Applied Geochemistry (v.21, #2)

Disposal of nuclear waste in deep geological formations is expected to induce thermal fluxes for hundreds of years with maximum temperature reaching about 100–150 °C in the nearfield argillaceous environment. The long-term behavior of clays subjected to such thermal gradients needs to be perfectly understood in safety assessment considerations. In this respect, a Toarcian argillaceous unit thermally disturbed by the intrusion of a 1.1-m wide basaltic dike at the Perthus pass (Herault, France), was studied in detail as a natural analogue. The thermal imprint induced by the dike was evaluated by a mineralogical, chemical and K–Ar study of the <2 μm clay fraction of shale samples collected at increasing distance from the basalt. The data suggest that the mineral composition of the shales was not significantly disturbed when the temperature was below 100–150 °C. Closer to the dike at 150–300 °C, changes such as progressive dissolution of chlorite and kaolinite, increased content of the mixed layers illite–smectite with more illite layers, complete decalcification and subsequent increased content of quartz, were found.At the eastern contact with the dike, the mineral and chemical compositions of both the shales and the basalt suggest water–rock interactions subsequent to the intrusion with precipitation of palagonite and renewed but discrete deposition of carbonate. A pencil cleavage developed in the shales during the dike emplacement probably favored water circulation along the contact. Strontium isotopic data suggest that the fluids of probable meteoric origin, reacted with Bathonian and Bajocian limestones before entering the underlying Toarcian shales.By analogy with deep geological radioactive waste repositories, the results report discrete mineralogical variations of the clays when subjected to temperatures of 100–150 °C that are expected in deep storage conditions. Beyond 150 °C, significant mineralogical changes may alter the physical and chemical properties of the shales, especially of the clay fraction. Also, the development of structural discontinuities in the so-called thermally disturbed zone might be of importance as these discontinuities might become zones for preferential fluid circulation. Finally, the study emphasizes the use of Rb–Sr and K–Ar isotopic systems as tracers of local circulating fluids related to low-grade thermal imprints.

Computer simulation of CO2 trapped through mineral precipitation in the Rose Run Sandstone, Ohio by Biniam Zerai; Beverly Z. Saylor; Gerald Matisoff (223-240).
Equilibrium, path-of-reaction and kinetic modeling of CO2–brine–mineral reactions in the Rose Run Sandstone, one of Ohio’s deep saline aquifers, was conducted in order to investigate the factors that are likely to influence the capacity of this formation to trap injected CO2 as solid carbonate mineral phases. Equilibrium modeling was applied to investigate the impact of temperature, pressure, mineralogy, brine composition, and CO2 fugacity on mineral dissolution and precipitation, the amount of CO2 sequestered, and the form of sequestration. Path of reaction and kinetic modeling were used to evaluate intermediate products and reaction progress as a function of time, as well as to investigate the impact of brine-to-rock ratio. The results of equilibrium modeling demonstrate that dissolution of albite, K-feldspar, and glauconite, and the precipitation of dawsonite and siderite are potentially very important for mineral trapping of CO2. According to the path of reaction and kinetic modelling, the stability of carbonate rocks is controlled by the brine-to-rock ratio, the pH of the system, the fugacity of CO2, and the kinetic rate of dissolution. In kinetic modeling, with a brine-to-rock ratio of 1:25, reactive surface area of 10 cm2/g, 10 MPa f CO 2 , and 12% porosity (1.5–2 g of CO2 per kg of reacted rock), significant quantities (10–40 g) of carbonate minerals were precipitated from both the sandstone and mixed rock assemblages. The Rose Run Sandstone has the potential to store CO2 over millennia as a negatively buoyant aqueous solution and, ultimately, as immobile carbonate minerals.

