Applied Geochemistry (v.75, #C)

Geochemistry of barium in soils supplied with carbonatite rock powder and barium uptake to plants by Mona Bakke Myrvang; Elin Gjengedal; Michael Heim; Tore Krogstad; Åsgeir R. Almås (1-8).
Apatite-biotite-carbonatite (short form: carbonatite) rock powder originating from Lillebukt alkaline complex, N-Norway is a potential liming and multi-nutrient fertilizer. However, the elevated contents of potentially bio-toxic barium (Ba) (up to 5.5 g kg−1) is a matter of concern. A pot experiment was conducted in order to investigate the geochemistry of Ba in a sandy peat-containing growth media treated with carbonatite, with special focus on Ba-speciation in the pore water. These results were correlated with Ba uptake in Trifolium repens L. (white clover) and Fesctua arundinácea L. (tall fescue). The Ba-speciation and bioavailability as affected by treatment and plant growth was conducted using WHAM/Model VI version 7.0.4 and the diffusive gradients in thin films (DGT) technique, respectively. The increasing cation exchange capacity (CEC) in the growth media, and colloidal fulvic acids (FA) decreased the Ba2+ concentration in pore water. Both the Ba detected in DGT (Badgt), and exchangeable Ba (Baexc) were higher in pots containing rock powder. A strong relationship between Baexc and diffusively bound Ba to FA (Badfa) with plant uptake was evident. The latter was likely initiated by organic material mobilization of Ba from carbonatite. The mobilized Ba bound to FA (Bafa) in pore water was mainly bound diffusively, and the Badfa fraction was thus made available to plants. Finally, the Badgt was increased in pots grown with white clover compared to tall fescue. Further investigation is required to determine if this is related to increased rhizosphere activity by plants or to the nitrogen fixating plant-microbe relationship.
Keywords: Barium; DGT; Rock powder; Soil organic matter; Speciation; WHAM;

Documenting the interaction between groundwater and rivers is fundamental to understanding hydrological systems. While many studies have examined the location and magnitude of groundwater inflows to rivers, much less is known about the transit times of water in catchments and from where in the aquifer the groundwater originates. Resolving those questions is vital for protecting riverine ecosystems, assessing the impact of contamination, and understanding the potential consequences of groundwater pumping. This study uses tritium (3H) to evaluate the mean transit times of water contributing to Deep Creek (southeast Australia), which is a chain-of-ponds river system. 3H activities of river water vary between 1.47 and 2.91 TU with lower 3H activities recorded during cease-to-flow periods when the river comprises isolated groundwater-fed pools. Regional groundwater 1–2.5 km away from Deep Creek at depths of 7.5–46.5 m has 3H activities of between <0.02 and 0.84 TU. The variation in 3H activities suggest that the water that inflows into Deep Creek is dominated by near-river shallow groundwater with the deeper groundwater only providing significant inflows during drier periods. If the water in the catchment can be represented by a single store with a continuum of ages, mean transit times of the river water range between <1 and 31 years whereas those of the groundwater are at least 75 years and mainly >100 years. Alternatively the variation in 3H activities can be explained by mixing of a young near-river water component with up to 50% older groundwater. The results of this study reinforce the need to protect shallow near-river groundwater from contamination in order to safeguard riverine ecosystems and also illustrate the potential pitfalls in using regional bores to characterise the geochemistry of near-river groundwater.
Keywords: Tritium; Groundwater; River water; Residence times;

Growth in global metal demand has fostered a new age of unconventional mining on the seafloor. In situ pulverization and extraction of seafloor massive sulfide (SMS) deposits is economically attractive due to minimal overburden and high ore grades. However, important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation. Data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining.Laboratory experiments were performed to evaluate the effects of pH, temperature, dissolved oxygen, and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater. These minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow. The rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22 °C are given in the form: R sp = K ( m O 2 ) a ( m H + ) b where Rsp is the specific rate (moles m−2 sec−1), k is the rate constant, oxygen and proton concentrations are expressed in molalities (m), and their reaction orders as a and b, respectively. The specific rate laws obtained for each sulfide studied are: R sp ( pyrrhotite ) = − 10 − 7.27 ( m O 2 ( aq ) ) 0.51 ± 0.08 ( m H + ) 0.08 ± 0.03 R sp ( chalcopyrite ) = − 10 − 9.38 ( m O 2 ( aq ) ) 1.16 ± 0.03 ( m H + ) 0.36 ± 0.09 When used to quantitatively predict maximum acid generation rates, these rate laws indicate that acid production from in situ SMS mining is insufficient to exceed the buffering capacity of advecting seawater. We also calculated the residence times of crushed sulfides in seawater with low PO2 (0.10 atm, pH of 8, 23 °C) and find that, depending on grain size, mining waste may persist near the seafloor for years. The implications are positive in terms of slow acid production, but potentially problematic considering the potential ecological effects of an unnatural influx of particulates.
Keywords: Seafloor mining; Hydrothermal vents; Seafloor massive sulfides; Chalcopyrite; Pyrrhotite;

