Applied Geochemistry (v.17, #3)

Biomass combustion is an important primary source of particles with adsorbed biomarker compounds in the global atmosphere. The introduction of natural product organic compounds into smoke occurs primarily by direct volatilization/steam stripping and by thermal alteration based on combustion temperature. Although the molecular compositions of organic matter in smoke particles are highly variable, the molecular tracers are generally still source specific. Dehydroabietic acid is typically the major tracer for conifer smoke in the atmosphere. Degradation products from biopolymers (e.g. levoglucosan from cellulose, methoxyphenols from lignin) are also excellent tracers. Additional markers of thermally-altered and directly-emitted natural products in smoke have been defined which aids the assessment of the organic matter types and input from biomass combustion to aerosols. The precursor to product approach of compound characterization by organic geochemistry has also been applied successfully to provide source specific tracers for studying the chemistry and dispersion of ambient aerosols and the intermingling of natural with anthropogenic emissions and with smoke plumes. A brief review of the organic matter composition in aerosols derived from the major sources is also given, with emphasis on the detection of biomass burning components. These major sources are the natural background from biogenic detritus (e.g. plant wax, microbes, etc.) and anthropogenic particle emissions (e.g. oils, soot, synthetics, compounds, etc.). The emissions of organic constituents in coal smoke particulate matter are also reviewed and depend on combustion temperature, ventilation, burn time, and coal rank (geologic maturity). The components of peat and brown coal and to a lesser degree semi-bituminous coal consist mainly of hydrocarbons, biomarkers, and aromatic components, quite similar to burning of contemporary biomass. Dispersion from the source and long range transport of smoke particulate matter with the associated organic compounds is also discussed.

Geochemical constraints for the origin of thermal waters from western Turkey by Avner Vengosh; Cahit Helvacı; İsmail H. Karamanderesi (163-183).
The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO3 type with high CO2 content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na–SO4 type that is also associated with metamorphic rocks of the Menderes Massif with H2S addition; and (4) a Ca–Mg–HCO3–SO4 type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Çanakkale–Tuzla (Br/Cl<sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water–rock interactions; (2) intensification of reactions due to high dissolved CO2 and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ11B=2.3‰ to 18.7‰; n=6) can indicate either leaching of B from the rocks, or B(OH)3 degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. 87Sr/86Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (87Sr/86Sr ratio as high as 0.719479) and carbonate rocks (low 87Sr/86Sr ratio of 0.707864).

Factor analysis applied to regional geochemical data: problems and possibilities by Clemens Reimann; Peter Filzmoser; Robert G. Garrett (185-206).
A large regional geochemical data set of C-horizon podzol samples from a 188,000 km2 area in the European Arctic, analysed for more than 50 elements, was used to test the influence of different variants of factor analysis on the results extracted. Due to the nature of regional geochemical data (neither normal nor log-normal, strongly skewed, often multi-modal data distributions), the simplest methods of factor analysis with the least statistical assumptions perform best. As a result of this test it can generally be suggested to use principal factor analysis with an orthogonal rotation for such data. Selecting the number of factors to extract is difficult, however, the scree plot provides some useful help. For the test data, a low number of extracted factors gave the most informative results. Deleting or adding just 1 element in the input matrix can drastically change the results of factor analysis. Given that selection of elements is often rather based on availability of analytical packages (or detection limits) than on geochemical reasoning this is a disturbing result. Factor analysis revealed the most interesting data structures when a low number of variables were entered. A graphical presentation of the loadings and a simple, automated mapping technique allows extraction of the most interesting results of different factor analyses in one glance. Results presented here underline the importance of careful univariate data analysis prior to entering factor analysis. Outliers should be removed from the dataset and different populations present in the data should be treated separately. Factor analysis can be used to explore a large data set for hidden multivariate data structures.

Modelling the interaction of bentonite with hyperalkaline fluids by David Savage; David Noy; Morihiro Mihara (207-223).
Many designs for geological disposal facilities for radioactive and toxic wastes envisage the use of cement together with bentonite clay as engineered barriers. However, there are concerns that the mineralogical composition of the bentonite will not be stable under the hyperalkaline pore fluid conditions (pH > 12) typical of cement and its properties will degrade over long time periods. The possible extent of reaction between bentonite and cement pore fluids was simulated using the reaction-transport model, PRECIP. Key minerals in the bentonite (Na-montmorillonite, analcite, chalcedony, quartz, calcite) were allowed to dissolve and precipitate using kinetic (time-dependent) reaction mechanisms. Simulations were carried out with different model variants investigating the effects of: temperature (25 and 70 °C); cement pore fluid composition; dissolution mechanism of montmorillonite; rates of growth of product minerals; solubilities of product minerals; and aqueous speciation of Si at high pH. Simulations were run for a maximum of 3.2 ka. The results of all simulations showed complex fronts of mineral dissolution and growth, driven by the relative rates of these processes for different minerals. Calcium silicate hydrate (CSH) minerals formed closest to the cement-bentonite boundary, whereas zeolites and sheet silicates formed further away. Some growth of primary bentonite minerals (analcite, chalcedony, calcite and montmorillonite) was observed under certain conditions. Most alteration was associated with the fluid of highest pH, which showed total removal of primary bentonite minerals up to 60 cm from the contact with cement after ∼1 ka. The maximum porosity increase observed was up to 80–90% over a narrow zone 1–2 cm wide, close to the cement pore fluid- bentonite contact. All simulations (except that with alternative aqueous speciation data for Si) showed total filling of porosity a few cms beyond this interface with the cement, which occurred after a maximum of 3.2 ka. Porosity occlusion was principally a function of the growth of CSH minerals such as tobermorite. There was very little difference in the alteration attained using different model variants, suggesting that bentonite alteration was not sensitive to the changes in parameters under the conditions studied, so that transport of pore fluid through the bentonite governed the amount of alteration predicted. Principal remaining uncertainties associated with the modelling relate to assumptions concerning the evolution of surface areas of minerals with time, and the synergy between changing porosity and fluid flow/diffusion.

