Applied Geochemistry (v.73, #C)
pH effect on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite by Zhifen Wang; Hai Wang; Qingmei Li; Minhong Xu; Yuhua Guo; Jinying Li; Tao Wu (1-7).
In a high-level radioactive waste (HLW) repository, pH has an impact on the solubility, migration, and adsorption of radionuclides. Thus, understanding the effects of pH on the diffusion of radionuclides is essential for long-term disposal of HLW. In this work, the diffusion behaviors of Re(VII) and Se(IV) in compacted Gaomiaozi (GMZ) bentonite at different pH have been investigated by a through-diffusion method. The effective diffusion coefficient, i.e., D e values of Re(VII) and Se(IV) were in the range of (1.0–2.4) × 10−11 m2/s at pH 3.0–10.0 and (0.38–2.3) × 10−11 m2/s at pH 3.0–9.0. In the case of Re(VII), the D e values remained almost unchanged probably because ReO4 − was the dominant species in the pH range of 3.0–10.0. In the case of Se(IV), whose predominant species were HSeO3 − at pH < 9.0 and SeO3 2− at pH ≥ 9.0, the D e values decreased by a factor of 3–6 at pH 9.0, i.e., D e (pH < 9.0)/D e (pH 9.0) ≈ 3–6, implying that the species with a higher valence state had a stronger anion exclusion effect. The decrease in D e values can be explained by the diffusion species of Se(IV). Additionally, the rock capacity factor α decreased with the increase of pH. HSeO3 − was absorbed on GMZ bentonite with distribution coefficient K d values in the range of (1.0–2.5) × 10−4 m3/kg at pH ≤ 8.0, whereas SeO3 2− was negligibly sorbed at pH > 8.0.
Keywords: Diffusion; Se(IV); Re(VII); pH; GMZ bentonite;
Reaction time scales for sulphate reduction in sediments of acidic pit lakes and its relation to in-lake acidity neutralisation by Stefan Peiffer (8-12).
Sulphate reduction is a key reaction to remove acidity from water bodies affected by acid mine drainage. In this study, 35S―SO4 2− reduction rates determined in sediments from a variety of acidic lignite pit lakes have been compiled. The rates decreased with pH and are strongly dependent on carbon substrate. The rates were fitted to a Monod model adapted to the specific conditions of acidic pit lakes (APL) sediments: i) sulphate reduction rate is independent from sulphate concentration due to the high concentration typically observed in APL systems (10–30 mM), ii) the observed pH dependency of sulphate reduction was accounted for by an inhibition function Finihibt which considers the occurrence of low cell numbers of sulphate reducing bacteria at pH values < 4.75. Simulated steady-state sulphate reduction rates are predicting measured rates at carbon substrate concentrations of <10 μM. Estimated steady-state reaction time scales range between 2.4 h at pH 7 and 41 h at pH 3 at a carbon half-saturation constant of KC−S = 100 μM and are increasing with increasing KC−S values. Time scales at low pH are too long to allow for significant generation of alkalinity during the time of residence of groundwater passing through the top and hence most reactive zone of APL sediments which has important implications for the remediation of acidic pit lakes.
Keywords: Sulphate reduction rates; Acid pit lakes; Monod kinetics; Modelling; Acidity; Internal neutralisation; Residence time; Sediments;
Effectiveness of various dispersed alkaline substrates for the pre-treatment of ferriferous acid mine drainage by Tsiverihasina V. Rakotonimaro; Carmen Mihaela Neculita; Bruno Bussière; Gérald J. Zagury (13-23).
