Hydrochemical characteristics and seasonal influence on the pollution by acid mine drainage in the Odiel river Basin (SW Spain)

The Odiel river Basin is heavily affected by acid mine drainage (AMD) from the sulphide mining areas in the Iberian Pyrite Belt (IPB). A thorough study has been conducted along this fluvial system, monitoring the seasonal influence on the pollution level and its hydrochemical characteristics. From 2002 to 2006, surface water samples were collected at 91 different points throughout the Odiel river Basin and analyzed by field and laboratory methods for dissolved metals and metalloids. Acid mine drainage affects 37% of the length of the drainage network, which shows a great diversity of geochemical conditions as well as significant variations through the hydrological year. Unaffected streams show different water types depending on the lithological substrate and the marine aerosol influence. Mean concentrations in the contaminated streams are very high: 231 mg/L of Fe, 135 mg/L of Al, 56 mg/L of Zn, 16 mg/L of Cu, etc. Four types of contaminated streams were recognized based on hydrochemical and physicochemical characteristics. There are important seasonal variations depending on the precipitation regimen, level of pollution and proximity to the AMD sources. In the more contaminated samples the M/Fe ratio (M = metals other than Fe) decreases during the summer season. Slightly contaminated samples show an inverse evolution as this ratio increases in spring and summer due to substantial Fe precipitation. A recomparison of contaminant loads suggests that the Odiel river Basin (including the Tinto river) accounts for 15% of the global gross flux of dissolved Zn and 3% of the global gross flux of dissolved Cu transported by rivers into the ocean.     

Assessing the concentration,speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine,Vermont, USA

The authors determine the composition of a river that is impacted by acid-mine drainage, evaluate dominant physical and geochemical processes controlling the composition, and assess dissolved metal speciation and toxicity using a combination of laboratory, field and modeling studies. Values of pH increase from 3.3 to 7.6 and the sum of dissolved base metal (Cd + Co + Cu + Ni + Pb + Zn) concentrations decreases from 6270 to 100 μg/L in the dynamic mixing and reaction zone that is downstream of the river’s confluence with acid-mine drainage. Mixing diagrams and PHREEQC calculations indicate that mixing and dilution affect the concentrations of all dissolved elements in the reach, and are the dominant processes controlling dissolved Ca, K, Li, Mn and SO₄ concentrations. Additionally, dissolved Al and Fe concentrations decrease due to mineral precipitation (gibbsite, schwertmannite and ferrihydrite), whereas dissolved concentrations of Cd, Co, Cu, Ni, Pb and Zn decrease due to adsorption onto newly formed Fe precipitates.     

Dynamics of contaminants in phosphogypsum of the fertilizer industry of Huelva (SW Spain): From phosphate rock ore to the environment

The dynamics of trace elements from phosphate rock ore to the environment in a phosphoric acid plant located in SW Spain and the impact of phosphogypsum wastes were investigated through total digestion and BCR-sequential extraction. Based on total concentration, element transfer factors as criteria for examining the potential environmental risk of waste with respect to ore rock were calculated, and it was observed that most trace elements are only transferred into phosphogypsum at rates of 2–12%. However, based on those concentrations that are likely to be most readily mobile in the environment, phosphogypsum acts as a higher emission source of contaminants than the original rock. About 100 million tonnes of phosphogypsum are stack-piled in a dump of 1200 ha over salt-marshes of an estuary formed by the confluence of the Tinto and Odiel rivers. Phosphogypsum has been applied, at the recommended rate of 20–25 t/ha since 1978–2001, to improve fertility and reduce Na saturation in agricultural soils of the Guadalquivir river valley (140 km²). Phosphogypsum capacity as a source of mobile contaminants in three environmental scenarios (water leaching, exposure to oxidising and reducing conditions) was quantified by combining sequential extraction and waste mass. The amounts of mobile contaminants that could be released for every tonne of phosphogypsum are approximately 7 × 10² g Sr, 1.1 × 10² g Fe, 55 g Y, 30 g Ce, 12 g Cr, 11 g Ti, 5 g Zn, 4 g each of Cu and Pb, 3 g each of V and Cd, 2 g each of As and Ni and 1 g U. Multiplying these amounts by 100 Mt and 20–25 t/ha, it is possible to calculate risk assessments of phosphogypsum for both estuarine zones, e.g. in a hypothetical stack collapse and waste spilling, and agricultural soils, respectively.     

