Spatial variations in water composition at a northern Canadian lake impacted by mine drainage

Release of acid drainage from mine-waste disposal areas is a problem of international scale. Contaminated surface water, derived from mine wastes, orginates both as direct surface runoff and, indirectly, as subsurface groundwater flow. At Camp Lake, a small Canadian Shield lake that is in northern Manitoba and is ice-covered 6 months of the year, direct and indirect release of drainage from an adjacent sulfide-rich tailings impoundment has severely affected the quality of the lake water. Concentrations of the products from sulfide oxidation are extremely high in the pore waters of the tailings impoundment. Groundwater and surface water derived from the impoundment discharge into a semi-isolated shallow bay in Camp Lake. The incorporation of this aqueous effluent has altered the composition of the lake water, which in turn has modified the physical limnology of the lake. Geochemical profiles of the water column indicate that, despite its shallow depth (6 m), the bay is stratified throughout the year. The greatest accumulation of dissolved metals and SO₄ is in the lower portion of the water column, with concentrations up to 8500 mg L⁻¹ Fe, 20,000 mg L⁻¹ SO₄, 30 mg L⁻¹ Zn, 100 mg L⁻¹ Al, and elevated concentrations of Cu, Cd, Pb and Ni. Meromictic conditions and very high solute concentrations are limited to the bay. Outside the bay, solute concentrations are lower and some stratification of the water column exists. Identification of locations and composition of groundwater discharge relative to lake bathymetry is a fundamental aspect of understanding chemical evolution and physical stability of mine-impacted lakes.     

The Diavik waste rock project: Initial geochemical response from a low sulfide waste rock pile

Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053 wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1 year. Sulfate concentrations reached 2000 mg L⁻¹ when ambient temperatures were >10 °C, and decreased as ambient temperatures declined to <0 °C. The pH decreased to <5, concomitant with an alkalinity minimum of <1 mg L⁻¹ (as total CaCO₃), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were <0.36 and <0.11 mg L⁻¹, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13 mg L⁻¹), Ni (<7.0 mg L⁻¹), Co (<1.5 mg L⁻¹), Zn (<0.5 mg L⁻¹), Cd (<0.008 mg L⁻¹) and Cu (<0.05 mg L⁻¹) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite [Fe_1−xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite [CuFeS₂] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO₄, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock.     

The relationship between fluid flow and mineral weathering in heterogeneous unsaturated porous media: A physical and geochemical characterization of a waste-rock pile

The relationships between factors that control subsurface flow and the timing, duration, and intensity of acidity generation and leaching of metals from waste-rock dumps are investigated. A 12 m high waste-rock pile that had been constructed in 1994 at Key Lake, Saskatchewan, Canada was disassembled, sampled and characterized in 2000. Physical properties that control water flow were characterized by measuring soil–water suction, volumetric water content, and the grain-size distribution at 60 randomized sites within the pile. Grain-size distribution was also measured at an additional 20 grid locations within the pile. Paste pH, pore-water geochemistry, mineralogy, and water-soluble extractions were used to investigate geochemical processes and sulfide oxidation at each of the 20 grid locations. A field-based soil–water characteristic curve could not be developed from the spatially variable and hysteretic field data; consequently, the grain-size distribution was used as a relative measure of subsurface flow and of the tendency to contain water under unsaturated conditions. The geochemical characterization demonstrated that marcasite underwent preferential weathering relative to pyrite and chalcopyrite, that dolomite was the main buffering carbonate mineral, and that gypsum, jarosite, and Fe oxyhydroxides were the main secondary (supergene) minerals. The pore waters contained up to 78,000 mg L⁻¹ SO₄, 690 mg L⁻¹ Ni and 1400 mg L⁻¹ U (800, 11.7 and 6 mM, respectively), suggesting that significant weathering had occurred. The pore water chemistry varied considerably between sampling sites. However, neither a correlation of pore-water chemistry with grain-size distribution nor a spatial relationship within the sampled grid was discernible.     