Dispersed alkaline substrates (DAS) have been successfully used in passive treatment of highly contaminated acid mine drainage (AMD) to limit coating and clogging issues. However, further optimization of DAS systems is still needed, especially for their long-term efficiency during the treatment of ferriferous AMD. In the present study, three types of DAS comprised of natural alkaline materials (wood ash, calcite, dolomite), in different proportions (20%v/v, 50%v/v, 80%v/v), and a substrate with high surface area (wood chips) were tested in 9 batch reactors. The testing was carried out, in duplicate, for a period of 91 days, to evaluate the comparative performance of the mixtures for iron pre-treatment in ferriferous AMD (2500 mg/L Fe, at pH 4). Results showed increasing of pH (between 4.15 and 7.12), regardless of the proportion of alkaline materials in the DAS mixtures. Among the tested mixtures, wood ash type DAS were more effective for Fe removal (99.9%) than calcite or dolomite type DAS (up to 66%). All tested DAS had limited efficiency for sulfate removal and an additional treatment unit, such as a sulfate-reducing biochemical reactor, is needed. Moreover, due to the similar performances of the calcite and dolomite DAS, they could be potentially substituted and rather be used in a polishing treatment unit. Based on these findings, the most promising mixture was the 50% wood ash type DAS (WA50-DAS).
Keywords: Ferriferous acid mine drainage; Iron pre-treatment; Dispersed alkaline substrate; Calcite; Dolomite;
Impact of Na―SO4 dominated ionic strength on trace metal removal products in vertical flow bioreactors by J.A. LaBar; R.W. Nairn (24-34).
Vertical flow bioreactors (VFBR) are often used as a component of passive treatment systems (PTS) to treat mine drainage. One of the primary purposes of VFBR is to remove trace metals from mine drainage and retain them in the organic substrate. Elevated ionic strength may impact the performance of VFBR and affect their ability to remove trace metals. A paired-comparison study was performed to determine how products of trace metal removal may change when ionic strength is elevated due to increased concentrations of common contributors to TDS, specifically sodium and sulfate. A sequential extraction procedure (SEP) and acid-volatile sulfide/simultaneously extracted metals analyses (AVS/SEM) were used to determine dominant Cd, Mn, Ni, Pb, and Zn removal products in bench-scale VFBR. Elevated ionic strength resulted in more Pb being retained in the substrates as an insoluble sulfide and less Mn being removed via adsorption to the substrates. An increase in ionic strength had a greater impact on adsorption when sulfate reduction was inhibited, with percentages of Mn and Zn removed via this mechanism decreasing by at least half. This finding could be particularly significant at the start of VFBR operation when adsorption is expected to be the primary removal mechanism.
Keywords: Passive treatment; Total dissolved solids; Sulfate reduction;
Assessing groundwater-surface water connectivity using radon and major ions prior to coal seam gas development (Richmond River Catchment, Australia) by Marnie L. Atkins; Isaac R. Santos; Damien T. Maher (35-48).
Coal seam gas (CSG, or coal bed methane) mining is rapidly growing, with poorly understood impacts on groundwater and surface water systems. Here, we use chemical tracers to investigate groundwater-surface water connectivity in an Australian river system (Richmond River Catchment, New South Wales) prior to CSG extraction but after ∼ 50 exploratory CSG wells were drilled. We performed four surveys of 29 interconnected creek and river sites, over contrasting hydrological conditions. Radon was used to determine if a surface water segment was gaining groundwater. Radon observations over four seasons revealed that 28 out of 77 surface water segments were clearly gaining groundwater, 5 were possibly gaining groundwater and 44 were undetermined. This is equivalent to gaining segments in 333 km (39%) of surface water from the 864 km being investigated. High spatial and temporal variability in groundwater gaining segments was found. Na/Cl ratios were used to determine the fraction of groundwater in surface water. Overall, the groundwater contribution in surface waters was 14–24% higher in post flood conditions than during the other three surveys of baseflow and moderate flow conditions. The results serve as a regional baseline assessment of river water chemistry and groundwater-surface water connectivity prior to the planned development of CSG fields. Our geochemical tracer approach allows for a quick qualitative assessment of groundwater-surface water connectivity in poorly gauged river systems and can define priority locations where groundwater extraction for CSG mining should be carefully managed.