Quantification of methylmercury and geochemistry of mercury in sediments from a contaminated area of Descoberto (MG), Brazil by Louise Aparecida Mendes; Jorge Carvalho de Lena; Cláudia Magalhães do Valle; Peter Marshall Fleming; Cláudia Carvalhinho Windmöller (32-43).
In 2002, metallic Hg was found buried in a rural area of Descoberto city, Brazil. The origin of the Hg was a gold mining explotation plant established nearly one century ago. Although a number of studies have been conducted in order to assess the contamination of the area, none of them investigated the presence of methylated Hg in the hydric system. In this work methylmercury (CH3Hg+) was determined using gas chromatography-pyrolysis-atomic fluorescence detection (CG-pyro-AFS) in material from rain sedimentation boxes and stream sediments near the contaminated area. Total Hg concentration (HgT) along with the chemical speciation by thermo-desorption were performed. HgT in material from the sedimentation boxes was found to be very high, up to 41,580 μg kg−1, even in the rainy season, when in general HgT were much lower than in dry season. The samples from the Grama and Rico streams show a range of HgT from 5.8 to 266 μg kg−1. The thermo-desorption analysis showed predominance of Hg2+, possibly linked to organic sulfur, suggested by a good positive correlation between Hg2+, HgT, organic mater (OM) and total S. The CH3Hg+ concentration in stream sediment samples ranged from <0.07 to 1.87 μg kg−1 and in the samples of sedimentation boxes the concentrations were 1.33 and 8.0 μg kg−1 during dry season. The sample with the highest percentage of HgT as Hg2+ (98%) presented also the highest percentage of CH3Hg+ (0.7%). These are high values, showing that care should be taken to avoid the transport of this material to the hydrological system. Further studies on the transfer through the food chain would be very important.
Keywords: Methylmercury; Speciation; Sediment contamination; Descoberto city– Brazil; Hg Geochemistry;

AERYN: A simple standalone application for visualizing and enhancing elemental maps by Vincent Mouchi; Quentin G. Crowley; Teresa Ubide (44-53).
Interpretation of high spatial resolution elemental mineral maps can be hindered by high frequency fluctuations, as well as by strong naturally-occurring or analytically-induced variations. We have developed a new standalone program named AERYN (Aspect Enhancement by Removing Yielded Noise) to produce more reliable element distribution maps from previously reduced geochemical data. The program is Matlab-based, designed with a graphic user interface and is capable of rapidly generating elemental maps from data acquired by a range of analytical techniques. A visual interface aids selection of appropriate outlier rejection and drift-correction parameters, thereby facilitating recognition of subtle elemental fluctuations which may otherwise be obscured. Examples of use are provided for quantitative trace element maps acquired using both laser ablation (LA-) ICP-MS and electron probe microanalysis (EPMA) of the cold-water coral Lophelia pertusa. We demonstrate how AERYN allows recognition of high frequency elemental fluctuations, including those which occur perpendicular to the maximum concentration gradient. Such data treatment compliments commonly used processing methods to provide greater flexibility and control in producing elemental maps from micro-analytical techniques.
Keywords: LA-ICP-MS; EDS; WDS; Element distribution maps; Processing; Matlab; Biominerals;

Voltammetric characterization of Boda Albitic Claystone. Comparison with previous Mössbauer spectroscopy results by Michel Perdicakis; Yuliang Xu; Károly Lazar; Zoltán Mathe; Lise Rouillard (54-62).
We used ‘Voltammetry of MicroParticles’ to investigate electrochemically samples from the Boda Albitic Claystone, a potential site for deep underground disposal of nuclear waste in Hungary. The samples were collected near the surface and at a depth of 1050 m. The electrochemical characterization in acidic media allowed the identification of Fe2+ and Fe3+ ions associated with clays, hematite and pyrite. The cathodic detection of pyrite does not raise special difficulties, but the anodic one is complex and depends on the time of the samples exposure to the atmosphere. This work completes a previous Mössbauer spectroscopy study by clarifying the presence, or absence, of extremely small amounts of pyrite in some samples.Display Omitted
Keywords: Underground nuclear waste disposal; Voltammetry of clays; Voltammetry of microparticles; Boda Albitic Claystone; Pyrite; Hematite;