Environmental assessment of abandoned mine tailings in Adak,Västerbotten district (northern Sweden)

Sulfide-rich mine tailings in Adak that are exposed to weathering cause acid mine drainage characterized by low pH (2–4) and high SO₄ (up to 800 mg L⁻¹). Surface water, sediment and soil samples collected in this study contain higher concentrations of As, Cu, Fe and Zn, compared to the target and/or intervention limits set by international regulatory agencies. In particular, high As concentrations in water (up to 2900 μg L⁻¹) and sediment (up to 900 mg kg⁻¹) are of concern. There is large variability in trace element concentrations, implying that both physical (grain size) and chemical factors (pH, secondary phases as sulfides, Al-oxides or clay minerals) play an important role in their distribution. The low pH keeps the trace elements dissolved, and they are transported farther downstream. Trace element partition coefficients are low (log K_d = 0.3–4.3), and saturation indices calculated with PHREEQC are <0 for common oxide and sulfidic minerals. The sediment and soil samples indicate an enhanced pollution index (up to 17), and high enrichment factors for trace elements (As up to 38,300; Zn up to 800). Finally, leaves collected from different plant types indicate bioaccumulation of several elements (As, Al, Cu, Fe and Zn). However, some of the plants growing in this area (e.g., Salix, Equisétum) are generally resistant to metal toxicity, and hence, liming and phytoremediation could be considered as potential on-site remediation methods.     

Microcosm assessment of the biogeochemical development of sulfur and oxygen in oil sands fluid fine tailings

Bitumen recovery from Alberta oil sands generates fluid fine tailings, which are retained in tailings ponds where solids settle and release process water. The recovered water is recycled for bitumen extraction, while the resulting tailings are incorporated into various landforms for reclamation, with one option being conversion of tailings basins to viable end pit lakes. Tailings ponds commonly host diverse microbial communities, including SO₄-reducing prokaryotes. The highly reducing nature of the hydrogen sulfide produced by these prokaryotes may impact the biogeochemical cycling of key nutrients. However, the behavioral dynamics of hydrogen sulfide production in ponds containing fluid fine tailings remain to be clearly explained. In this study, microcosms are used as analogues of the sediment–water interface of a tailings pond undergoing reclamation to determine sulfide generation patterns and the behavior of O₂. In the microcosms, hydrogen sulfide fluxes correlated positively with biotic activity, reaching levels of over 2 × 10³ nmol cm⁻² s⁻¹, leading to Fe sulfide formation. Depth-related hydrogen sulfide profiles in the microcosms were comparable to those encountered in situ, in Syncrude’s West In-Pit, an active tailing pond. Oxygen diffusion across the fluid fine tailing sediment–water interface was controlled to different degrees by both biotic and abiotic processes. The results have implications for quantitatively estimating the impact of hydrogen sulfide production, O₂ availability, and biogeochemical cycling of key nutrients important for the success of life in fluid fine tailings-affected ecosystems. This paper shows that this production of hydrogen sulfide may be a self-limiting process, which will begin to decrease after a period of time.     

Interaction of diamond mine waste and surface water in the Canadian Arctic

Factors controlling the chemical composition of water interacting with finely-crushed kimberlite have been investigated by sampling pore waters from processed kimberlite fines stored in a containment facility. Discharge water from the diamond recovery plant and surface water from the containment facility, which acts as plant intake water, were also sampled. All waters sampled are pH-neutral, enriched in SO₄, Mg, Ca, and K, and low in Fe. Pore-water samples, representing the most concentrated waters, are characterized by the highest SO₄ (up to 4080 mg l⁻¹), Mg (up to 870 mg l⁻¹), and Ca (up to 473 mg l⁻¹). The water discharged from the processing plant has higher concentrations of all major dissolved constituents than the intake water. The dominant minerals present in the processed fines and the kimberlite ore are serpentine and olivine, with small amounts of Ca sulphate and Fe sulphide restricted to mud xenoclasts. Reaction and inverse modeling suggest that much of the water-rock interaction takes place within the plant and involves the dissolution of chrysotile and Ca sulphate, and precipitation of silica and Mg carbonate. Evapoconcentration also appears to be a significant process affecting pore water composition in the containment facility. The reaction proposed to be occurring during ore processing involves the dissolution of CO_2(g) and may represent an opportunity to sequester atmospheric CO₂ through mineral carbonation.     