Keywords: Groundwater; Radon; Major ions; Coal seam gas; Catchment; Unconventional gas;
Controls on the δ13CDIC and alkalinity budget of a flashy subtropical stream (Manoa River, Hawaii) by Benjamin Hagedorn; Aly I. El-Kadi; Robert B. Whittier (49-58).
Hawaiian streams are flashy in nature because watersheds are small and steep and receive intense and unevenly distributed rainfall. As a result, stream chemistry is characterized by considerable spatiotemporal variability. To examine how rainfall and streamflow affect the solute content of the Manoa River in Hawaii, time-series geochemical data collected during 17 sampling campaigns in spring-fall of 2010 were evaluated in a coupled δ13CDIC/major ion inversion model. Spatially, the stream is characterized by a distinct shift from a low HCO3 (43 mg/L), low pCO2 (3760 ppmv) and heavy δ13CDIC (−6.5‰) fingerprint in the upper reaches to a high HCO3 (91 mg/L), high pCO2 (8961 ppmv) and light δ13CDIC (−11.7‰) signature in the lowlands. These trends are attributed to (1) progressive weathering of exposed aluminosilicates, and (2) downstream enrichment in CO2 from organic matter decay in the soil zone. Solute (i.e., nitrate) yields from nitric acid weathering are generally low (<1% of TDS), even in the developed lowlands, where runoff of nitrate-enriched urban effluent has historically been documented. Data furthermore indicate a significant positive correlation between δ13CDIC and rainfall rates in the mid-stream section of the river which is consistent with an atmospheric CO2 dilution effect during high rainfall events. This dilution effect needs to be accounted for to reliably describe the role of volcanic island river systems in global assessments of silicate weathering and CO2 degassing.
Keywords: DIC; Hawaii; Streamflow; Carbon isotope; Volcanic island; Urban land cover;
Reactive transport modelling of groundwater-bentonite interaction: Effects on exchangeable cations in an alternative buffer material in-situ test by I. Wallis; A. Idiart; R. Dohrmann; V. Post (59-69).
Bentonite clays are regarded a promising material for engineered barrier systems for the encapsulation of hazardous wastes because of their low hydraulic permeability, swelling potential, ability to self-seal cracks in contact with water and their high sorption potential. SKB (Svensk Kärnbränslehantering) has been conducting long term field scale experiments on potential buffer materials at the Äspö Hard Rock Laboratory for radioactive waste disposal in Sweden.The Alternative Buffer Material (ABM) test examined buffer properties of eleven different clay materials under the influence of groundwater and at temperatures reaching up to 135 °C, replicating the heat pulse after waste emplacement. Clay materials were emplaced into holes drilled in fractured granite as compacted rings around a central heater element and subsequently brought into contact with groundwater for 880 days. After test termination, and against expectations, all clay materials were found to have undergone large scale alterations in the cation exchange population. A reactive-diffusive transport model was developed to aid the interpretation of the observed large-scale porewater chemistry changes. It was found, that the interaction between Äspö groundwater and the clay blocks, together with the geochemical nature of the clays (Na vs Ca-dominated clays) exerted the strongest control on the porewater chemistry. A pronounced exchange of Na by Ca was observed and simulated, driven by large Ca concentrations in the contacting groundwater. The model was able to link the porewater alterations to the fracture network in the deposition hole. The speed of alterations was in turn linked to high diffusion coefficients under the applied temperatures, which facilitated the propagation of hydrochemical changes into the clays. With diffusion coefficients increased by up to one order of magnitude at the maximum temperatures, the study was able to demonstrate the importance of considering temperature-dependant diffusion in understanding and predicting geochemical alterations of engineered barriers systems after relatively short exposure times following waste emplacement.Display Omitted
Keywords: Reactive transport modelling; Exchangeable cations; Groundwater interaction; Temperature-dependant diffusion; Bentonites; ABM; In-situ scale;
Sequestration of molybdate during transformation of 2-line ferrihydrite under alkaline conditions by Soumya Das; Joseph Essilfie-Dughan; M. Jim Hendry (70-80).