Soil mercury distribution in adjacent coniferous and deciduous stands highly impacted by acid rain in the Ore Mountains, Czech Republic by Tomáš Navrátil; James B. Shanley; Jan Rohovec; Filip Oulehle; Martin Šimeček; Jakub Houška; Pavel Cudlín (63-75).
Forests play a primary role in the cycling and storage of mercury (Hg) in terrestrial ecosystems. This study aimed to assess differences in Hg cycling and storage resulting from different vegetation at two adjacent forest stands - beech and spruce. The study site Načetín in the Czech Republic's Black Triangle received high atmospheric loadings of Hg from coal combustion in the second half of the 20th century as documented by peat accumulation rates reaching 100 μg m−2 y−1. In 2004, the annual litterfall Hg flux was 22.5 μg m−2 y−1 in the beech stand and 14.5 μg m−2 y−1 in the spruce stand. Soil concentrations and pools of Hg had a strong positive relation to soil organic matter and concentrations of soil sulfur (S) and nitrogen (N). O-horizon Hg concentrations ranged from 245 to 495 μg kg−1 and were greater in the spruce stand soil, probably as a result of greater dry Hg deposition. Mineral soil Hg concentrations ranged from 51 to 163 μg kg−1 and were greater in the beech stand soil due to its greater capacity to store organic carbon (C). The Hg/C ratio increased with depth from 0.3 in the O-horizon to 3.8 μg g−1 in the C horizon of spruce soil and from 0.7 to 2.7 μg g−1 in beech soil. The Hg/C ratio was greater at all mineral soil depths in the spruce stand. The organic soil Hg pools in beech and spruce stands (6.4 and 5.7 mg m−2, respectively) were considerably lower than corresponding mineral soil Hg pools (39.1 and 25.8 mg m−2). Despite the important role of S in Hg cycling, differences in soil Hg distribution at both stands could not be attributed to differences in soil sulfur speciation.
Keywords: Mercury; Carbon; Sulfur; Nitrogen; Soil pools; Oxalate extractable Al and Fe; Hg/C ratio; C/S ratio; Black triangle;

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO2 as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO2, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca2+ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca2+ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca2+ condition coinciding with the formation of CaCO3. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca2+ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO2 sequestration methods in a controlled large-scale environment.
Keywords: CO2 sequestration; Mixed-flow bio-reactor; Steady state; Freshwater cyanobacteria; Synechococcus sp.; Calcite; Air bubbling;

The Athabasca Oil Sands contain one of the world's largest oil reserves consisting of approximately 168 billion barrels of currently recoverable bitumen. With 20% recoverable through open pit mining methods, this extraction process produces a considerable amount of fluid fine tailings (FFT) waste material, which must be deposited on site in tailings ponds. These ponds allow the waste sand, clay and residual bitumen to settle out of the water column, allowing for the water to be recycled for use again in the extraction process. It is vital to gain a better understanding of the processes contributing to the development of physicochemical gradients (pH, Eh, Oxygen etc…) that form in these tailings ponds over time, with the goal of remediation and subsequent construction of end-pit lake systems once oil extraction has ceased. To differentiate between the impacts of biotic and abiotic processes in fresh (newly processed material) and mature FFT (∼38 year old tailings) over a 52-week study, a specific experimental design was utilized in accordance with novel microsensor profiling techniques. The sulfide diffusive fluxes within mature biotic systems measured 37.6 μmol m−2 day−1 at the onset of the experiment, decreasing over time, as FeS mineralization progressed. In addition, DO fluxes also showed strong correlation to the physical affects of consolidation, and overall biological consumption of O2 at the FFT-water interface. This holistic study comparing different tailings pond materials provides insight regarding biotransformation and physicochemical controls effecting sediment oxygen demand associated with reclaimed wetlands and end pit lake development.
Keywords: Athabasca; Oil sands; Tailings pond; Hydrogen sulfide; Methane; Oxygen; Flux; Microcosm; Microsensor; Physicochemical; Diffusion; SOD;