Arsenic behaviour in gold-ore mill tailings,Massif Central,France: hydrogeochemical study and investigation of in situ redox signatures

Historical Au-ore exploitation at the Chéni mine in the Massif Central, France, generated 525,000 tonnes of finely ground mill tailings deposited in a heap that has spread with time into three settling basins. The tailings, which are rich in quartz (80%), mica and clay minerals (10% of illite, smectite, kaolinite and chlorite), feldspars (5%) but poor in carbonates (<1%), also contain sulphides (around 5%, mainly pyrite and arsenopyrite). Arsenic content of the tailings is around 6 g kg. This paper describes the geochemistry of drainage waters, with special attention paid to in situ values of the three major redox couples, namely Fe(II)/Fe(III), As(III)/As(V) and S(IV)/S(VI). The water samples range from acidic and oxidized (pH 2.9, E_h +700 mV) to moderate pH and weakly reducing (pH 7.6, E_h 15 mV). The waters are rich in SO₄ and Ca and have variable As (0.05–95 mg L⁻¹) and Fe concentrations (0.07–141 mg L⁻¹). Reduced As(III) species predominate over As(V) species (As(III)/As(V) up to 21), whereas oxidized forms of Fe and S are favoured (Fe(II)/Fe(III) up to 0.5, and S(IV)/S(VI) up to 1).Thermodynamic calculations were performed with the PHREEQC and EQ3NR codes based on a revised As database to evaluate saturation indices (SI) of the waters in relation to the main minerals and define which redox couples control the redox state of the system. The important role of carbonates, though only present in small amounts, explains the acid buffering generated by the oxidation of sulphides for waters in the pH 7–7.5 range. Measured E_h appears to fall between the calculated E_h of the Fe(II)/Fe(III) couple and that of the As(III)/As(V) couple, illustrating redox disequilibrium.     

Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications

In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L⁻¹) are accompanied by high concentrations of Fe (3–13.7 mg L⁻¹) and PO₄ (1–6.5 mg L⁻¹). Ground water that is rich in Mn (1–5.3 mg L⁻¹) contains <50 μg L⁻¹ of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L⁻¹ m⁻¹. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L⁻¹) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L⁻¹ in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.     

Fate of organic matter during aquifer storage and recovery (ASR) of reclaimed water in a carbonate aquifer

Understanding the fate of injected organic matter and the consequences of subsequent redox processes is essential to assess the viability of using reclaimed water in aquifer storage and recovery (ASR). A full-scale field trial was undertaken at Bolivar, South Australia where two ASR cycles injected approximately 3.6 × 10⁵ m³ of reclaimed water into a carbonate aquifer over a 3-a period. Organic C within reclaimed water was predominantly in the dissolved fraction, ranging from 1 to 2 mmol L⁻¹ (10–20 mg L⁻¹), markedly higher than potable supply and stormwater previously reported as source waters for ASR. Between 20% and 24% of the injected dissolved organic C (DOC) was mineralised through reaction with injected O₂ and NO₃. Furthermore, this was achieved mainly within the first 4 m of aquifer passage. Despite the presence of residual DOC, SO₄ reduction was not induced within the bulk of the injected plume. It was only near the ASR well during an extended storage phase where deeply reduced (methanogenic) conditions developed, indicating variable redox zones within the injectant plume. The quality of water recovered from the ASR well indicated that the organic C content of reclaimed water does not restrict its application as a recharge source for ASR.     

Trace metal distribution and mobility in drill cuttings and produced waters from Marcellus Shale gas extraction: Uranium,arsenic, barium