Hematite is a thermodynamically stable iron oxide under the aerobic conditions present in most natural surface soils and sediments. Most studies to date have focused on the capacity of hematite to adsorb trace metals and metalloids, but structural incorporation of trace metals within hematite is less recognized. This study assessed the incorporation of molybdenum within the structure of hematite during the phase transformation of 2-line ferrihydrite under alkaline conditions (pH ∼10). Extended X-ray absorption fine structure analyses show molybdenum incorporated into hematite, with two Mo-O shells having a coordination number (CN) of 3 and average bond distances of 1.78 ± 0.01 and 2.08 ± 0.02 Å, respectively, as well as two Mo-Fe shells with a CN of 3 and average bond distances of 3.10 ± 0.02 Å and 3.44 ± 0.02 Å, respectively. This observation suggests the tetrahedrally-coordinated Mo in the molybdate that adsorbs onto the 2-line ferrihydrite changes to an octahedrally-coordinated Mo within the hematite with Mo possibly substituting for Fe in the hematite structure. Our findings suggest that molybdenum partitioning (low concentrations) to iron oxides in the environment can occur due to structural incorporation as well as adsorption.
Keywords: Molybdenum; Hematite; Ferrihydrite; Mine tailings; Structural incorporation;
Subsurface variations in arsenic mineralogy and geochemistry following long-term weathering of gold mine tailings by Stephanie L. DeSisto; Heather E. Jamieson; Michael B. Parsons (81-97).
Variations in arsenic (As) mineralogy and geochemical controls on its mobility were evaluated in subsurface tailings at the historical Montague and Goldenville mine sites in Nova Scotia, Canada. Tailings at these sites contain some of the highest As concentrations in Nova Scotia and are located in close proximity to local communities. Pore water in the subsurface tailings is characterized by circumneutral to alkaline pH (6.2 to 8.7) and mildly reducing to oxidizing redox conditions (+130 mV to +347 mV). Bulk chemistry, scanning electron microscopy, and synchrotron micro-X-ray diffraction analyses showed As mineral hosts differ with depth. The deepest tailings (max. 2 m) are in direct contact with partially decomposed vegetation, which supports reducing conditions and the precipitation of authigenic As and Fe sulfides. Under reducing conditions, dissolved As concentrations are also controlled by desorption of As from dissolution of Fe and Mn oxides and the sorption or co-precipitation of As with carbonates. These geochemical controls differ from those influencing dissolved As concentrations under oxidizing conditions. In the near surface, As mobility is controlled by oxidative dissolution of primary arsenopyrite, precipitation of secondary Fe arsenates, Fe oxyhydroxides and Mn oxides, secondary Ca-Fe arsenates, and sorption onto Fe oxyhydroxides and gangue minerals. Some of these mineral species are stable under different conditions yet occur in close association, indicating the importance of microenvironments. The results of this study show that the weathering characteristics of these tailings vary with depth, leading to the formation of new As hosts that are distinct from those observed in the near surface. Identification of these As hosts provides an understanding of current controls on As mobility and has implications for future reprocessing and/or remediation efforts.
Keywords: Arsenic; Mine waste; Mineralogy; Gold; Remediation; Nova Scotia;
Identifying calcium-containing mineral species in the JEB Tailings Management Facility at McClean Lake, Saskatchewan by Peter E.R. Blanchard; Andrew P. Grosvenor; John Rowson; Kebbi Hughes; Caitlin Brown (98-108).