Behaviour of rare earth elements at the freshwater–seawater interface of two acid mine rivers: the Tinto and Odiel (Andalucia, Spain)

The concentrations of dissolved and suspended particulate rare-earth elements (REE) are reported in acid-sulphate waters from the Odiel and Tinto rivers. Shale normalized patterns are typically convex and high REE concentrations (e.g., Ce=0.43–65 μg.l⁻¹) are present in the waters. The REE content of the suspended load is greater by a factor of up to 3000. In the Odiel river, REE patterns of the particulates are essentially convex and sub-parallel to those of the waters; speciation calculations indicate that SO₄ complexes play a dominant role in controlling the REE distributions. In the Tinto river, the REE patterns of the suspended load are slightly fractionated and a negative Ce anomaly is apparent in several samples, reflecting the local influence of phosphogypsum deposits.Contrasting with normal estuaries, REE are not intensely removed in the low chlorinity zone. A remobilization in relation to Fe reduction is observed in the Tinto river.     

Spatial-seasonal dynamics of chromophoric dissolved organic matter in Lake Taihu, a large eutrophic, shallow lake in China

From 2005 to 2009, the spatial distribution and the seasonal dynamics of chromophoric dissolved organic matter (CDOM) were explored in Lake Taihu in eastern China. The spatial-seasonal dynamics of CDOM absorption and three CDOM composition variables, including spectral slope (S), spectral slope ratio (S_R) and the M value, defined as the ratio of absorption at 250 nm/365 nm, were analyzed and discussed. Furthermore, river input processes and degradation of phytoplankton were studied to assess their impact on CDOM composition and the factors involved in the spatial-seasonal variability of CDOM. The CDOM absorption coefficient at a wavelength of 350 nm, a(350), ranged from 1.37-9.55 m⁻¹ with a mean of 3.33 ± 1.32 m⁻¹. Spatially, higher a(350) values, but lower spectral slope, spectral slope ratio and M values, were recorded in the northern algae dominated bays while lower values were recorded in southeastern macrophyte dominated bays. The a(350) was significantly higher in inshore waters than in offshore waters. Values of S, S_R and M decreased during the flood and algal bloom season in spring and summer whereas a(350) increased. In the three river profiles, the gradual decrease of a(350) along the trajectory from the river mouth into the lake during the flood season showed the contribution of allochthonous CDOM. A laboratory phytoplankton degradation experiment was conducted to determine the contribution of CDOM production from phytoplankton. The significant increase of a(350) with time in the CDOM production experiment underlines the importance of autochthonous CDOM production during the algal bloom season. In summary, the significant increase of a(350) in spring and summer (algal bloom season) may be due to both the allochthonous CDOM input from the surrounding rivers and the autochthonous production of CDOM from degrading phytoplankton.     

Refining the estimation of metal loads dissolved in acid mine drainage by continuous monitoring of specific conductivity and water level

