Applied Geochemistry (v.19, #1)

A general model is presented for geochemical processes occurring in the unsaturated zone of a carbonate-depleted, pyritic tailings deposit. Quantification of slow geochemical reactions, using published, empirical rate laws from small-scale experiments on monomineralic samples, and geochemical equilibrium reactions successfully reproduced the relative rates of field processes in the case study, Impoundment 1 in Kristineberg. Reproduction of absolute rates was achieved by scaling down all laboratory-derived mineral weathering rates by two orders of magnitude. The sensitivity of the modelled groundwater composition and pH to rates of pH-buffering processes and redox reactions indicated that inclusion and accurate quantification of all dominant geochemical processes on the field scale is necessary for reliable prediction of groundwater composition and pH.

Groundwater is the most important source of water supply in Datong city. However, the levels of shallow Quaternary groundwaters from urbanized areas have been declining continuously and groundwater quality deteriorating in recent years. Understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Datong. Mineral hydrolysis of alumino-silicate minerals such as plagioclase and clinopyroxene, is the primary process controlling the concentration of H4SiO4 in the study area. Speciation calculations using the geochemical modeling code PHREEQC indicated that hydrolysis of bedrock, mainly composed of basalt and metamorphic rocks, is the major hydrogeochemical process controlling groundwater chemistry. The study area can be divided into 3 hydrogeochemical zones: A. Recharge (unimpacted) zone, B. Intermediate (industry-impacted) zone, and C. Discharge (agriculture-impacted) zone. Ion exchange and industrial and/or agricultural contamination contribute to the increase of Na+ from Zone A to Zone C, where the concentration of NO3 - is up to 461.5 mg/l with a mean value of 101.5 mg/l, indicating that agricultural practice seriously affects groundwater. Sulfate concentration in groundwaters in an alluvial fan at Datong is extremely high, up to 1172.9 mg/l, and shows a close relationship with the concentrations of trace elements, especially Ni and Co, indicating that coal mining is the main contamination source for groundwater from the alluvial fan, in addition to agricultural activities.

Modeling sorption of divalent metal cations on hydrous manganese oxide using the diffuse double layer model by Jennifer W. Tonkin; Laurie S. Balistrieri; James W. Murray (29-53).
Manganese oxides are important scavengers of trace metals and other contaminants in the environment. The inclusion of Mn oxides in predictive models, however, has been difficult due to the lack of a comprehensive set of sorption reactions consistent with a given surface complexation model (SCM), and the discrepancies between published sorption data and predictions using the available models. The authors have compiled a set of surface complexation reactions for synthetic hydrous Mn oxide (HMO) using a two surface site model and the diffuse double layer SCM which complements databases developed for hydrous Fe (III) oxide, goethite and crystalline Al oxide. This compilation encompasses a range of data observed in the literature for the complex HMO surface and provides an error envelope for predictions not well defined by fitting parameters for single or limited data sets. Data describing surface characteristics and cation sorption were compiled from the literature for the synthetic HMO phases birnessite, vernadite and δ-MnO2. A specific surface area of 746 m2g−1 and a surface site density of 2.1 mmol g−1 were determined from crystallographic data and considered fixed parameters in the model. Potentiometric titration data sets were adjusted to a pHIEP value of 2.2. Two site types (≡XOH and ≡YOH) were used. The fraction of total sites attributed to ≡XOH (α) and pK a2 were optimized for each of 7 published potentiometric titration data sets using the computer program FITEQL3.2. pK a2 values of 2.35±0.077 (≡XOH) and 6.06±0.040 (≡YOH) were determined at the 95% confidence level. The calculated average α value was 0.64, with high and low values ranging from 1.0 to 0.24, respectively. pK a2 and α values and published cation sorption data were used subsequently to determine equilibrium surface complexation constants for Ba2+, Ca2+, Cd2+, Co2+, Cu2+, Mg2+, Mn2+, Ni2+, Pb2+, Sr2+ and Zn2+. In addition, average model parameters were used to predict additional sorption data for which complementary titration data were not available. The two-site model accounts for variability in the titration data and most metal sorption data are fit well using the pK a2 and α values reported above. A linear free energy relationship (LFER) appears to exist for some of the metals; however, redox and cation exchange reactions may limit the prediction of surface complexation constants for additional metals using the LFER.

Bulk and molecular stable C isotopic compositions and biomarker distributions provide evidence for a diverse community of algal and bacterial organisms in the sedimentary organic matter of a carbonate section throughout the Permian–Triassic (P/Tr) transition at the Idrijca Valley, Western Slovenia. The input of algae and bacteria in all the Upper Permian and Lower Scythian samples is represented by the predominance of C15–C22 n-alkanes, odd C-number alkylcyclohexanes, C27 steranes and substantial contents of C21–C30 acyclic isoprenoids. The occurrence of odd long-chain n-alkanes (C22–C30) and C29 steranes in all the samples indicate a contribution of continental material. The decrease of Corg and Ccarb contents, increase of Rock-Eval oxygen indices, and 13C-enrichment of the kerogen suggest a decrease in anoxia of the uppermost Permian bottom water. The predominance of odd C-number alkylcycloalkanes, C27 steranes, and C17 n-alkanes with δ 13C values ∼−30‰, and 13C-enrichment of the kerogens in the lowermost Scythian samples are evidence of greater algal productivity. This increased productivity was probably sustained by a high nutrient availability and changes of dissolved CO2 speciation associated to the earliest Triassic transgression. A decrease of Corg content in the uppermost Scythian samples, associated to a 13C-depletetion in the carbonates (up to 4‰) and individual n-alkanes (up to 3.4‰) compared to the Upper Permian samples, indicate lowering of the primary productivity (algae, cyanobacteria) and/or higher degradation of the organic matter.