Emerging airborne contaminants in India: Platinum Group Elements from catalytic converters in motor vehicles by Indra S. Sen; Arijeet Mitra; Bernhard Peucker-Ehrenbrink; Sarah E. Rothenberg; Sachchida Nand Tripathi; Michael Bizimis (100-106).
Platinum Group Element (PGE) pollution on the Indian subcontinent is a growing concern because vehicle sales in India have rapidly increased over the last decade, and it is well known that automobile catalytic converters are one of the major source of anthropogenic PGE in the environment. Despite the rapid growth of the Indian automobile industry, the sources and magnitude of PGE contamination in Indian airborne particles are unknown. In this study we report PGE and mercury (Hg) concentrations, as well as osmium isotope ratios (187Os/188Os) of airborne particles (PM10) collected in Kanpur, a large industrial city in India. We estimate that 61 ± 22%, 32 ± 24%, and 7 ± 3% of the total Os fraction are derived from eroding upper continental crust, catalytic converters fitted in the exhaust system of motor vehicles, and fossil fuel combustion, respectively. Only one sample had a ten times higher (∼76%) than average contribution from fossil fuel. Unlike Os, Pt is predominantly (84 ± 10%) derived from anthropogenic sources. Platinum Group Element and Hg concentrations are not well correlated. However, the highest concentration of particulate Hg corresponds to the most radiogenic 187Os/188Os isotope ratios (4.6). Our results further indicated that PGE/Ir ratios could be successfully used to quantify the relative proportions of natural and anthropogenic PGE sources in aerosol samples. Since PGE and Hg data on Indian environmental samples are scarce, this study provides an interpretive framework that calls for additional assessments of PGE and Hg concentrations in environmental samples from India.

Determination of in situ formation water chemistry is an essential component of reservoir management. This paper details the use of thermodynamic computer models to calculate reservoir pH and restore produced water analyses for prediction of scale formation. Bakken produced water samples were restored to formation conditions and calculations of scale formation performed. In situ pH is controlled by feldspar-clay equilibria. Calcite scale is readily formed due to changes in pH during pressure drop from in situ to surface conditions. The formation of anhydrite and halite scale, which has been observed, was predicted only for the most saline samples. In addition, the formation of anhydrite and/or halite may be related to the localized conditions of increased salinity as water is partitioned into the gas phase during production.
Keywords: Formation water chemistry; Bakken Shale; Scale formation; Geochemical models;

The present research tests the application of geochemical atlas of soils and stream sediments in the investigation of weathering and erosion processes on volcanic islands. The composition of surface soils collected in six catchments from Santiago Island (Cape Verde) are compared with bedload stream deposits sourced by these catchment areas in order to evaluate the spatial variability of these exogenous processes. The geochemistry of bedload stream deposits is between that of the fresh rocks and the topsoils of their source areas. Relative to average soil composition, bedload deposits are depleted in most of less-mobile elements (e.g., Al, Fe, La, Sc) and strongly enriched in Na and, usually, Ca. When the topsoil weathering intensity in the catchment areas is highly variable and the composition of bedload deposits is substantially different from the average soil composition, bedload deposits should incorporate significant amounts of poorly-weathered rocks and sectors from erosion occur within the drainage basin. Ratios of non-mobile elements allow the identification of highly vulnerable and erosion-protected sectors within the catchments. It is proposed that the catchments of the rivers in the SW flanking side of Santiago Island include sectors where lava shields formed during the post-erosional eruptive phases are capable of an effective protection to erosion. Conversely, the NE-facing part of the island is highly dissected and any younger post-erosional cover was either completely eroded away, or never existed in the first place. Simple compositional parameters derived from the databases of geochemical maps of soil and stream sediments provide important information for the analyses of weathering, erosion and denudation processes at the catchment scale.
Keywords: Volcanic island; Santiago Island (Cape Verde); Topsoils; Bedload stream sediments; Weathering; Erosion;

Stability, transformations and stabilizing potential of an amorphous manganese oxide and its surface-modified form in contaminated soils by Zuzana Michálková; Michael Komárek; Martina Vítková; Magdalena Řečínská; Vojtěch Ettler (125-136).
A surface-modified amorphous manganese oxide (SM-AMO) was prepared to increase the stability of a previously studied promising stabilizing agent and to compare its immobilizing efficiency with respect to contaminating metals with the original material. To synthesize the SM-AMO, the AMO surface was synthetically covered with a coating of MnCO3 because newly formed rhodochrosite precipitates were previously found to increase the stability of AMO particles in soils. A preliminary experiment evaluating the long-term stability of both materials in pure water suggested higher stability for the SM-AMO particles, showing a smaller release of Mn compared to the original AMO. An adsorption kinetics study focused on As, Cd, Pb and Zn showed lower adsorption rates and adsorption capacity for Zn, probably as a result of partial surface passivation. In comparison to these results for simple controlled systems, different effects were recorded when the two materials were applied to contaminated soils. When incubated in soil, a constantly lower mass loss was recorded in the case of SM-AMO. There were no significant differences in the release of Mn and DOC into the soil solution or in the stabilizing efficiency with regard to contaminating metal(loid)s between the original and surface-modified materials. Concerning the potential solid phase transformations in soil conditions, we observed a gradual equilibration between the surface composition of both materials. While the newly formed rhodochrosite precipitated on the AMO surface, the MnCO3 coatings on SM-AMO gradually dissolved. Both amendments also effectively supported microbial activity, especially in the more contaminated soil sample. Thus, despite the smaller mass loss, the effectiveness of both materials is comparable in the long term.
Keywords: Remediation; Stabilization; Immobilization; Mn oxide; Rhodochrosite;

