⁻¹). The same main mineral types were found in the different systems: 2:1 minerals, feldspars, chlorite and quartz; with a few particles of oxides. Differences in their relative concentrations in the rivers, essentially reflecting the geological differences of the respective catchment areas, made it possible to trace the origin of the mineral colloids present in lake surface waters throughout the year. The relationship between colloid size and mineralogy was assessed, showing that different minerals have distinct size distributions. In the rivers, where almost no aggregates were observed, PSD would essentially be the result of the nature (particle formation and mineralogy) of the colloids, whereas in the lake, aggregation and sedimentation processes would also play a role, particularly in summer.     

Organic complexation of rare earth elements in natural waters: Evaluating model calculations from ultrafiltration data

The Stockholm Humic Model (SHM) and Humic Ion-Binding Models V and VI were compared for their ability to predict the role of dissolved organic matter (DOM) in the speciation of rare earth elements (REE) in natural waters. Unlike Models V and VI, SHM is part of a speciation code that also allows us to consider dissolution/precipitation, sorption/desorption and oxidation/reduction reactions. In this context, it is particularly interesting to test the performance of SHM. The REE specific equilibrium constants required by the speciation models were estimated using linear free-energy relationships (LFER) between the first hydrolysis constants and the stability constants for REE complexation with lactic and acetic acid. Three datasets were used for the purpose of comparison: (i) World Average River Water (Dissolved Organic Carbon (DOC) = 5 mg L⁻¹), previously investigated using Model V, was reinvestigated using SHM and Model VI; (ii) two natural organic-rich waters (DOC = 18-24 mg L⁻¹), whose REE speciation has already been determined with both Model V and ultrafiltration studies, were also reinvestigated using SHM and Model VI; finally, (iii) new ultrafiltration experiments were carried out on samples of circumneutral-pH (pH 6.2-7.1), organic-rich (DOC = 7-20 mg L⁻¹) groundwaters from the Kervidy-Naizin and Petit-Hermitage catchments, western France. The results were then compared with speciation predictions provided by Model VI and SHM, successively. When applied to World Average River Water, both Model VI and SHM yield comparable results, confirming the earlier finding that a large fraction of the dissolved REE in rivers occurs as organic complexes This implies that the two models are equally valid for calculating REE speciation in low-DOC waters at circumneutral-pH. The two models also successfully reproduced ultrafiltration results obtained for DOC-rich acidic groundwaters and river waters. By contrast, the two models yielded different results when compared to newly obtained ultrafiltration results for DOC-rich (DOC > 7 mg L⁻¹) groundwaters at circumneutral-pH, with Model VI predictions being closer to the ultrafiltration data than SHM. Sensitivity analysis indicates that the “active DOM parameter” (i.e., the proportion of DOC that can effectively complex with REE) is a key parameter for both Model VI and SHM. However, a survey of ultrafiltration results allows the “active DOM parameter” to be precisely determined for the newly ultrafiltered waters studied here. Thus, the observed discrepancy between SHM predictions and ultrafiltration results cannot be explained by the use of inappropriate “active DOM parameter” values in this model. Save this unexplained discrepancy, the results presented in this study demonstrate that both Model VI and SHM can provide reliable estimates of REE speciation in organic-rich waters. However, it is essential to know the proportion of DOM that can actively complex REE before running these two speciation models.     

Arsenic speciation and turnover in intact organic soil mesocosms during experimental drought and rewetting

Wetlands are significant sources and sinks for arsenic (As), yet the geochemical conditions and processes causing a release of dissolved arsenic and its association with the solid phase of wetland soils are poorly known. Here we present experiments in which arsenic speciation was determined in peatland mesocosms in high spatiotemporal resolution over 10 months. The experiment included a drought/rewetting treatment, a permanently wet, and a defoliated treatment. Soil water content was determined by the TDR technique, and arsenic, iron and sulfate turnover from mass balancing stocks and fluxes in the peat, and solid phase contents by sequential extractions. Arsenic content ranged from 5 to 25 mg kg⁻¹ and dissolved concentrations from 10 to 300 μg L⁻¹, mainly in form of As(III), and secondarily of As(V) and dimethylated arsenic (DMA). Total arsenic was mainly associated with amorphous iron hydroxides (R² > 0.95, α < 0.01) and deeper into the peat with an unidentified residual fraction. Arsenic release was linked to ferrous iron release and primarily occurred in the intensely rooted uppermost soil. Volumetric air contents of 2-13 % during drought eliminated DMA from the porewater and suppressed its release after rewetting for >30 d. Dissolved As(III) was oxidized and immobilized as As(V) at rates of up to 0.015 mmol m⁻³ d⁻¹. Rewetting mobilized As(III) at rates of up to 0.018 mmol m⁻³ d⁻¹ within days. Concurrently, Fe(II) was released at depth integrated rates of up 20 mmol m⁻³ d⁻¹. The redox half systems of arsenic, iron, and sulfur were in persistent disequilibrium, with H₂S being a thermodynamically viable reductant for As(V) to As(III). The study suggests that rewetting can lead to a rapid release of arsenic in iron-rich peatlands and that methylation is of lesser importance than co-release with iron reduction, which was largely driven by root activity.     