CO2 soil flux at Vulcano (Italy): comparison between active and passive methods by Maria Luisa Carapezza; Domenico Granieri (73-88).
Carbon dioxide soil flux has been used for many years to monitor Italian active volcanoes and both the active Dynamic Concentration (DCM) and the passive Accumulation Chamber (ACM) methods are employed. These two methods have been compared by means of 218 simultaneous flux measurements carried out in the La Fossa area of Vulcano Island, where a large variation of CO2 soil release occurs. Results indicate that DCM overestimates CO2 flux and is proportional to it only in high flux zones (flux higher than 100 gm−2 day−1). Using ACM fluxes and the Stefan–Maxwell equation, the measured CO2/depth curves in the soil could be reproduced. In high flux points CO2 is transported mostly by viscous flow up to a very shallow depth and then by diffusive flow, which is the dominant gas transport mechanism in low flux points. Carbon dioxide soil flux values are controlled by proximity to active gas releasing fractures, by changes in the barometric pressure and by variations in soil permeability.

Factors affecting trace element concentrations in soils developed on recent marine deposits from northern France by Thibault Sterckeman; Francis Douay; Denis Baize; Hervé Fourrier; Nicolas Proix; Christian Schvartz (89-103).
Total concentrations of 18 trace and 2 major elements (Al, Fe) as well as physico-chemical characteristics (texture, organic C, pH, CaCO3, CEC) were measured in cultivated soil samples (fraction <2 mm) from 23 surface and 38 deeper horizons developed in recent marine deposits. Correlations between trace element contents and other soil parameters were compared in order to reveal those soil factors affecting the trace element distributions. Whatever the horizon type, Co, Cr, In, Ni and V are almost exclusively associated with the fine (<20 μm) mineral fraction and do not show a direct association with organic matter. Bismuth, Sn, Tl and Zn also show a close association with the fine mineral fraction of the deep horizons, but seem to be partly associated with organic matter in the ploughed horizons. In the deep horizons, the association of Cd, Cu and Pb with the fine mineral fraction is clearly less close, while these elements appear to be associated with organic matter in the ploughed horizons. The Mn content increases with that of the fine mineral fraction up to a certain point. Arsenic, Mo and Sb contents are poorly correlated with the fine solid phase of the deep horizons. The distribution of Hg does not appear to be associated with any specific soil component. The ploughed horizons are highly enriched with Pb, Cu, Cd, Hg and Se (+84% to +225%) and moderately enriched with Tl, Mn, Sb, Bi, Sn and Zn (+7% to +48%). There is no surface enrichment of As, Co, Cr, In, Mo, Ni and V. It is possible to model most of the trace element distributions with pedotransfer functions of the physico-chemical characteristics.

Hydrogeochemical and hydrogeological investigations of thermal waters in the Emet area (Kütahya, Turkey) by Ünsal Gemici; Gültekin Tarcan; Mümtaz Çolak; Cahit Helvacı (105-117).
Metamorphic rocks host the majority of the thermal waters of the Emet area. Only Dereli springs are hosted by non-metamorphic carbonates and ophiolitic rocks. The carbonated rocks of the lower parts of the Neogene sequence are also secondary reservoir rocks. The measured surface temperatures of thermal waters are between 33 and 54 °C. Most of the thermal waters are characterized as Ca–Mg–SO4–HCO3 type although there are a few Ca–Na–HCO3, Na–Ca–SO4 and Ca–Mg–HCO3 waters. Calcium concentrations in the thermal waters are 89–354 mg/kg. High SO4 contents of the thermal waters (up to 1309 mg/kg) are related to rocks and minerals in the Red Unit below the Emet borate deposits. Although the SO4 concentrations are high and SO4 is the major anion, gypsum and anhydrite are undersaturated for all of the thermal waters indicating that dissolution of SO4 is still taking place in the reservoir. Thermal waters are oversaturated at outlet conditions with respect to calcite, chalcedony, dolomite and quartz. According to the activity diagrams thermal waters are likely to form illite as an alteration product in the reservoir and Ca and Mg contents are controlled by exchange with smectite. Reservoir temperatures obtained by silica geothermometers and assessments of the saturation states of minerals are more appropriate for Emet geothermal waters. Assessments of the various geothermometers suggest that reservoir temperature is around 75–87 °C.