Carbon dioxide enhanced oil recovery (CO2-EOR) has been widely applied to the process of carbon capture, utilization, and storage (CCUS). Here, we investigate CO2–oil–water–rock interactions under reservoir conditions (100 °C and 24 MPa) in order to understand the fluid–rock interactions following termination of a CO2-EOR project. Our experimental results show that CO2-rich fluid remained the active fluid controlling the dissolution–precipitation processes in an oil-undersaturated sandstone reservoir; e.g., the dissolution of feldspar and calcite, and the precipitation of kaolinite as well as solid phases comprising O, Si, Al, Na, C, and Ti. Mineral dissolution rates were reduced in the case that mineral surfaces were coated by oil. Mineral wettability and composition, and oil saturation were the main controls on the exposed surface area of grains, and mineral wettability in particular led to selective dissolution. In addition, the permeability of the reservoir decreased substantially due to the precipitation of kaolinite and solid-phase particles, and due to the clogging of less soluble mineral particles released by the dissolution of K-feldspar and carbonate cement, whereas porosity increased. The results provide insight into potential formation damage resulting from CO2-EOR projects.
Keywords: CO2–oil–brine–rock interaction; Permeability; Wettability; CCUS; Songliao basin;

Mineralogical controls on porosity and water chemistry during O2-SO2-CO2 reaction of CO2 storage reservoir and cap-rock core by Julie K. Pearce; Alexandra Golab; Grant K.W. Dawson; Lydia Knuefing; Carley Goodwin; Suzanne D. Golding (152-168).
Reservoir and cap-rock core samples with variable lithology's representative of siliciclastic reservoirs used for CO2 storage have been characterized and reacted at reservoir conditions with an impure CO2 stream and low salinity brine. Cores from a target CO2 storage site in Queensland, Australia were tested. Mineralogical controls on the resulting changes to porosity and water chemistry have been identified. The tested siliciclastic reservoir core samples can be grouped generally into three responses to impure CO2-brine reaction, dependent on mineralogy. The mineralogically clean quartzose reservoir cores had high porosities, with negligible change after reaction, in resolvable porosity or mineralogy, calculated using X-ray micro computed tomography and QEMSCAN. However, strong brine acidification and a high concentration of dissolved sulphate were generated in experiments owing to minimal mineral buffering. Also, the movement of kaolin has the potential to block pore throats and reduce permeability. The reaction of the impure CO2-brine with calcite-cemented cap-rock core samples caused the largest porosity changes after reaction through calcite dissolution; to the extent that one sample developed a connection of open pores that extended into the core sub-plug. This has the potential to both favor injectivity but also affect CO2 migration. The dissolution of calcite caused the buffering of acidity resulting in no significant observable silicate dissolution. Clay-rich cap-rock core samples with minor amounts of carbonate minerals had only small changes after reaction. Created porosity appeared mainly disconnected. Changes were instead associated with decreases in density from Fe-leaching of chlorite or dissolution of minor amounts of carbonates and plagioclase. The interbedded sandstone and shale core also developed increased porosity parallel to bedding through dissolution of carbonates and reactive silicates in the sandy layers. Tight interbedded cap-rocks could be expected to act as baffles to fluids preventing vertical fluid migration. Concentrations of dissolved elements including Ca, Fe, Mn, and Ni increased during reactions of several core samples, with Mn, Mg, Co, and Zn correlated with Ca from cap-rock cores. Precipitation of gypsum, Fe-oxides and clays on seal core samples sequestered dissolved elements including Fe through co-precipitation or adsorption. A conceptual model of impure CO2-water-rock interactions for a siliciclastic reservoir is discussed.Display Omitted
Keywords: CO2 geological storage; Sandstone reservoir; Cap-rock; SO2 impurities; O2 impurities; CO2-water-rock interactions; Micro CT; Precipice Sandstone;

Statistical analysis of geochemical compositions: Problems, perspectives and solutions by Peter Filzmoser; Karel Hron; Raimon Tolosana-Delgado (169-170).