The accurate estimation of metal loads transported by streams is necessary to calculate reliable mass transfers of metals between compartments, both at local and global scales. This estimation is particularly relevant in the case of the Tinto and Odiel Rivers (SW Spain) due to their significant contribution to the total metal transfer from continents to the ocean. At a local scale, the metal load transported by streams plays a key role in predicting the biogeochemical evolution of water reservoirs affected by Acid Mine Drainage (AMD). This work uses the relationships between specific conductivity (SC) and dissolved elements to calculate the metal load of the River Meca, a tributary of the Odiel. The SC and the water level were continuously monitored from April 2009 to June 2010. Water samples were also collected and measurements of the discharge were carried out manually once a month. The relationships between the SC and the concentration of dissolved elements are, in general, very good (R² > 0.90). However, some key elements such as Fe show a very poor correlation. A simple methodology based on the MIX code (a maximum likelihood method to estimate mixing ratios) was used to elucidate their different behaviours. During the dry period (April–December, 2009) the Fe concentration was lower than that deduced from the SC recorded value due to the precipitation of Fe-oxihydroxides, which also reduced the concentrations of As, Cr, Pb and, to a lesser extent, Cu. At the same time Na, Sr, Ca and Li were enriched because of the higher interaction with the riverbed materials. Correlations between the SC and the metal concentration improved significantly when each period was considered separately. A second dry period (April–June 2010) shows high SC values, although no dissolution/precipitation of solid phases is evidenced. This indicates that SC alone is not enough to predict the dissolved metal loads in Mediterranean AMD streams. The metal load transported by the River Meca was determined for the hydrological year 2009/10 as 1933 ± 129 tonnes of Fe, 990 ± 155 of Al and 378 ± 41 of Zn.     

Sr isotopic composition of Paleoproterozoic C-rich carbonate rocks: The Tulomozero Formation,SE Fennoscandian Shield

Rb–Sr systematics has been studied in ¹³C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ¹⁸O/¹⁶O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ⁸⁷Sr/⁸⁶Sr values in the least altered (Mn/Sr < 2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ⁸⁷Sr/⁸⁶Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.     

Sources and flux of trace elements in river water collected from the Lake Qinghai catchment,NE Tibetan Plateau

River waters play a significant role in supplying naturally- and anthropogenically-derived materials to Lake Qinghai, northeastern Tibetan Plateau. To define the sources and controlling processes for river water chemistry within the Lake Qinghai catchment, high precision ICP-MS trace element concentrations were measured in water samples collected from the Buha River weekly in 2007, and from other major rivers in the post-monsoon (late October 2006) and monsoon (late July 2007) seasons. The distributions of trace elements vary in time and space with distinct seasonal patterns. The primary flux in the Buha River is higher TDS and dissolved Al, B, Cr, Li, Mo, Rb, Sr and U during springtime than those during other seasons and is attributed to the inputs derived from both rock weathering and atmospheric processes. Among these elements, the fluxes of dissolved Cr, B and Rb are strongly influenced by eolian dust input. The fluxes of dissolved Li, Mo, Sr and U are also influenced by weathering processes, reflecting the sensitivity of chemical weathering to monsoon conditions. The anthropogenic sources appear to be the dominant contribution to potentially harmful metals (Ni, Cu, Co, Zn and Pb), with high fluxes at onset of the main discharge pulses due, at least partially, to a runoff washout effect. For other major rivers, except for Ba, concentrations of trace elements are higher in the monsoon than in the post-monsoon season. A total of 38.5 ± 3.1 tons of potentially harmful elements are transported into the lake annually, despite human activities within the catchment being limited. Nearly all river water samples contain dissolved trace elements below the World Health Organization guidelines for drinking water, with the exception of As and B in the Daotang River water samples collected in late July probably mobilized from underlying lacustrine sediments.     

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.     

Eolian deposition of trace elements onto Taylor Valley Antarctic glaciers

The major ion and trace element chemistry in four 60-110 cm deep snow pits was examined from three Antarctic Taylor Valley glaciers (Commonwealth, Canada and Howard), all located within 20 km of the Ross Sea. Taylor Valley (TV) (77°30′S, 163°15′E) is part of the McMurdo Dry Valleys (MDV), the largest ice-free area in Antarctica. Snow chemistry of these glaciers is strongly influenced by cross-valley (SW-NE and NE-SW) winds that deposit eolian materials, including mineral dust and soluble salts. Arsenic, Cd, Cu, Mo, Pb, Sb and Sn in recent TV glacier snow are almost exclusively derived from eolian dust. The relative magnitude of element variations with depth relate to the exposure of snow sites to prevalent winds. Eolian deposition is a primary source of these elements to Canada Glacier snow and supra and proglacial streams. Eolian processes are likely very important to the delivery and availability of nutrients and, potentially toxic elements to dry valley Antarctic ecosystems.     