The JEB Tailings Management Facility (TMF) is central to reducing the environmental impact of the McClean Lake uranium mill facility that is operated by AREVA Resources Canada. This facility has been designed around the idea that elements of concern (e.g., U, As, Ni, Se, Mo) will be controlled through equilibrium with precipitants. Confirming the presence of calcium-containing carbonates in the JEB TMF is the first step in determining if gypsum (CaSO4·2H2O) controls the concentration of HCO3 − (aq), limiting the formation of soluble uranyl bicarbonate complexes. A combination of X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and microprobe X-ray fluorescence (XRF) mapping was used to analyze a series of tailings samples from the JEB TMF. Calcium carbonate in the form of calcite (CaCO3), aragonite (CaCO3), and dolomite (CaMg(CO3)2) were identified by analysing Ca K-edge μ-XANES spectra coupled with microprobe XRF mapping. This is the first observation of these phases in the JEB TMF. The combination of μ-XANES and XRF mapping provided a greater sensitivity to low concentration calcium species compared to the other techniques used, which were only sensitive to the major species present (e.g., gypsum).Display Omitted
Keywords: Uranium mining; Calcium; Tailings; Synchrotron radiation; X-ray absorption spectroscopy; X-ray microprobe;
The effect of Si and Al concentrations on the removal of U(VI) in the alkaline conditions created by NH3 gas by Yelena P. Katsenovich; Claudia Cardona; Robert Lapierre; Jim Szecsody; Leonel E. Lagos (109-117).
Remediation of uranium in the deep unsaturated zone is a challenging task, especially in the presence of oxygenated, high-carbonate alkalinity soil and pore water composition typical for arid and semi-arid environments of the western regions of the U.S. This study evaluates the effect of various pore water constituencies on changes of uranium concentrations in alkaline conditions, created in the presence of reactive gases such as NH3 to effectively mitigate uranium contamination in the vadose zone sediments. This contaminant is a potential source for groundwater pollution through slow infiltration of soluble and highly mobile uranium species towards the water table. The objective of this research was to evaluate uranium sequestration efficiencies in the alkaline synthetic pore water solutions prepared in a broad range of Si, Al, and bicarbonate concentrations typically present in field systems of the western U.S. regions and identify solid uranium-bearing phases that result from ammonia gas treatment. In previous studies (Szecsody et al. 2012; Zhong et al. 2015), although uranium mobility was greatly decreased, solid phases could not be identified at the low uranium concentrations in field-contaminated sediments. The chemical composition of the synthetic pore water used in the experiments varied for silica (5–250 mM), Al3+ (2.8 or 5 mM), HCO3 − (0–100 mM) and U(VI) (0.0021–0.0084 mM) in the solution mixture. Experiment results suggested that solutions with Si concentrations higher than 50 mM exhibited greater removal efficiencies of U(VI). Solutions with higher concentrations of bicarbonate also exhibited greater removal efficiencies for Si, Al, and U(VI). Overall, the silica polymerization reaction leading to the formation of Si gel correlated with the removal of U(VI), Si, and Al from the solution. If no Si polymerization was observed, there was no U removal from the supernatant solution. Speciation modeling indicated that the dominant uranium species in the presence of bicarbonate were anionic uranyl carbonate complexes (UO2(CO3)2 −2 and UO2(CO3)3 −4) and in the absence of bicarbonate in the solution, U(VI) major species appeared as uranyl-hydroxide (UO2(OH)3 − and UO2(OH)4 −2) species. The model also predicted the formation of uranium solid phases. Uranyl carbonates as rutherfordine [UO2CO3], cejkaite [Na4(UO2)(CO3)3] and hydrated uranyl silicate phases as Na-boltwoodite [Na(UO2)(SiO4)·1.5H2O] were anticipated for most of the synthetic pore water compositions amended from medium (2.9 mM) to high (100 mM) bicarbonate concentrations.
Keywords: Uranium; Silica; Ammonia gas; Vadose zone; Removal efficiency;
Use of mineral/solution equilibrium calculations to assess the potential for carnotite precipitation from groundwater in the Texas Panhandle, USA by Anthony J. Ranalli; Douglas B. Yager (118-131).