Compositional multivariate statistical analysis of thermal groundwater provenance: A hydrogeochemical case study from Ireland by Sarah Blake; Tiernan Henry; John Murray; Rory Flood; Mark R. Muller; Alan G. Jones; Volker Rath (171-188).
Thermal groundwater is currently being exploited for district-scale heating in many locations world-wide. The chemical compositions of these thermal waters reflect the provenance and circulation patterns of the groundwater, which are controlled by recharge, rock type and geological structure. Exploring the provenance of these waters using multivariate statistical analysis (MSA) techniques increases our understanding of the hydrothermal circulation systems, and provides a reliable tool for assessing these resources.Hydrochemical data from thermal springs situated in the Carboniferous Dublin Basin in east-central Ireland were explored using MSA, including hierarchical cluster analysis (HCA) and principal component analysis (PCA), to investigate the source aquifers of the thermal groundwaters. To take into account the compositional nature of the hydrochemical data, compositional data analysis (CoDa) techniques were used to process the data prior to the MSA.The results of the MSA were examined alongside detailed time-lapse temperature measurements from several of the springs, and indicate the influence of three important hydrogeological processes on the hydrochemistry of the thermal waters: 1) salinity and increased water-rock interaction; 2) dissolution of carbonates; and 3) dissolution of sulfides, sulfates and oxides associated with mineral deposits. The use of MSA within the CoDa framework identified subtle temporal variations in the hydrochemistry of the thermal springs, which could not be identified with more traditional graphing methods, or with a standard statistical approach. The MSA was successful in distinguishing different geological settings and different annual behaviours within the group of springs. This study demonstrates the usefulness of the application of MSA within the CoDa framework in order to better understand the underlying controlling processes governing the hydrochemistry of a group of thermal springs in a low-enthalpy setting.
Keywords: Hydrochemistry; Compositional data analysis; Principal component analysis; Low-enthalpy geothermal; Thermal springs; Ireland;

The aim of this contribution is to explore the relationship among some concepts, often considered to be unrelated, such as weathering reactions, compositional data and fractals by means of distribution analysis.Weathering reactions represent the necessary transfer of heat and entropy to the environment in geochemical cycles. Compositional data express the relative abundance of chemical elements/species in a given total (i.e. volume or weight). Fractals are temporal or spatial objects with self-similarity and scale-invariance, so that internal structures repeat themselves over multiple levels of magnification or scales of measurement.Gibbs's free energy and the application of the Law Mass Action can be used to model weathering reactions, under the hypothesis of chemical equilibrium. Compositional data are obtained in the analytical phase after the determination of the concentrations of chemicals in sampled solid, liquid or gaseous materials. Fractals can be measured by using their fractal dimensions.In this paper, the presence of fractal structures is observed when the frequency distribution of isometric log-ratio coordinates is investigated, showing the logarithm of the cumulative number of samples exceeding a certain coordinate value plotted against the coordinate value itself. Isometric log-ratio coordinates (or balances) were constructed by using the sequential binary partition (SBP) method. The balances were identified to maintain, as far as possible, the similarity with a corresponding weathering reaction affecting the Arno river catchment (Tuscany, central Italy) as described by the Law of Mass Action. The emergence of fractal structures indicates the presence of dissipative systems, which require complexity, large numbers of inter-connected elements and stochasticity.
Keywords: Weathering reactions; Isometric coordinates; Distribution analysis; Fractals; Dissipative structures;

Interpretation of regional scale, multivariate geochemical data is aided by a statistical technique called “clustering.” We investigate a particular clustering procedure by applying it to geochemical data collected in the State of Colorado, United States of America. The clustering procedure partitions the field samples for the entire survey area into two clusters. The field samples in each cluster are partitioned again to create two subclusters, and so on. This manual procedure generates a hierarchy of clusters, and the different levels of the hierarchy show geochemical and geological processes occurring at different spatial scales. Although there are many different clustering methods, we use Bayesian finite mixture modeling with two probability distributions, which yields two clusters. The model parameters are estimated with Hamiltonian Monte Carlo sampling of the posterior probability density function, which usually has multiple modes. Each mode has its own set of model parameters; each set is checked to ensure that it is consistent both with the data and with independent geologic knowledge. The set of model parameters that is most consistent with the independent geologic knowledge is selected for detailed interpretation and partitioning of the field samples.
Keywords: Clustering; Geochemical data; Compositional data; Finite mixture model; Bayesian modeling; Regional geochemistry;