Trace element fractionation and transport in boreal rivers and soil porewaters of permafrost-dominated basaltic terrain in Central Siberia

The chemical status of ∼40 major and trace elements (TE) and organic carbon (OC) in pristine boreal rivers draining the basaltic plateau of Central Siberia (Putorana) and interstitial solutions of permafrost soils was investigated. Water samples were filtered in the field through progressively decreasing pore size (5 μm → 0.22 μm → 0.025 μm → 10 kDa → 1 kDa) using cascade frontal filtration technique. Most rivers and soil porewaters exhibit 2-5 times higher than the world average concentration of dissolved (i.e., <0.22 μm) iron (0.03-0.4 mg/L), aluminum (0.03-0.4 mg/L), OC (10-20 mg/L) and various trace elements that are usually considered as immobile in weathering processes (Ti, Zr, Ga, Y, REEs). Ultrafiltration revealed strong relationships between concentration of TE and that of colloidal Fe and Al. According to their partition during filtration and association with colloids, two groups of elements can be distinguished: (i) those weakly dependent on ultrafiltration and that are likely to be present as truly dissolved inorganic species (Li, Na, K, Si, Mn, Mo, Rb, Cs, As, Sb) or, partially (20-30%) associated with small size Fe- and Al-colloids (Ca, Mg, Sr, Ba) and to small (<1-10 kDa) organic complexes (Co, Ni, Cu, Zn), and (ii) elements strongly associated with colloidal iron and aluminum in all ultrafiltrates largely present in 1-100 kDa fraction (Ga, Y, REEs, Pb, V, Cr, Ti, Ge, Zr, Th, U). TE concentrations and partition coefficients did not show any detectable variations between different colloidal fractions for soil porewaters, suprapermafrost flow and surface streams. TE concentration measurements in river suspended particles demonstrated significant contribution (i.e., ?30%) of conventionally dissolved (<0.22 μm) forms for usually “immobile” elements such as divalent transition metals, Cd, Pb, V, Sn, Y, REEs, Zr, Hf, Th. The Al-normalized accumulation coefficients of TE in vegetation litter compared to basalts achieve 10-100 for B, Mn, Zn, As, Sr, Sn, Sb, and the larch litter degradation is able to provide the major contribution to the annual dissolved flux of most trace elements. It is hypothesized that the decomposition of plant litter in the topsoil horizon leads to Fe(III)-, Al-organic colloids formation and serves as an important source of elements in downward percolating fluids.     

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.     

Features of acid–saline systems of Southern Australia

The discovery of layered, SO₄-rich sediments on the Meridiani Planum on Mars has focused attention on understanding the formation of acid–saline lakes. Many salt lakes have formed in southern Australia where regional groundwaters are characterized by acidity and high salinity and show features that might be expected in the Meridiani sediments. Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the formations come to the surface or interact with oxidised meteoric water, acid groundwaters result. In this paper examples of such waters around Lake Tyrrell, Victoria, and Lake Dey-Dey, South Australia, are reviewed. The acid–saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases Ra. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (ρ ∼ 1.04) regional water flows out over the denser (ρ ∼ 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. The studied groundwater systems were discovered by U exploration programs following up radiometric anomalies related to this Ra. Evaporation concentrates the lesser soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the more soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the salt-lake. These brines are near-neutral in pH.     