Development of unconventional shale gas wells can generate significant quantities of drilling waste, including trace metal-rich black shale from the lateral portion of the drillhole. We carried out sequential extractions on 15 samples of dry-drilled cuttings and core material from the gas-producing Middle Devonian Marcellus Shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal. Maximum whole rock concentrations of uranium (U), arsenic (As), and barium (Ba) were 47, 90, and 3333 mg kg⁻¹, respectively. Sequential chemical extractions suggest that although silicate minerals are the primary host for U, as much as 20% can be present in carbonate minerals. Up to 74% of the Ba in shale was extracted from exchangeable sites in the shale, while As is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation. For comparison, U and As concentrations were also measured in 43 produced water samples returned from Marcellus Shale gas wells. Low U concentrations in produced water (<0.084–3.26 μg L⁻¹) are consistent with low-oxygen conditions in the wellbore, in which U would be in its reduced, immobile form. Arsenic was below detection in all produced water samples, which is also consistent with reducing conditions in the wellbore minimizing oxidation of As-bearing sulfide minerals.Geochemical modeling to determine mobility under surface storage and disposal conditions indicates that oxidation and/or dissolution of U-bearing minerals in drill cuttings would likely be followed by immobilization of U in secondary minerals such as schoepite, uranophane, and soddyite, or uraninite as conditions become more reducing. Oxidative dissolution of arsenic containing sulfides could release soluble As in arsenate form under oxic acidic conditions. The degree to which the As is subsequently immobilized depends on the redox conditions along the landfill flow path. The results suggest that proper management of drill cuttings can minimize mobilization of these metals by monitoring and controlling Eh, pH and dissolved constituents in landfill leachates.     

An Apatite II permeable reactive barrier to remediate groundwater containing Zn,Pb and Cd

Phosphate-induced metal stabilization involving the reactive medium Apatite II™ [Ca_10−xNa_x(PO₄)_6−x(CO₃)_x(OH)₂], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO₄ and NO₃. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River, Idaho. Microbially mediated SO₄ reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb₁₀(PO₄)₆(OH,Cl)₂] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L⁻¹), has reduced Zn to near background in this region (about 100 μg L⁻¹), and has reduced SO₄ by between 100 and 200 mg L⁻¹ and NO₃ to below detection (50 μg L⁻¹). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).     

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

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

An Apatite II permeable reactive barrier to remediate groundwater containing Zn,Pb and Cd

Phosphate-induced metal stabilization involving the reactive medium Apatite II™ [Ca_10−xNa_x(PO₄)_6−x(CO₃)_x(OH)₂], where x < 1, was used in a subsurface permeable reactive barrier (PRB) to treat acid mine drainage in a shallow alluvial groundwater containing elevated concentrations of Zn, Pb, Cd, Cu, SO₄ and NO₃. The groundwater is treated in situ before it enters the East Fork of Ninemile Creek, a tributary to the Coeur d’Alene River, Idaho. Microbially mediated SO₄ reduction and the subsequent precipitation of sphalerite [ZnS] is the primary mechanism occurring for immobilization of Zn and Cd. Precipitation of pyromorphite [Pb₁₀(PO₄)₆(OH,Cl)₂] is the most likely mechanism for immobilization of Pb. Precipitation is occurring directly on the original Apatite II. The emplaced PRB has been operating successfully since January of 2001, and has reduced the concentrations of Cd and Pb to below detection (2 μg L⁻¹), has reduced Zn to near background in this region (about 100 μg L⁻¹), and has reduced SO₄ by between 100 and 200 mg L⁻¹ and NO₃ to below detection (50 μg L⁻¹). The PRB, filled with 90 tonnes of Apatite II, has removed about 4550 kg of Zn, 91 kg of Pb and 45 kg of Cd, but 90% of the immobilization is occurring in the first 20% of the barrier, wherein the reactive media now contain up to 25 wt% Zn. Field observations indicate that about 30% of the Apatite II material is spent (consumed).     

Genesis of the Saishitang skarn type copper deposit,West Qinling,Qinghai Province: Evidence from fluid inclusions and stable isotopes