Altered wallrocks of vein-type Pb–Zn–Sb mineralization, Siegerland district, Rheinisches Schiefergebirge, have been investigated by a combination of inorganic and organic geochemical methods, including major and trace element analysis, vitrinite reflectance measurements, C isotope and elemental analysis of kerogen. Alteration features of the siliciclastic pelitic-psammitic Lower Devonian wallrocks are increased K/Na ratios, significant desilicification and relative immobility of a number of elements, notably Al, Ti, Zr, Cr, V. Wallrock kerogens display elevated vitrinite reflectance values, decrease in H/C atomic ratios coupled with increase in S/C atomic ratios and heavier C isotope compositions, compared to the unaltered precursor sedimentary rocks. Interaction processes between the hydrothermal fluids and the respective wallrocks, related to injection of high-temperature silica-undersaturated solutions, are dominated by quartz dissolution coupled with sericitization reactions. Heat transfer due to fluid infiltration/convection and wallrock reactions caused fluid cooling, which promoted the sequential deposition of quartz and stibnite/sulphosalts within the vein systems. Hydrocarbons, detected in ore assemblages of Pb–Zn and Sb mineralization, were most probably derived from the Lower Devonian very low-grade (meta)sedimentary rocks. High maturity levels and pronounced, typical organic alteration patterns indicate that thermochemical SO2− 4 reduction (TSR) played an important role in precipitation of metal sulphides. The present study demonstrates that a combination of inorganic and organic investigations on fluid–rock interaction processes is particularly useful for deciphering precipitation mechanisms of base metal sulphides.

Experimental soil acidification by Mária Dubiková; Philippe Cambier; Vladimı́r S̆ucha; Mária C̆aplovic̆ová (245-257).
A laboratory experiment was designed to investigate the interaction of acid mine drainage (AMD) with soils. The study used 3 undisturbed cambisol columns from the mining region of Banská S̆tiavnica, S̆obov (Slovakia). Columns were acidified with AMD emerging from the mine wastes rich in pyrite, with pure H2SO4 solutions of equivalent concentration (0.15 M H2SO4) or with distilled water. The experiment was performed for 30 weeks. The pH of H2SO4 leachates decreased from 4.1 to 3.3 and was accompanied by increasing amounts of dissolved Al. Progressive acidification increased leaching of major cations Ca, Mg, and K. Acidification of the soil with AMD produced leachates with a pH close to 3 and a high content of different cations and sulphates. Almost 100% of the Fe and 25% of the Al added with AMD were retained in the soil column. Leaching of alkali-earth and alkali cations from the soil by AMD was much lower than by H2SO4 solution. Analyses of solid samples from the 3 columns used selective extractions, XRD and analytical electron microscopy. These showed the different formations and transformations of minerals in acidified columns: amorphous Fe-oxyhydroxides were formed in both AMD and H2SO4 leached soils in the form of a characteristic “mohair” structure rich in Al and Si when AMD was applied. Clay minerals were hardly affected in these samples, in contrast with samples from the H2SO4 acidified column where dissolution of vermiculite, transformation of chlorite, and precipitation of Si gel were observed. Thick Fe-coatings on particles and within the pores of soils leached with AMD change their micro-aggregation and so inhibit the dissolution of phyllosilicates.

Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa, Argentina by P.L Smedley; H.B Nicolli; D.M.J Macdonald; A.J Barros; J.O Tullio (259-284).
Groundwaters from Quaternary loess aquifers in northern La Pampa Province of central Argentina have significant quality problems due to high concentrations of potentially harmful elements such as As, F, NO3-N, B, Mo, Se and U and high salinity. The extent of the problems is not well-defined, but is believed to cover large parts of the Argentine Chaco-Pampean Plain, over an area of perhaps 106 km2. Groundwaters from La Pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km. Dissolved As spans over 4 orders of magnitude (<4–5300 μg l−1) and concentrations of F have a range of 0.03–29 mg l−1, B of 0.5–14 mg l−l, V of 0.02–5.4 mg l−1, NO3–N of <0.2–140 mg l−1, Mo of 2.7–990 μg l−1 and U of 6.2–250 μg l−1. Of the groundwaters investigated, 95% exceed 10 μg As l−1 (the WHO guideline value) and 73% exceed 50 μg As l−1 (the Argentine national standard). In addition, 83% exceed the WHO guideline value for F (1.5 mg l−1), 99% for B (0.5 mg l−1), 47% for NO3-N (11.3 mg l−1), 39% for Mo (70 μg l−1), 32% for Se (10 μg l−1) and 100% for U (2 μg l−1). Total dissolved solids range between 730 and 11400 mg l−1, the high values resulting mainly from evaporation under ambient semi-arid climatic conditions. The groundwaters are universally oxidising with high dissolved-O2 concentrations. Groundwater pHs are neutral to alkaline (7.0–8.7). Arsenic is present in solution predominantly as As(V). Groundwater As correlates positively with pH, alkalinity (HCO3), F and V. Weaker correlations are also observed with B, Mo, U and Be. Desorption of these elements from metal oxides, especially Fe and Mn oxides under the high-pH conditions is considered an important control on their mobilisation. Mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility. Weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved As and other trace elements. Concentrations of As and other anions and oxyanions appear to be particularly high in groundwaters close to low-lying depressions which act as localised groundwater-discharge zones. Concentrations up to 7500 μg l−1 were found in saturated-zone porewaters extracted from a cored borehole adjacent to one such depression. Concentrations are also relatively high where groundwater is abstracted from close to the water table, presumably because this zone is a location of more active weathering reactions. The development of groundwaters with high pH and alkalinity results from silicate and carbonate reactions, facilitated by the arid climatic conditions. These factors, together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of As and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal.

Evidence of hydrological control of Sr behavior in stream water (Strengbach catchment, Vosges mountains, France) by Dominique Aubert; Anne Probst; Peter Stille; Daniel Viville (285-300).
Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.

Rates of sulfate reduction and metal sulfide precipitation in a permeable reactive barrier by S.G Benner; D.W Blowes; C.J Ptacek; K.U Mayer (301-320).
A full-scale reactive barrier, utilizing bacterially mediated SO4 reduction to promote metal sulfide precipitation and alkalinity generation, was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. Monitoring of groundwater chemistry over a 3-a period allows assessment of long-term reactive barrier performance. The overall rate of SO4 removal within the barrier declined with time by 30% from an initial rate of 58 to 40 mmol l−1 a−1 38 months after installation. Over the same time, the rate of Fe removal declined by 50% from 38 to 18 mmol l−1 a−1. The degree of SO4 reduction and Fe sulfide precipitation within the barrier is both spatially and temporally variable. Spatial differences are primarily the result of different residence times due to hydraulic conductivity variations of the treatment material. Temporal variations are likely the result of a decline in organic C availability and reactivity over time and seasonal variations in the rate of SO4 reduction. Temperatures in the aquifer fluctuate from a low of 2 °C in the winter to a high of 16 °C in the summer and the rate of SO4 reduction in the summer is nearly twice as great as the winter rate. An effective activation energy (E a ) of 40 kJ mol−1 can account for the temperature-induced changes. In Year 3, the barrier removed >1000 mg/l SO4 and >250 mg/l Fe, demonstrating the long-term viability of this remedial approach.

Mineralogical, petrographical, and geochemical studies of the weathering profile have been carried out at Omai Au mine, Guyana. The area is underlain by felsic to mafic volcanic and sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleoproterozoic greenstone belts of the Guiana Shield. Tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith. The top of the weathering profile consists of lateritic gravel or is masked by the Pleistocene continental-deltaic Berbice Formation. Mineralogical composition of regolith consists mainly of kaolinite, goethite and quartz, and subordinately sericite, feldspar, hematite, pyrite, smectite, heavy minerals, and uncommon mineral phases (nacrite, ephesite, corrensite, guyanaite). A specific feature of the weathering profile at Omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon. Chemical changes during the weathering processes depend on various physicochemical and structural parameters. Consequently, the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile, but rather an index that measures the degree of supergene alteration that has affected each analyzed sample, independently of the depth of sampling. Thus, the mineralogical index of alteration (MIA) can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock. It can also aid in establishing a quantitative relationship between intensity of weathering and mobility (leaching or accumulation) of each element in each analyzed sample. At Omai, some major and trace elements that are commonly considered as immobile (ex: TiO2, Zr, etc.) during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance. In addition, due to higher “immobile element” ratios, the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of ∼20 units is necessary for correct classification. In contrast, these elements could aid in defining the material source in sedimentary rocks affected by weathering. Generally, the rare-earth element (REE) patterns of the bedrock are preserved in the saprolith horizon. This can represent a potentially useful tool for geochemical exploration in tropical terrains. Strong negative Ce and Tb anomalies are displayed by weathered pillowed andesites, which are explained by the influence of the water/rock ratio.