Heavy metal anomalies in the Tinto and Odiel River and estuary system,Spain

The Tinto and Odiel rivers drain 100 km from the Rio Tinto sulphide mining district, and join at a 20-km long estuary entering the Atlantic Ocean. A reconnaissance study of heavy metal anomalies in channel sand and overbank mud of the river and estuary by semi-quantitative emission dc-arc spectrographic analysis shows the following upstream to downstream ranges in ppm (μg g^?1): As 3,000 to <200, Cd 30 to <0.1, Cu 1,500 to 10, Pb 2,000 to <10, Sb 3000 to <150, and Zn 3,000 to <200. Organic-rich (1.3–2.6% total organic carbon, TOC), sandysilty overbank clay has been analyzed to represent suspended load materials. The high content of heavy metals in the overbank clay throughout the river and estuary systems indicates the importance of suspended sediment transport for dispersing heavy metals from natural erosion and anthropogenic mining activities of the sulfide deposit. The organic-poor (0.21–0.37% TOC) river bed sand has been analyzed to represent bedload transport of naturally-occurring sulfide minerals. The sand has high concentrations of metals upstream but these decrease an order of magnitude in the lower estuary. Although heavy metal contamination of estuary mouth beach sand has been diluted to background levels estuary mud exhibits increased contamination apparently related to finer grain size, higher organic carbon content, precipitation of river-borne dissolved solids, and input of anthropogenic heavy metals from industrial sources. The contaminated estuary mud disperses to the inner shelf mud belt and offshore suspended sediment, which exhibit metal anomalies from natural erosion and mining of upstream Rio Tinto sulphide lode sources (Pb, Cu, Zn) and industrial activities within the estuary (Fe, Cr, Ti). Because heavy metal contamination of Tinto-Odiel river sediment reaches or exceeds the highest levels encountered in other river sediments of Spain and Europe, a detailed analysis of metals in water and suspended sediment throughout the system, and epidemiological analysis of heavy metal effects in humans is appropriate.     

Major and trace element geochemistry of Lake Bogoria and Lake Nakuru,Kenya, during extreme draught

The physico-chemical properties of water samples from the two athalassic endorheic lakes Bogoria and Nakuru in Kenya were analysed. Surface water samples were taken between July 2008 and October 2009 in weekly intervals from each lake. The following parameters were determined: pH, salinity, electric conductivity, dissolved organic carbon (DOC), the major cations (FAAS and ICP-OES) and the major anions (IC), as well as certain trace elements (ICP-OES). Samples of superficial sediments were taken in October 2009 and examined using Instrumental Neutron Activation Analysis (INAA) for their major and trace element content including rare earth elements (REE). Both lakes are highly alkaline with a dominance of Na > K > Si > Ca in cations and HCO₃ > CO₃ > Cl > F > SO₄ in anions. Both lakes also exhibited high concentrations of Mo, As and fluoride. Due to an extreme draught from March to October 2009, the water level of Lake Nakuru dropped significantly. This created drastic evapoconcentration, with the total salinity rising from about 20‰ up to 63‰. Most parameters (DOC, Na, K, Ca, F, Mo and As) increased with falling water levels. A clear change in the quality of DOC was observed, followed by an almost complete depletion of dissolved Fe from the water phase. In Lake Bogoria the evapoconcentration effects were less pronounced (total salinity changed from about 40‰ to 48‰). The distributions of REE in the superficial sediments of Lake Nakuru and Lake Bogoria are presented here for the first time. The results show a high abundance of the REE and a very distinct Eu depletion of Eu/Eu* = 0.33–0.45.     