The Saishitang skarn type copper deposit, located in the southeast part of the Dulan–Ela Mountain Triassic volcanic–magmatic arc and forearc accretionary wedge, belongs to the Tongyugou–Saishitang tin–copper polymetallic ore field in West Qinling, Qinghai province. Based on the contact/crosscutting relationships, mineral associations and mineralization characteristics, hydrothermal fluid evolution can be divided into three stages: skarn (I), quartz sulfide (II) and polymetallic sulfide-bearing quartz–calcite vein (III). The quartz sulfide stage (II) can be further divided into a massive sulfide stage (II-1) and a layered sulfide stage (II-2). This paper presents detailed analysis of fluid inclusions, H–O, S and Pb isotope compositions of rock samples from each of the above three stages as well as analysis of fluid inclusions from quartz diorite. The homogenization temperature, salinity, density and pressure of fluid inclusions in quartz diorite and typical transparent minerals showed a tendency of gradual decline in these evolutionary stages. The ore-forming fluid can be classified as a Na⁺–Ca^2 +–SO₄^2 − Cl⁻ system with a minor proportion of a Na⁺–Ca^2 +–NO₃⁻ SO₄^2 − system, which likely resulted from mixing of magmatic and formation water. The H–O isotope composition indicates that the proportion of formation water increased during the ore-forming process, and meteoric water was mixed in the late quartz–calcite vein stage. The δ³⁴S (CDT) values (− 6.45–5.57‰) and Pb isotope compositions show that the ore-forming materials were mainly derived from magmatic fluid. Ore-forming fluid was boiling during the main ore-forming stage (II-1) due to pressure decrease. Consequently, the physical and chemical conditions (i.e., pH, Eh, f_O2, f_S2) changed, and metallic elements (including Cu) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. According to the integrated analysis of ore features, mineral associations, alteration characteristics, ore-forming environment and fluid evolutionary process, it is concluded that the Saishitang deposit is a typical skarn deposit.     

Dissolution kinetics of soil clays in sulfuric acid solutions: Ionic strength and temperature effects

Significant amounts of sulfuric acid (H₂SO₄) rich saline water can be produced by the oxidation of sulfide minerals contained in inland acid sulfate soils (IASS). In the absence of carbonate minerals, the dissolution of phyllosilicate minerals is one of very few processes that can provide long-term acid neutralisation. It is therefore important to understand the acid dissolution behavior of naturally occurring clay minerals from IASS under saline–acidic solutions. The objective of this study was to investigate the dissolution of a natural clay-rich sample under saline–acidic conditions (pH 1–4; ionic strengths = 0.01 and 0.25 M; 25 °C) and over a range of temperatures (25–45 °C; pH 1 and pH 4). The clay-rich sample referred to as Bottle Bend clay (BB clay) used was from an IASS (Bottle Bend lagoon) in south-western New South Wales (Australia) and contained smectite (40%), illite (27%), kaolinite (26%) and quartz (6%). Acid dissolution of the BB clay was initially rapid, as indicated by the fast release of cations (Si, Al, K, Fe, Mg). Relatively higher Al (pH 4) and K (pH 2–4) release was obtained from BB clay dissolution in higher ionic strength solutions compared to the lower ionic strength solutions. The steady state dissolution rate (as determined from Si, Al and Fe release rates; R_Si, R_Al, R_Fe) increased with decreasing solution pH and increasing temperature. For example, the highest log R_Si value was obtained at pH 1 and 45 °C (−9.07 mol g⁻¹ s⁻¹), while the lowest log R_Si value was obtained at pH 4 and 25 °C (−11.20 mol g⁻¹ s⁻¹). A comparison of these results with pure mineral dissolution rates from the literature suggests that the BB clay dissolved at a much faster rate compared to the pure mineral samples. Apparent activation energies calculated for the clay sample varied over the range 76.6 kJ mol⁻¹ (pH 1) to 37.7 kJ mol⁻¹ (pH 4) which compare very well with the activation energy values for acidic dissolution of monomineralic samples e.g. montmorillonite from previous studies. The acid neutralisation capacity (ANC) of the clay sample was calculated from the release of all structural cations except Si (i.e. Al, Fe, K, Mg). According to these calculations an ANC of 1.11 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 (I = 0.25 M, 25 °C) compared to an ANC of 0.21 kg H₂SO₄/tonne clay/day at pH 4 (I = 0.25 M, 25 °C). The highest ANC of 6.91 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 and at 45 °C (I = 0.25 M), which is more than three times higher than the ANC provided under the similar solution conditions at 25 °C. In wetlands with little solid phase buffering available apart from clay minerals, it is imperative to consider the potential ANC provided by the dissolution of abundantly occurring phyllosilicate minerals in devising rehabilitation schemes.     