Medieval lead making on Mont-Lozère Massif (Cévennes-France): Tracing ore sources using Pb isotopes by Sandrine Baron; Jean Carignan; Sarah Laurent; Alain Ploquin (241-252).
This study aims to document the origin of metallurgical activities on the Mont-Lozère Massif (Cévennes Mountains, South of Massif Central, France), which is the largest medieval site of Pb–Ag metallurgical activities in France. These activities are characterised by more than 70 sites comprising numerous dispersed slags. Related charcoal samples dated by 14C have yielded a medieval, ca. XI–XII th. C. age. Numerous ore deposits, now mostly old mine tailings, surround the granitic massif and are possible candidates for suppliers of the old smelting activities. In Western Europe, Pb–Ag mines were of primordial importance for the medieval monetary system. Mines were intensively coveted by Lords, Bishops and Kings, and their exploitation was strictly regulated by each owner. The scarcity of ancient manuscripts makes it impossible to use a historical approach to determine the ore source regions related to a given metallurgical activity.The ore sources for this metallurgical activity were traced by comparing the Pb isotopic composition of slags and that of galena deposits surrounding the Mont-Lozère. The range of Pb isotopic compositions measured in slag samples is restricted and included in the field defined by galena from surrounding ores in Pb–Pb isotope diagrams. It is shown that, although direct tracing of the ore supplies is not straightforward, many ancient mine areas must be excluded, whereas some others seem highly likely. These old mines are not necessarily the closest to the smelting sites, suggesting that the choice of metal making location was constrained by several factors, including type of wood (here beech) and water proximity, but more importantly land ownership.

The Dalaman and Köyceğiz thermal springs are from karstic limestones belonging to Upper Cretaceous to Burdigalian Beydağları autochthon and Carboniferous to Lutetian Lycian nappes. They have measured temperatures of 24– 41 °C, specific electrical conductivities of 14,310–45,600 μS/cm, and are dominated by Na (1550–8500 mg/kg) and Cl (2725–15,320 mg/kg). The heat source of the geothermal systems of the area is tectonic related and the occurrence of the thermal springs is related to the young normal faults. Meteoric waters and seawaters recharge the reservoir rocks, are heated at depth with increasing geothermal gradient, and move up to the surface through the fractures and faults by convection trend and emerge as thermal springs. While thermal waters move up to the surface, they mix with different proportions of seawater and cold fresh waters. The seawater contribution to the thermal waters varies from 24% to 78%. Lake waters in the area are connected with thermal waters. Consequently, their chemical composition is influenced by the chemistry of thermal waters. Chemical equilibrium modelling based on measured outlet temperatures and measured pH shows that all the waters are oversaturated with respect to quartz and K-mica and undersaturated with respect to Al(OH)3, anorthite, gypsum, siderite and SiO2(a). Albite, alunite, aragonite, Ca-montmorillonite, calcite, chalcedony, chlorite, dolomite, Fe(OH)3(a), fluorite, gypsum, illite, K-feldspar, kaolinite and sepiolite minerals are mostly oversaturated or undersaturated. Mineral saturation studies of the thermal springs indicate that dolomite, chalcedony and quartz are most likely to cause scaling at outlet conditions. Assessments from various chemical geothermometers, and Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature is around 65–90 °C. The temperatures obtained from quartz, quartz-steam loss, Mg/Li geothermometers and mineral equilibrium diagrams give the most reasonable results.

Reliable quantification of mineral weathering rates is a key to assess many environmental problems. In this study, the authors address the applicability of pure mineral laboratory rate laws for dissolution of mill tailings samples. Mass-normalised sulfide and aluminosilicate mineral dissolution rates, determined in oxygenated batch experiments, were found to be different between two samples from the same ∼50-year-old, carbonate-depleted mill tailings deposit. Consideration of difference in particle surface area and mineralogy between the samples resolved most of this discrepancy in rates. While the mineral surface area normalised dissolution rates of pyrite in a freshly crushed pure pyrite specimen and a sulfide concentrate derived from the tailings were within the range of abiotic literature rates of oxidation by dissolved molecular O2, as were rates of sphalerite and chalcopyrite dissolution in the tailings, dissolution rates of pyrite and aluminosilicates in the tailings generally differed from literature values. This discrepancy, obtained using a consistent experimental method and scale, is suggested to be related to difficulties in quantifying individual mineral reactive surface area in a mixture of minerals of greatly varying particle size, possibly due to factors such as dependence of surface area-normalised mineral dissolution rates on particle size and time, or to non-proportionality between rates and BET surface area.