A threshold in soil formation at Earth’s arid-hyperarid transition

The soils of the Atacama Desert in northern Chile have long been known to contain large quantities of unusual salts, yet the processes that form these soils are not yet fully understood. We examined the morphology and geochemistry of soils on post-Miocene fans and stream terraces along a south-to-north (27° to 24° S) rainfall transect that spans the arid to hyperarid transition (21 to ∼2 mm rain y⁻¹). Landform ages are ? 2 My based on cosmogenic radionuclide concentrations in surface boulders, and Ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2.1 My. A chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall. In the arid (21 mm rain y⁻¹) soil, total mass loss to weathering of silicate alluvium and dust (−1030 kg m⁻²) is offset by net addition of salts (+170 kg m⁻²). The most hyperarid soil has accumulated 830 kg m⁻² of atmospheric salts (including 260 kg sulfate m⁻² and 90 kg chloride m⁻²), resulting in unusually high volumetric expansion (120%) for a soil of this age. The composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in-soil chemical transformation and loss. Long-term rates of atmospheric solute addition were derived from the ion inventories in the driest soil, divided by the landform age, and compared to measured contemporary rates. With decreasing rainfall, the soil salt inventories increase, and the retained salts are both more soluble and present at shallower depths. All soils generally exhibit vertical variation in their chemistry, suggesting slow and stochastic downward water movement, and greater climate variability over the past 2 My than is reflected in recent (∼100 y) rainfall averages. The geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold: in wetter soils, the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 10⁵ to 10⁶ years, while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion. The specific geochemistry of hyperarid soils is a function of atmospheric sources, and is expected to vary accordingly at other hyperarid sites. This work identifies key processes in hyperarid soil formation that are likely to be independent of location, and suggests that analogous processes may occur on Mars.     

Temporal trends of dissolved weathering products released from a high Arctic coal mine waste rock pile in Svalbard (78°N)

It is well known that oxidation of sulphide-containing coal mine waste has considerable environmental impacts due to generation of acid mine drainage (AMD) containing high dissolved metal concentrations. This study is the first to evaluate seasonal trends in the release of AMD from high arctic coal mine waste rock. Runoff from an abandoned coal mine waste pile in Svalbard (78°N) was studied during the entire 3–4 month period with running water in 2005. Temporal variation in concentrations and fluxes of dissolved elements were quantified based on daily water sampling and used to evaluate weathering processes and estimate element budgets on a daily, seasonal and annual basis. Apart from alkali- and alkaline earth metals; Fe, Al, Mn, Zn and Ni were found to be the most abundant metals in the runoff. Element concentrations were highly correlated and suggest that the processes of sulphide oxidation, ion exchange and silicate weathering occurring within the waste pile were linked throughout the measuring period. Observed pH values varied from 2.8 to 5.2 and SO₄ concentrations from 21 to 1463 mg L⁻¹. Manganese and Al concentrations were observed above phytotoxic levels (up to 4 and 23 mg L⁻¹, respectively) and were considered the most critical elements in terms of environmental impact. Throughout the summer a total dissolved quantity of 58 kg Mn, 238 kg Al and 13,700 kg SO₄ was released from the pile containing approximately 200,000 m³ of pyritic waste material (<1% FeS₂). The highest concentrations of metals, lowest pH values and a very high daily release of H₂SO₄ (up to twice as high as the following month) were observed during the first week of thaw. This is considered a result of an accumulation of weathering products, generated within the waste pile during winter and released as a pollution-flush during early spring. Similar accumulation/flush sequences were observed later in the summer where rain events following relatively long dry periods caused high daily metal fluxes and on some occasions also elevated dissolved metal concentrations. Despite highly variable weather/climate conditions during the rest of the summer the investigated waste rock pile acted like a relative constant pollution-source during this period. Future investigations regarding the environmental impact of mine waste in the region should include measurements of bioavailable metals in order to provide further details on the seasonal trends in environmental impact.     

Iron-oxide crystallinity increases during soil redox oscillations

An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH)₃.Fe²⁺_(aq), log K^o = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O₂ that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic ⁵⁷Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in E_h (from 200 to 700 mV) and pFe²⁺ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during the reduction half-cycles. Similar intra-cycle oscillations were observed in the HCl and acid ammonium oxalate (AAO) extractable pools. The cumulative response of soil solids during multiple redox oscillations included: (1) a decrease in most HCl and AAO extractable metals and (2) a transformation of SRO Fe (as nano-goethite) to micro-crystalline goethite and micro-crystalline hematite. This may be the first direct demonstration that Fe oxide crystallinity increases during redox oscillations—an a priori unexpected result.     