This study investigated the potential for the uranium mineral carnotite (K2(UO2)2(VO4)2·3H2O) to precipitate from evaporating groundwater in the Texas Panhandle region of the United States. The evolution of groundwater chemistry during evaporation was modeled with the USGS geochemical code PHREEQC using water-quality data from 100 groundwater wells downloaded from the USGS National Water Information System (NWIS) database. While most modeled groundwater compositions precipitated calcite upon evaporation, not all groundwater became saturated with respect to carnotite with the system open to CO2. Thus, the formation of calcite is not a necessary condition for carnotite to form. Rather, the determining factor in achieving carnotite saturation was the evolution of groundwater chemistry during evaporation following calcite precipitation. Modeling in this study showed that if the initial major-ion groundwater composition was dominated by calcium-magnesium-sulfate (>70 precent Ca + Mg and >50 percent SO4 + Cl) or calcium-magnesium-bicarbonate (>70 percent Ca + Mg and <70 percent HCO3 + CO3) and following the precipitation of calcite, the concentration of calcium was greater than the carbonate alkalinity (2mCa+2 > mHCO3 − + 2mCO3 −2) carnotite saturation was achieved. If, however, the initial major-ion groundwater composition is sodium-bicarbonate (varying amounts of Na, 40–100 percent Na), calcium-sodium-sulfate, or calcium-magnesium-bicarbonate composition (>70 percent HCO3 + CO3) and following the precipitation of calcite, the concentration of calcium was less than the carbonate alkalinity (2mCa+2 < mHCO3 - + 2mCO3 −2) carnotite saturation was not achieved. In systems open to CO2, carnotite saturation occurred in most samples in evaporation amounts ranging from 95 percent to 99 percent with the partial pressure of CO2 ranging from 10−3.5 to 10−2.5 atm. Carnotite saturation occurred in a few samples in evaporation amounts ranging from 98 percent to 99 percent with the partial pressure of CO2 equal to 10−2.0 atm. Carnotite saturation did not occur in any groundwater with the system closed to CO2.
Keywords: Geochemical modeling; Groundwater; PHREEQC; Texas panhandle; Uranium mineralization; Carnotite;
Organic carbon source tracing and DIC fertilization effect in the Pearl River: Insights from lipid biomarker and geochemical analysis by Mingxing Yang; Zaihua Liu; Hailong Sun; Rui Yang; Bo Chen (132-141).
The photosynthetic conversion of dissolved inorganic carbon (DIC) into organic carbon (OC) by using aquatic phototrophs in rivers may serve as a potential carbon sink, especially in the carbonate rock areas, thereby offering a clue for finding the missing carbon sink. However, primary-produced autochthonous OC is erroneously considered as terrestrial-derived allochthonous OC. Thus, carbonate weathering-related carbon sink is underestimated if only DIC concentrations sampled at river mouths are considered, and the transformation of DIC to autochthonous OC is neglected. Therefore, distinguishing sources of autochthonous and allochthonous OC is vital in the assessment of carbon sink. In this study, source-specific biomarkers, in association with chemical compositions and phytoplankton proxies in water samples collected from the Pearl River, were analyzed to determine OC sources. Results showed that biomarkers in the Pearl River were quite abundant, and the calculated average autochthonous OC was approximately 65% of the total OC, indicating intense in-river primary productivity. Moreover, phytoplankton biomass and DIC concentration were positively related, indicating the DIC fertilization effect on aquatic photosynthesis. High total suspended solid (TSS) on the water surface blocked the sunlight and then reduced phytoplankton production. However, in situ photosynthesis of phytoplankton could also produce autochthonous OC, even larger than the allochthonous source at sites with high DIC, and even with higher TSS concentrations. These findings comprehensively elucidated the formation of autochthonous OC based on the coupling action of rock weathering and photosynthetic activity in the riverine system, suggesting a potential direction for finding the missing carbon sink.