Mineral springs from Daylesford, Australia discharge at ambient temperatures, have high CO2 contents, and effervesce naturally. Mineral waters have high HCO3 and Na concentrations (up to 4110 and 750 mg/L, respectively) and CO2 concentrations of 620–2520 mg/L. Calcium and Mg concentrations are 61–250 and 44–215 mg/L, respectively, and Si, Sr, Ba, and Li are the most abundant minor and trace elements. The high P CO2 of these waters promotes mineral dissolution, while maintaining low pH values, and geochemical modelling indicates that the CO2-rich mineral water must have interacted with both sediments and basalts. Amorphous silica concentrations and silica geothermometry indicate that these waters are unlikely to have been heated above ambient temperatures and therefore reflect shallow circulation on the order of several hundreds of metres. Variations in minor and trace element composition from closely adjacent spring discharges indicate that groundwater flows within relatively isolated fracture networks. The chemical consistency of individual spring discharges over at least 20 a indicates that flow within these fracture networks has remained isolated over long periods. The mineral water resource is at risk from mixing with potentially contaminated surface water and shallow groundwater in the discharge areas. Increased δ 2H values and Cl concentrations, and lower Na concentrations indicate those springs that are most at risk from surface contamination and overpumping. Elevated NO3 concentrations in a few springs indicate that these springs have already been contaminated during discharge.

Arsenic concentration variability in public water system wells in Minnesota, USA by Melinda L. Erickson; Randal J. Barnes (305-317).
There is significant random and systematic variability in As concentrations in numerous public water system wells in Minnesota. Arsenic concentrations fluctuate above and below the USA’s As drinking water Maximum Contaminant Level (MCL) of 10 μg/L. The average As concentration is commonly within one standard deviation of the MCL. Results of intensive sampling conducted over the course of approximately 1 year at 3 public water system wells is consistent with the analysis of historic As measurements. In some cases, significant As concentration variability was noted over a short period of time. In these wells, the As concentration was less than 10 μg/L shortly after pumping started, but the As concentration increased over time to a level exceeding 10 μg/L. In these wells, the As concentration decreased to below 10 μg/L again when pumping was briefly restarted after being stopped for 4 h. The As concentration variability is likely due to As adsorption reactions between Fe oxides in the well borehole and in the aquifer near the well borehole during periods when the pump is not in operation. When it is crucial to accurately determine true average As concentrations – for example, at one of the many wells that fluctuate above and below the regulatory As limit of 10 μg/L – it is worthwhile collecting samples frequently during pumping to more accurately determine the average As concentration. Determining a reliable average depends on the standard deviation (SD) of the measurements, with more measurements required if the SD is larger.

To evaluate the extent of human impact on a pristine Antarctic environment, natural baseline levels of trace metals have been established in the basement rocks of the Larsemann Hills, East Antarctica.From a mineralogical and geochemical point of view the Larsemann Hills basement is relatively homogeneous, and contains high levels of Pb, Th and U. These may become soluble during the relatively mild Antarctic summer and be transported to lake waters by surface and subsurface melt water. Melt waters may also be locally enriched in V, Cr, Co, Ni, Zn and Sn derived from weathering of metabasite pods. With a few notable exceptions, the trace metal concentrations measured in the Larsemann Hills lake waters can be entirely accounted for by natural processes such as sea spray and surface melt water input. Thus, the amount of trace metals released by weathering of basement lithologies and dispersed into the Larsemann Hills environment, and presumably in similar Antarctic environments, is, in general, not negligible, and may locally be substantial.The Larsemann Hills sediments are coarse-grained and contain minute amounts of clay-size particles, although human activities have contributed to the generation of fine-grained material at the most impacted sites. Irrespective of their origin, these small amounts of fine-grained clastic sediments have a relatively small surface area and charge, and are not as effective metal sinks as the abundant, thick cyanobacterial algal mats that cover the lake floors. Thus, the concentration of trace metals in the Larsemann Hills lake waters is regulated by biological activity and thawing–freezing cycles, rather than by the type and amount of clastic sediment supply.