Sediment pore-water interactions associated with arsenic and uranium transport from the North Cave Hills mining region,South Dakota,USA

The extent of historical U mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota, USA. While previous studies reported watershed sediment and surface water As and U concentrations up to 90× established background concentrations, it was unclear whether or how localized changes in sediment redox behavior may influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid–liquid As and U distributions as a function of sediment depth. Pore-water and solid phase As and U concentrations, Fe speciation, Eh and pH were measured to ascertain specific geochemical conditions responsible for As and U remobilization and transport behavior. At a mine overburden sedimentation pond adjacent to the mine sites, high total aqueous As and U concentrations (4920 and 674 μg/L, respectively) were found within surface water during summer sampling; however pond dredging prior to autumn sampling resulted in significantly lower aqueous As and U concentrations (579 and 108 μg/L, respectively); however, both As and U still exceeded regional background concentrations (20 and 18 μg/L, respectively). At a wetlands-dominated deposition zone approximately 2 km downstream of the sedimentation pond, pore-water geochemical conditions varied seasonally. Summer conditions promoted reducing conditions in pore water, resulting in active release of As(III) to the water column. Autumn conditions promoted oxidizing conditions, decreasing pore-water As (As_pw) 5× and increasing U_pw 10×. Peak U pore-water concentrations (781 μg/L) were 3.5× greater than determined for the surface water (226 μg/L), and approximately 40× background concentrations. At the Bowman–Haley reservoir backwaters 45 km downstream from the mine sites, As and U pore-water concentrations increased significantly between the summer and autumn deployments, attributed to increased Fe reduction processes. Geochemical modeling suggests solid-phase Fe reduction promotes the liberation of pore-water As and U via suppressing the formation of thioarsenite. Intermittent hydrological processes facilitate As and U transport and deposition throughout the watershed, while biogeochemical-influenced redox changes cycle As and U between pore and surface water within localized environments.     

Stoichiometry of a dissolution reaction of a trioctahedral vermiculite at pH 2.7

Transformation of vermiculite to hydroxy-interlayered vermiculites (HIV) significantly modifies the physicochemical properties of the original mineral. HIV is a common phase in acid soils, nevertheless its formation remains poorly understood. The main goal of this paper was to clarify the kinetics and process of interlayer aluminization of pure vermiculite using an experimental design. For this purpose, we monitored the dissolution of Na-saturated vermiculite in dilute HCL at pH 2.7, at 50 °C for 672 h in stirred flow-through reactors. Both reacted samples at different dissolution steps, and the leaching of elements, were investigated. The main result was a rapid change to hydroxy-interlayered vermiculite, with a decrease in CEC and a progressive displacement of d(0 0 1) reflection near 1.4 nm after K saturation, resulting from formation of hydroxy-interlayer material. Vermiculite was found to dissolve non-stoichiometrically for 500 h; after that, the release rate for Si, Mg and Al became stoichiometric with respect to vermiculite chemistry. By contrast, Fe sustained non-stoichiometric release throughout the whole experiment. At the steady state, i.e., after 500 h, a dissolution rate of 8.8 ± 0.1 × 10⁻¹¹ mol vermiculite m⁻² s⁻¹ was found with respect to Si. Both Al and Fe precipitated in the interlayer space, and their amounts calculated at the end of the experiment were 3.74 × 10⁻⁴ mol g⁻¹ of vermiculite for Al and 8.74 × 10⁻⁵ for Fe. The rate of interlayer aluminization increased for 60 h and then regularly decreased. Al-interlayering stopped after 288 h, but Fe still precipitated in the interlayer space.A comparison with the same mineral incubated for three years in acid soils revealed that the reaction was proton-promoted. The same pattern of CEC decrease and interlayer aluminization was observed, but the kinetics were slower due to soil environmental conditions.