Element chemostratigraphy of the Devonian/Carboniferous boundary – A compositional approach by K. Fačevicová; O. Bábek; K. Hron; T. Kumpan (211-221).
The Devonian/Carboniferous (D/C) boundary is a critical interval in the Phanerozoic history, which is associated with vigorous climatic perturbations, continental glaciation, global sea-level fall and rapidly increased extinction rates in marine realms. In many sections world-wide, these global changes left a marked lithological signature, in particular the Hangenberg black shale (products of deep-shelf anoxia) and the overlying Hangenberg sandstone (sudden siliciclastic influx into predominantly carbonate depositional environments). Both layers bear a distinct geochemical signature. Even though either or both of these two lithologies are absent at many sections, their correlative counterparts can be indicated by subtle geochemical markers. We studied elemental geochemistry of fourteen D/C boundary sections in six key areas across Europe with the aim to select globally correlatable elemental proxy for the D/C boundary. Analysis of raw/log-transformed geochemical data (EDXRF, c.p.s. units), presenting the standard approach here, indicates that concentrations of terrigenous elements (Al, K, Rb, Ti and Zr) are mainly controlled by diluted Ca (carried by marine calcium carbonate) in limestone facies and, accordingly, their variations can be related to carbonate production in the sea rather than to terrigenous input from continent. Nevertheless, due to the relative nature of geochemical observations, reliance solely on statistical processing of raw data might lead to incomplete picture of multivariate data structure and/or biased interpretations. For this reason, the aim of this contribution is to discuss the logratio alternatives of the standard statistical methods, which may better reflect the relative nature of the data. For this purpose, principal component analysis was employed to reveal main geochemical patterns and while the geochemical signature of the D/C boundary was further analysed using Q-mode clustering that leads to predicative orthonormal logratio coordinates – balances. The comprehensive picture of the multivariate data structure provided by these statistical tools makes them a primary choice for exploratory compositional data analysis. At the same time, it turns out that the standard and compositional approaches have synergic effects. This fact can be extensively used in further geochemical studies.
Keywords: Element geochemistry; Compositional biplot; Dendrogram; Logratio coordinates; Devonian/Carboniferous boundary; Variation matrix;

Compositional data analysis of Holocene sediments from the West Bengal Sundarbans, India: Geochemical proxies for grain-size variability in a delta environment by R.P. Flood; M.R. Bloemsma; G.J. Weltje; I.D. Barr; S.M. O’Rourke; J.N. Turner; J.D. Orford (222-235).
This paper is part of a special issue of Applied Geochemistry focusing on reliable applications of compositional multivariate statistical methods. This study outlines the application of compositional data analysis (CoDa) to calibration of geochemical data and multivariate statistical modelling of geochemistry and grain-size data from a set of Holocene sedimentary cores from the Ganges-Brahmaputra (G-B) delta. Over the last two decades, understanding near-continuous records of sedimentary sequences has required the use of core-scanning X-ray fluorescence (XRF) spectrometry, for both terrestrial and marine sedimentary sequences. Initial XRF data are generally unusable in ‘raw-format’, requiring data processing in order to remove instrument bias, as well as informed sequence interpretation. The applicability of these conventional calibration equations to core-scanning XRF data are further limited by the constraints posed by unknown measurement geometry and specimen homogeneity, as well as matrix effects. Log-ratio based calibration schemes have been developed and applied to clastic sedimentary sequences focusing mainly on energy dispersive-XRF (ED-XRF) core-scanning. This study has applied high resolution core-scanning XRF to Holocene sedimentary sequences from the tidal-dominated Indian Sundarbans, (Ganges-Brahmaputra delta plain). The Log-Ratio Calibration Equation (LRCE) was applied to a sub-set of core-scan and conventional ED-XRF data to quantify elemental composition. This provides a robust calibration scheme using reduced major axis regression of log-ratio transformed geochemical data. Through partial least squares (PLS) modelling of geochemical and grain-size data, it is possible to derive robust proxy information for the Sundarbans depositional environment. The application of these techniques to Holocene sedimentary data offers an improved methodological framework for unravelling Holocene sedimentation patterns.
Keywords: Compositional data; Ganges-Brahmaputra delta; XRF; Calibration; Geochemistry; Grain-size; Partial least squares;