Keywords: Lipid biomarker; Carbon source; Carbonate weathering; In-river primary production; Missing carbon sink; DIC fertilization effect; Pearl River;
Geothermal solute flux monitoring and the source and fate of solutes in the Snake River, Yellowstone National Park, WY by R. Blaine McCleskey; Jacob B. Lowenstern; Jonas Schaper; D. Kirk Nordstrom; Henry P. Heasler; Dan Mahony (142-156).
The combined geothermal discharge from over 10,000 features in Yellowstone National Park (YNP) can be can be estimated from the Cl flux in the Madison, Yellowstone, Falls, and Snake Rivers. Over the last 30 years, the Cl flux in YNP Rivers has been calculated using discharge measurements and Cl concentrations determined in discrete water samples and it has been determined that approximately 12% of the Cl flux exiting YNP is from the Snake River. The relationship between electrical conductivity and concentrations of Cl and other geothermal solutes was quantified at a monitoring site located downstream from the thermal inputs in the Snake River. Beginning in 2012, continuous (15 min) electrical conductivity measurements have been made at the monitoring site. Combining continuous electrical conductivity and discharge data, the Cl and other geothermal solute fluxes were determined. The 2013–2015 Cl fluxes (5.3–5.8 kt/yr) determined using electrical conductivity are comparable to historical data. In addition, synoptic water samples and discharge data were obtained from sites along the Snake River under low-flow conditions of September 2014. The synoptic water study extended 17 km upstream from the monitoring site. Surface inflows were sampled to identify sources and to quantify solute loading. The Lewis River was the primary source of Cl, Na, K, Cl, SiO2, Rb, and As loads (50–80%) in the Snake River. The largest source of SO4 was from the upper Snake River (50%). Most of the Ca and Mg (50–55%) originate from the Snake Hot Springs. Chloride, Ca, Mg, Na, K, SiO2, F, HCO3, SO4, B, Li, Rb, and As behave conservatively in the Snake River, and therefore correlate well with conductivity (R2 ≥ 0.97).
Keywords: Yellowstone National Park; Grand Teton National Park; Geothermal; Electrical conductivity; Specific conductance;
Nitrate distribution and potential attenuation mechanisms of a municipal water supply bedrock aquifer by Tomás Opazo; Ramón Aravena; Beth Parker (157-168).
The Silurian bedrock aquifer constitutes a major aquifer system for groundwater supply across the Ontario province in Canada. The application of natural and industrial fertilizers near urban centers has led to groundwater NO3 −-N concentrations that sometimes have exceeded the drinking water limit, posing a threat to the usage of groundwater for the human consumption. Therefore, there is a growing interest and concern about how nitrate is being leached, transported and potentially attenuated in bedrock aquifers. This study assesses the local distribution of groundwater NO3 − in the up-gradient area of two historically impacted municipal wells, called Carter Wells, in the City of Guelph, Canada, in order to evaluate the potential nitrate attenuation mechanisms, using both groundwater geochemical and isotopic analysis (3H, δ15N-NO3, δ18O-NO3, δ18O-SO4, δ34S-SO4) and a detailed vertical hydrogeological and geochemical bedrock characterization. The results indicate that probably the main source of nitrate to the Carter Wells is the up-gradient Arkell Research Station (ARS), an agricultural research facility where manure has been historically applied. The overburden and bedrock groundwater with high NO3 concentrations at the ARS exhibits a manure-related δ15N and δ18O signature, isotopically similar to the high nitrate in the down-gradient groundwater from domestic wells and from the Carter Wells. The nitrate spatial distribution appears to be influenced and controlled by the geology, in which more permeable rock is found in the Guelph Formation which in turn is related to most of the high NO3 − groundwater. The presence of an underlying low permeability Eramosa Formation favors the development of oxygen-depleted conditions, a key factor for the occurrence of denitrification. Groundwater with low NO3 −-N concentrations associated with more oxygen-limited conditions and coincident with high SO4 2− concentrations are related to more enriched δ15N and δ18O values in NO3 − and to more depleted δ34S and δ18O values in SO4 2−, suggesting that denitrification coupled with pyrite oxidation is taking place. The presence of macro crystalized and disseminated pyrite especially in the Eramosa Formation, can support the occurrence of this attenuation process. Moreover, based on tritium analysis, some denitrification can occur in shallow bedrock and within relatively short residence times, associated with less permeable conditions in depth which facilitates oxygen consumption through sulfide oxidation. The role of denitrification mediated by organic carbon cannot be discarded at the study site. This study suggests that the geological configuration and particularly the presence of low permeability Eramosa Formation can play an important role on nitrate natural attenuation, which may serve as a decision factor on defining the bedrock water supply system for both domestic and municipal purposes.