Tailings dam seepage at the rehabilitated Mary Kathleen uranium mine,Australia

This study reports on the seepage of metals, metalloids and radionuclides from the Mary Kathleen uranium mill tailings repository. Since rehabilitation in the 1980s, the capped tailings have developed a stratified hydrochemistry, with acid (pH 3.7), saline, metal-rich (Fe, Mn, Ni, U ± As, Pb, Zn), oxygenated (1.05 mg L⁻¹ DO), radioactive waters in the upper tailings pile and near-neutral pH (pH 7.57), metal-poor, reduced (0.08 mg L⁻¹ DO) waters at depth. Seepage (∼0.5 L s⁻¹) of acid (pH 5.5), metal-rich (Fe, Mn ± Ni, U, Zn), radioactive (U-235, U-238, Ra-226, Ra-228, Ac-227) waters occurs from the base of the tailings dam retaining wall into the former evaporation pond and local drainage system. Oxygenation of the seepage waters causes the precipitation of Fe and coprecipitation and adsorption of other metals (U, Y), metalloids (As), rare earth elements (Ce, La) and radionuclides (U-235, U-238). By contrast, alkalis and alkaline–earth elements (Ca, K, Mg, Na, Sr), Mn, sulfate and to some degree metals (U, Zn, Ni), rare earth elements (Ce, La) and radionuclides (U-235, U-238, Ra-226, Ra-228) remain in solution until pH neutralisation and evaporation lead to their precipitation in efflorescences and sulfate-rich evaporative sediments. While the release of contaminant loads from the waste repository through seepage is insignificant (e.g. ∼5 kg of U per year), surface waters downstream of the tailings impoundment possess TDS, U and SO₄ concentrations that exceed Australian water quality guideline values in livestock drinking water. Thus, in areas with a semi-arid climate, even insignificant load releases of contaminants from capped tailings repositories can still cause the deterioration of water quality in ephemeral creek systems.     

Enhancement of in situ biodegradation of organic compounds in groundwater by targeted pump and treat intervention

This study demonstrates the value of targeted pump and treatment (PAT) to enhance the in situ biodegradation of organic contaminants in groundwater for improved restoration. The approach is illustrated for a plume of phenolic compounds in a sandstone aquifer, where PAT is used for hydraulic containment and removal of dissolved phase contaminants from specific depth intervals. Time-series analysis of the plume hydrochemistry and stable isotope composition of dissolved species (δ³⁴S-SO₄, δ¹³C-CH₄, δ¹³C-TDIC (TDIC = Total Dissolved Inorganic Carbon)) in groundwater samples from high-resolution multilevel samplers were used to deduce changes in the relative significance of biodegradation processes and microbial activity in the plume, induced by the PAT system over 3 years. The PAT system has reduced the maximum contaminant concentrations (up to 6800 mg L⁻¹ total phenols) in the plume by 50% to ∼70% at different locations. This intervention has (i) stimulated in situ biodegradation in general, with an approximate doubling of contaminant turnover based on TDIC concentration, which has increased from <200 mg L⁻¹ to >350 mg L⁻¹, (ii) enhanced the activity of SO₄-reducing microorganisms (marked by a declining SO₄ concentration with corresponding increase in SO₄-δ³⁴S to values >7–14‰_V-CDT relative to background values of 1.9–6.5‰_V-CDT), and (iii) where the TDIC increase is greatest, has changed TDIC-δ¹³C from values of −10 to −15‰_V-PDB to ∼−20‰_V-PDB. This indicates an increase in the relative importance of respiration processes (including denitrification and anaerobic methane oxidation, AMO) that yield ¹³C-depleted TDIC over fermentation and acetoclastic methanogenesis that yield ¹³C-enriched TDIC in the plume, leading to higher contaminant turnover. The plume fringe was found to be a zone of enhanced biodegradation by SO₄-reduction and methanogenesis. Isotopically heavy methane compositions (up to −47.8‰_V-PDB) and trends between δ¹³C-TDIC and δ¹³C-CH₄ suggest that AMO occurs at the plume fringe where the contaminant concentrations have been reduced by the PAT system. Mass and isotope balances for inorganic carbon in the plume confirm the shift in spatial dominance of different biodegradation processes and significant increase in contribution of anaerobic respiration for contaminant biodegradation in zones targeted by the PAT system. The enhanced in situ biodegradation results from a reduction in organic contaminant concentrations in the plume to levels below those that formerly suppressed microbial activity, combined with increased supply of soluble electron acceptors (e.g. nitrate) into the plume by dispersion. An interruption of the PAT system and recovery of the dissolved organic contaminant concentrations towards former values highlights the dynamic nature of this enhancement on restoration and relatively rapid response of the aquifer microorganisms to changing conditions induced by the PAT system. In situ restoration using this combined engineered and passive approach has the potential to manage plumes of biodegradable contaminants over shorter timescales than would be possible using these methods independently. The application of PAT in this way strongly depends on the ability to ensure an adequate flux of dissolved electron acceptors into the plume by advection and dispersion, particularly in heterogeneous aquifers.     