Colloid-facilitated transport of contaminants could enhance the release rate of radionuclides from the cementitious near field of a repository for radioactive waste. In the current design of the planned Swiss repository for intermediate-level radioactive waste, a gas-permeable mortar is employed as backfill material for the engineered barrier. The main components of the material are hardened cement paste (HCP) and quartz aggregates. The chemical condition in the backfill mortar is controlled by the highly alkaline cement pore water present in the large pore space. The interaction of pore water with the quartz aggregates is expected to be the main source for colloids. Colloid transport is facilitated due to the high porosity of the backfill mortar. Batch-type studies have been performed to generate colloidal material in systems containing crushed backfill mortar or quartz in contact with artificial cement pore water (ACW) at pH 13.3. The chemical composition of the colloidal material corresponds to that of calcium silicate hydrates (CSH). Batch flocculation tests show that, after about 20 days reaction time, the concentration of the CSH-type colloids is typically below 0.1 mg l−1 due to reduced colloid stability in ACW. Uptake studies with Cs(I), Sr(II) and Th(IV) on a CSH phase (initial C:S ratio=1.09) have been carried out to assess the sorption properties of the colloidal material. The influence of uptake by colloids on radionuclide mobilisation is expressed in terms of sorption reduction on the immobile phase (HCP). Sorption reduction factors can be estimated on the assumption that the sorption properties of the colloidal material are either similar to those of the CSH phase or HCP, and that sorption is linear and reversible. A scaling factor accounts for the higher specific surface area of the colloidal material compared to the CSH phase and HCP. At colloid concentration levels typically encountered in highly alkaline cement pore waters, colloid-induced sorption reduction is predicted to be negligibly small even for strongly sorbing radionuclides, such as Th(IV). Thus, no significant impact of cementitious colloids on radionuclide mobilisation in the porous backfill mortar is anticipated.

Base-metals and organic content in stream sediments in the vicinity of a landfill by M.A. Gonçalves; J.M.F. Nogueira; J. Figueiras; C.V. Putnis; C. Almeida (137-151).
Landfill facilities are essential to modern societies as repositories for municipal solid wastes (MSW). However, irrespective of the regulations necessary to their construction there is always a certain risk of environmental impact. For this reason monitoring should be essential to their operation. In the present work, the environmental impact of such a landfill is studied, whose operation has been running for only 3 a. The approach used was to sample a series of stream sediments along two contaminated run-off water paths. These sediments were chemically analysed, and their mineralogy was also studied. Their most important feature is the existence of some contamination in Cu, Zn, and Pb above background levels in the first 200 m in the direction of water flow, whose concentration gradually decreases until background levels are reached. Although some artificial alloys and sulphides (sphalerite) were shown to be present, they could not account for the analysed concentrations, and cannot explain the negative gradient concentration by themselves. Sequential metal extraction methods together with solid phase extraction for the isolation of organic content, showed that metals adsorbed on the sediments are mostly associated with organic compounds. Although monocarboxylic organic acids were among the most important organic compounds present both in the contaminated waters and sediments, there is no clear correlation between these and base metal concentrations in the sediments. A stronger correlation exists if the total concentration of organic compounds is considered, which suggests that base metals can be easily partitioned into the sediments by the rapid adsorption onto their surfaces by organic compounds near the pollution source. The organic content identified decreases down stream as do base metal concentrations. This can be explained by the formation of complexes with organic acids as is extensively reported in natural and laboratory systems. The overall results point to the limited capacity for pollutant retention in this system, and suggest that in case of serious failure the contaminated plumes are likely to disperse rapidly into the environment.

In aquatic systems, the bioavailability of an element to microorganisms is greatly influenced by its chemical speciation. The goal of this work was to assess metal toxicity to a green algae (Pseudokirchneriella subcapitata) and a bacterium (Vibrio fisheri) as a function of size fractionation and chemical speciation (using the program MINTEQA2) in contaminated water of the Upper Vistula River. Water samples were collected at 1 reference site, 4 polluted sites and one polluted site on the Vistula's main tributary, the Przemsza River. Toxicity measurements were performed on unfiltered samples and, total dissolved (<1.2 μm), and truly dissolved (<1 kDa) fractions. Trace metal (Cd, Co, Cr, Cu, Mn, Pb, Zn) concentrations were measured in these samples and also in the colloidal fraction (1 kDa–1.2 μm). At the reference site, the low metal concentrations were in agreement with the absence of measurable toxicity. In the polluted section of the river, free metal concentrations were largely below the potential toxic levels for bacteria, which was in agreement with the absence of toxicity. Although Zn2+ was at potentially toxic-level concentrations in total dissolved and truly dissolved fractions in the polluted riverine section, toxicity for algae was observed, only in truly dissolved fractions from two stations. The absence of toxicity in most samples was related to metal association with particles and with low molecular weight ligands as well as the presence of organic ligands (phenol). The reason for toxic effects in two ultrafiltered samples is not clear, but may be related to the elimination of the colloidal organic fraction and thus the eradication of its protective effect occurring in natural samples.