Investigating relations between environmental toxins in Northern Irish soils and streams and Chronic Kidney Disease prevalence by Chloe E. Jackson; Jennifer M. McKinley; Ulrich Ofterdinger; Damian Fogarty; Peter M. Atkinson; Sherry Palmer (236-246).
The unknown aetiology of Chronic Kidney Disease (CKD) has attracted recent attention as a result of the increasing global prevalence and recent reviews of occupational and environmental exposure to nephrotoxins. The main focus of this research is to examine the potential relationship between environmental exposure to known nephrotoxins including arsenic, cadmium and lead and the potential health risk associated with the progressive dysfunction of the kidneys in renal impaired patients with CKD across Northern Ireland. In addition to these known nephrotoxins, co-abundance with several essential elements has been found to play a role as protecting mechanisms while others increase the uptake of nephrotoxic elements as a result of similar absorption mechanisms within the body. Key elements protecting the body from toxicity include selenium and zinc, whereas those which have been attributed to enhance the uptake of arsenic, cadmium and lead include iron and calcium. The compositional nature of the soil and stream geochemical data is explored to aid in the analysis of interactions between elements. Two approaches, one data-driven and the other knowledge-driven, are explored to investigate the associations between co-abundant elements. The bioaccessibility of these elements, which is the portion of the relevant toxin absorbed within the body, is also investigated to identify areas across Northern Ireland with an increased environmental hazard and potential health risk. The study uses a combination of datasets from the United Kingdom Renal Registry (UKRR) unknown aetiology subset, the soil and stream geochemical dataset from the Tellus Survey (GSNI) with the addition of a bioaccessibility subset. Findings suggest a relationship between the presence of elevated arsenic in stream waters and impaired renal function of the kidneys. Interactions between essential elements and potentially toxic elements could explain the regional variation of CKD of uncertain aetiology across Northern Ireland.
Keywords: Environmental toxins; Arsenic; Cadmium; Geochemistry; Bioaccessibility; Kidney disease data;

A multivariate spatial analysis was conducted on a suite of glacial till geochemical data collected over the Melville Peninsula, Nunavut, Canada. Previous studies demonstrated through the application of multivariate statistical techniques that the composition of the lake sediment geochemistry reflects the underlying geology in northern Canada. In this study, the application of minimum/maximum autocorrelation factor analysis (MAF) to glacial till geochemistry has extended the knowledge and description of the underlying geology through the recognition of spatially correlated factors that represent distinct lithologic features and glacial transport processes.Maps of posterior probabilities for the underlying lithologies were estimated based on the MA factors and compared with those from standard principal component analysis. The use of MAF provides a measured improvement in predictive mapping irrespective of the choice of lag separation.

The complexity of modern geochemical data sets is increasing in several aspects (number of available samples, number of elements measured, number of matrices analysed, geological-environmental variability covered, etc), hence it is becoming increasingly necessary to apply statistical methods to elucidate their structure. This paper presents an exploratory analysis of one such complex data set, the Tellus geochemical soil survey of Northern Ireland (NI). This exploratory analysis is based on one of the most fundamental exploratory tools, principal component analysis (PCA) and its graphical representation as a biplot, albeit in several variations: the set of elements included (only major oxides vs. all observed elements), the prior transformation applied to the data (none, a standardization or a logratio transformation) and the way the covariance matrix between components is estimated (classical estimation vs. robust estimation). Results show that a log-ratio PCA (robust or classical) of all available elements is the most powerful exploratory setting, providing the following insights: the first two processes controlling the whole geochemical variation in NI soils are peat coverage and a contrast between “mafic” and “felsic” background lithologies; peat covered areas are detected as outliers by a robust analysis, and can be then filtered out if required for further modelling; and peat coverage intensity can be quantified with the %Br in the subcomposition (Br, Rb, Ni).
Keywords: Centered log-ratio transformation; clr; Spurious correlation; Compositional data analysis;

Identifying felsic intrusions is an essential task in support of mineral exploration because the intrusions can be a source of energy and metals for magmatic-hydrothermal mineralization. In this paper, two models for mapping felsic intrusions are compared based on regional geochemical and geophysical data. Geochemical data as a type of compositional data which carry relative information should be preprocessed using log-ratio transformation. The first model, a factor ratio (F2/F1) model, was developed based on the chemical characteristics of the felsic intrusions, which are rich in K2O and high field strength elements (F2), but poor in Fe2O3 and compatible elements (F1). The second model, a hybrid model that combines principal component analysis and local singularity analysis, was based on the chemical and physical properties of the felsic intrusions. The results showed that (1) raw geochemical data should be processed using log-ratio transformation prior to multivariate data analysis to avoid spurious correlations between variables, and (2) the hybrid model performed better than the ratio of factors model for inferring felsic intrusions in the study area. The felsic intrusions mapped in this study provide information that can support further mineral exploration in the Dong Ujimqin Fe–Cu polymetallic district, Inner Mongolia, northern China.
Keywords: Compositional data; Factor ratio; Felsic intrusions; Singularity; Geoinformatics;