Arsenic immobilization in water and soil using acid mine drainage sludge

Adsorption onto Fe-containing minerals is a well-known remediation method for As-contaminated water and soil. In this study, the use of acid mine drainage sludge (AMDS) to adsorb As was investigated. AMDS is composed of amorphous particles and so has a large surface area (251.2 m² g⁻¹). Here, adsorption of both arsenite and arsenate was found to be almost 100%, under various initial AMDS dosages, with the arsenate adsorption rate being faster. The optimum pH for As adsorption onto AMDS was pH 7.0 and the maximum adsorption capacities for arsenite and arsenate were 58.5 mg g⁻¹ and 19.7 mg g⁻¹ AMDS, respectively. In addition, experiments revealed that AMDS dosages decreased As release from contaminated soil. Therefore, the AMDS used in this study was confirmed to be a suitable candidate for immobilizing both arsenite and arsenate in contaminated soils.     

Mobilization of arsenic from acid deposition – The Elbe River catchment,Czech Republic

Temporal changes of As concentration in surface waters were observed in some areas of the Czech Republic. Mobilized As originates from past atmospheric deposition. To understand the factors influencing As aqueous concentration and mobility the chemistry and runoff generation of a number of brooks, springs and rivers in the central part of the Elbe River catchment, Czech Republic, were monitored. Seasonal variations of As (from 0.5 to 10.5 μg L⁻¹), Fe (from 0.05 to 3.9 mg L⁻¹) and DOC (dissolved organic C – from 1.2 to 17.5 mg L⁻¹) were observed in monitored stream waters with maximum values of As and Fe in the summer months at pH values 7.6–7.8. The concentration of As in particles with a diameter < 60 μm correlates with the Fe concentration. There is no correlation between Fe and As in filtered samples (<0.45 μm). The As concentration in stream water colloids depends on an increase in DOC concentration and a decrease in ionic strength. The DOC stabilizes As in solution and reduces its re-adsorption on Fe colloids and consequently As concentration in the stream increases.     

Mechanisms controlling acid neutralization and metal mobility within a Ni-rich tailings impoundment

Low-quality pore waters containing high concentrations of dissolved H⁺, SO₄, and metals have been generated in the East Tailings Management Area at Lynn Lake, Manitoba, as a result of sulfide-mineral oxidation. To assess the abundance, distribution, and solid-phase associations of S, Fe, and trace metals, the tailings pore water was analyzed, and investigations of the geochemical and mineralogical characteristics of the tailings solids were completed. The results were used to delineate the mechanisms that control acid neutralization, metal release, and metal attenuation. Migration of the low-pH conditions through the vadose zone is limited by acid-neutralization reactions, resulting in the development of distinct pore-water pH zones at depth; the neutralization reactions involve carbonate (pH ⩾ 5.7), Al-hydroxide (pH ⩾ 4.0), and aluminosilicate solids. As the zone of low-pH pore water expands, the pH will then be primarily controlled by less soluble solids, such as Fe(III) oxyhydroxides (pH < 3.5) and the relatively more recalcitrant aluminosilicates (pH ⩾ 1.3). Precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxides and hydroxysulfates control the concentrations of dissolved Fe(III). Concentrations of dissolved SO₄ are principally controlled by the formation of gypsum and jarosite. Geochemical extractions indicate that the solid-phase concentrations of Ni, Co, and Zn are associated predominantly with reducible and acid-soluble fractions. The concentrations of dissolved trace metals are therefore primarily controlled by adsorption/complexation and (or) co-precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxide and hydroxysulfate minerals. Concentrations of dissolved metals with relatively low mobility, such as Cu, are also controlled by the precipitation of discrete minerals. Because the major proportion of metals is sequestered through adsorption and (or) co-precipitation, the metals are susceptible to remobilization if low-pH or reducing conditions develop within the tailings.