The leaching of major and trace elements from MSWI bottom ash as a function of pH and time by Joris J. Dijkstra; Hans A. van der Sloot; Rob N.J. Comans (335-351).
In this paper, the leaching behaviour of major components (Al, Ca, SO4, Mg, Si, Fe, Na and DOC) and trace elements (Ni, Zn, Cd, Cu, Pb, Mo and Sb) from MSWI bottom ash is studied as a function of time over a wide range of pH, under pH-controlled conditions. Equilibrium geochemical modelling using the modelling framework ORCHESTRA is used to enable a process-based interpretation of the results and to investigate whether ‘equilibrium’ is attained during the time scale of the experiments. Depending on the element and setpoint-pH value, net concentration increases or decreases of up to one order of magnitude were observed. Different concentration–time trends (increase or decrease) are observed in different pH ranges. The direction of the concentration–time trends depends on: (1) the shape of the ‘equilibrium’ solubility curve, and (2) the position of the setpoint-pH in the leaching test relative to the natural pH of the sample. Although the majority of the elements do not reach steady state, leached concentrations over a wide pH range have been shown to closely approach ‘equilibrium’ model curves within an equilibration time of 168 h. The different effects that leaching kinetics may have on the pH dependent leaching patterns have been identified for a wide range of elements, and can generally be explained in a mechanistic way. The results are in support of the currently prescribed equilibration time of 48 h in the European standard for the pH-static leaching test (TS14997). Finally, this study demonstrates that pH-static leaching experiments such as described in the European standards (TS14497 and TS14429), in combination with selective chemical extractions and a mechanistically based modelling approach, constitute a powerful set of tools for the characterization of leaching processes in waste materials over a wide range of conditions.

Chemical weathering rates for a mudstone obtained from a mining environment were investigated using a combination of batch reactors and hydrologically unsaturated column experiments. Results of tracer tests were combined with relationships between solute concentrations, mass fluxes, flow rates and residence times, and used to calculate element release rates and infer rate-controlling mechanisms for the two different experimental environments.The former is attributed to equilibrium-controlled release of particular elements (Si, Al), while the latter is ascribed to transport-controlled release of others (Mg, Mn, Ca, Na, K, S). Tracer tests using NaBr solutions revealed that some elements were also affected by ion exchange (Mg, Mn, Ca, Na), but these effects were temporary and did not mask underlying dissolution rate-controlling mechanisms. Analysis of characteristic diffusive lengths was used to distinguish between transport rates limited by the transfer of solutes between immobile and mobile water within the columns, and rates limited by slow diffusion across partially reacted mineral layers. The analysis suggests that transfer between immobile and mobile water limited the element release rates, with diffusion hindered by low diffusion coefficients within the unsaturated medium, and by low interfacial areas between mobile and immobile fluids. Results of the batch experiments showed different characteristics. Element concentrations either rose to a plateau or increased linearly with time. Rate-controlling mechanisms associated with these characteristics were equilibrium (Fe) and surface kinetic reactions (Si, Mg, Mn, Ca, K, S), respectively. Surface area-normalized element release rates for Mg, Mn, Ca and S are consistently a factor of 4 higher than those from the column experiments. This is a significant difference and cannot be attributed to differences in mineral preparation or to factors influencing individual minerals. It must, therefore, reflect the difference in the rate-controlling mechanism, that is, transport vs. surface-kinetic control.These results suggest that some proportion of the commonly recorded discrepancy between laboratory and field weathering rates is due to hydrological differences between the two environments, and that hydrological characterization of weathering environments is, therefore, as important as physical–geochemical characterization of reacting solids. An important practical implication of the work is that substantial reservoirs of solutes are held in immobile water in the unsaturated environment, and that these could be released by soil disturbance or flooding.

Predictions of mine-related water pollution are often based on laboratory assays of mine-site material. However, many of the factors that control the rate of element release from a site, such as pH, water–rock ratio, the presence of secondary minerals, particle size, and the relative roles of surface-kinetic and mineral equilibria processes can exhibit considerable variation between small-scale laboratory experiments and large-scale field sites.Monthly monitoring of mine effluent and analysis of natural geological material from four very different mine sites have been used to determine the factors that control the rate of element release and mineral sources and sinks for major elements and for the contaminant metals Zn, Pb, and Cu. The sites are: a coal spoil tip; a limestone-hosted Pb mine, abandoned for the last 200 a; a coal mine; and a slate-hosted Cu mine that was abandoned 150 a ago. Hydrogeological analysis of these sites has been performed to allow field fluxes of elements suitable for comparison with laboratory results to be calculated. Hydrogeological and mineral equilibrium control of element fluxes are common at the field sites, far more so than in laboratory studies. This is attributed to long residence times and low water–rock ratios at the field sites. The high water storativity at many mine sites, and the formation of soluble secondary minerals that can efficiently adsorb metals onto their surfaces provides a large potential source of pollution. This can be released rapidly if conditions change significantly, as in, for example, the case of flooding or disturbance.