The record of temperature, wind velocity and air humidity in the δD and δO of water inclusions in synthetic and Messinian halites

Deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally-determined during evaporation of a NaCl solution (35 g L⁻¹) as a function of water temperature and wind velocity. In the case of a null wind velocity, slopes of δD-δ¹⁸O trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23-47 °C. For wind velocities ranging from 0.8 to 2.2 m s⁻¹, slopes of the δD-δ¹⁸O trajectories linearly increase with increasing wind velocity at a given water temperature. These experimental results can be modeled by using Rayleigh distillation equations taking into account wind-related kinetics effects. Deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro-equilibration techniques.These isotopic compositions accurately reflect those of the surrounding residual waters during halite growth. Isotopic compositions of water inclusions in twenty natural halites from the Messinian Realmonte mine in Sicily suggest precipitation temperatures of that match the homogenization temperatures obtained by microthermometry (median = 34 ± 5 °C). The similarity between the measured and experimental slopes of the δD-δ¹⁸O evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits. Hydrogen and oxygen isotope compositions of water inclusions from Realmonte halite also define a linear trend whose extrapolation until intersection with the Mediterranean Meteoric Water Line allows the characterization of the water source with δD and δ¹⁸O values of −70 ± 10‰ and −11.5 ± 1.5‰, respectively. These results reveal that the huge amounts of salts deposited in Sicily result from the evaporation of seawater mixed with a dominant fraction (?50%) of meteoric waters most likely deriving from alpine fluvial discharge.     

Ab-initio structure, energy and stable Fe isotope equilibrium fractionation of some geochemically relevant H-O-Fe complexes

The hexa-aqua complexes [Fe(H₂O)_6−m−n(OH)_n]⁽²⁻ⁿ⁾⁺n = 0 → 3, m = 0 → 6 − n; [Fe(H₂O)_6−m−n(OH)_n]⁽³⁻ⁿ⁾⁺n = 0 → 4, m = 0 → 6 − n were investigated by ab-initio methods with the aim of determining their ground-state geometries, total energies and vibrational properties by treating their inner solvation shell as part of their gaseous precursor¹ (or “hybrid approach”). After a gas-phase energy optimization within the Density Functional Theory (DFT), the molecules were surrounded by a dielectric representing the Reaction Field through an implicit Polarized Continuum Model (PCM). The exploration of several structural ligand arrangements allowed us to quantify the relative stabilities of the various ionic species and the role of the various forms of energy (solute-solvent electronic interaction, cavitation, dispersion, repulsion, liberation free energy) that contribute to stabilize the aqueous complexes. A comparison with experimental thermochemistries showed that ab-initio gas-phase + solvation energies are quite consistent with experimental evidence and allow the depiction of the most stable form in solution and the eventual configurational disorder of water/hydroxyl species around central cations. A vibrational analysis performed on the ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe and ⁵⁸Fe isotopomers indicated important separative effects systematically affected by the extent of deprotonation. The role of the system’s redox state (fO₂) and acidity (pH) on the isotopic imprinting of the aqueous species in solution was investigated by coupling the separative effects with speciation calculations. The observed systematics provided a tool of general utility in the interpretation of the iron isotopic signature of natural waters. Applications to the interpretation of isotopic fractionation in solution dictated by redox equilibria and to the significance of the Fe-isotopic imprinting of Banded Iron Formations are given.     

The universal ratio of boron to chlorinity for the North Pacific and North Atlantic oceans

We report seawater boron concentration (mg kg⁻¹) and chlorinity (‰) values measured in seawater samples (n = 139) collected at various depths in the North Pacific and North Atlantic oceans and the East/Japan Sea (located in the western temperate North Pacific). Our results indicate that variations in seawater boron concentration are strongly coupled to variations in chlorinity (and salinity), yielding a mean boron to chlorinity ratio of 0.2414 ± 0.0009 mg kg⁻¹ ‰⁻¹ (boron to salinity ratio = 0.1336 ± 0.0005 mg kg⁻¹ ‰⁻¹). This ratio was surprisingly universal throughout the water column in the three marine basins and across widely different ocean surface regimes, but differs from the generally accepted ratio of 0.232 ± 0.005 mg kg⁻¹ ‰⁻¹ determined by Uppström (1974), which was based on only 20 measurements at four sites in the tropical Pacific Ocean. In converting total alkalinity to carbonate alkalinity (and vice versa) for thermodynamic calculations, the difference between these two ratios leads to a difference of 5 μmol kg⁻¹ in estimates for ocean surface waters, where the contribution of borate to total alkalinity is typically greatest. We suggest the use of the new boron to chlorinity ratio for predicting seawater boron concentrations using chlorinity (or salinity) data.     

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (ε_a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of ²H and ¹⁸O associated with evaporation of lake waters. The most negative ε_a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε_a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ¹⁸O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.