Keywords: Nitrate; Denitrification; Fractured rock; Isotopes;
Dissolution-precipitation reactions controlling fast formation of dolomite under hydrothermal conditions by German Montes-Hernandez; Nathaniel Findling; François Renard (169-177).
In laboratory experiments, the precipitation of dolomite at ambient temperature is virtually impossible due to strong solvation shells of magnesium ions in aqueous media and probably also due to the existence of a more intrinsic crystallization barrier that prevents the formation of long-range ordered crystallographic structures at ambient surface conditions. Conversely, dolomite can easily form at high temperature (>100 °C), but its precipitation and growth requires several days or weeks depending on experimental conditions. In the present study, experiments were performed to assess how a single heat-ageing step promotes the formation of dolomite under high-carbonate alkaline conditions via dissolution-precipitation reactions. This reaction pathway is relevant for the so-called hydrothermal dolomite frequently observed in carbonate platforms, but still ill-defined and understood. Our precipitation route is summarized by two main sequential reactions: (1) precipitation of Mg-calcite at low temperature (∼20 °C) by aqueous carbonation of synthetic portlandite (Ca(OH)2) in a highly alkaline medium (1 M of NaOH and 1 M of MgCl2), leading to precipitation of oriented nanoparticles of low- and high-Mg calcite (∼79 wt%) coexisting with aragonite (∼18 wt%) and brucite (∼3 wt%) after 24 h; (2) fast dolomitization process starting from 1 h of reaction by a single heat-ageing step from ∼20 to 200, 250 and 300 °C. Here, the Mg-calcite acts as a precursor that lowers the overall kinetics barrier for dolomite formation. Moreover, it is an important component in some bio-minerals (e.g. corals and seashells). Quantitative Rietveld refinements of XRD patterns, FESEM observations and FTIR measurements on the sequentially collected samples suggest fast dolomite precipitation coupled with dissolution of transient mineral phases such as low-Mg calcite (Mg < 4 mol%), high-Mg calcite (Mg > 4 mol%), proto-dolomite (or disordered dolomite; Mg > 40 mol%) and Ca-magnesite. In this case, the dolomite formation rate and the time-dependent mineral composition strongly depend on reaction temperature. For example, high-purity dolomitic material (87 wt% of dolomite mixed with 13 wt% of magnesite) was obtained at 300 °C after 48 h of reaction. Conversely, a lower proportion of dolomite (37 wt%), mixed with proto-dolomite (43 wt%), Ca-magnesite (16 wt%) and high-Mg calcite (4 wt%), was obtained at 200 °C after 72 h. The present experiments provide an additional mechanism for the massive dolomite formation in sedimentary environments (ex. deep sea organic-rich carbonate-sediments) if such sediments are subjected to significant temperature variations, for example by hot fluid circulations related to volcanic activity. In such systems, organic degradation increases the carbonate alkalinity (HCO3 −) necessary to induce the dolomitization process at low and high temperature.
Keywords: Dolomite; Mg-calcite; Ca-magnesite; Hydrothermal conditions; CO2; Carbonation;