Applied Geochemistry (v.20, #6)

Possibilities of reflectance spectroscopy for the assessment of contaminant elements in suburban soils by Yunzhao Wu; Jun Chen; Xinmin Wu; Qingjiu Tian; Junfeng Ji; Zhihao Qin (1051-1059).
Due to rapid economic development, high levels of potentially harmful elements are continuously being released into the suburban soils of the Nanjing area, China. Conventional methods for investigating soil potentially harmful element contamination based on raster sampling and chemical analysis are time consuming and relatively expensive. Reflectance spectroscopy within the visible-near-infrared region has been widely used to predict soil constituents due to its rapidity, convenience and accuracy. The objective of this study was to examine the possibility of using soil reflectance spectra as a rapid method to simultaneously assess contaminant metals (Ni, Cr, Cu, Hg, Pb, Zn) and As in the suburban soils of the Nanjing area. One hundred and twenty soil samples were collected for chemical analyses and spectral measurements. Prediction of contaminant elements was achieved by a partial least-square regression (PLSR) approach. According to their relationships with Fe, the seven contaminant elements could be categorized into two groups. The results showed that the prediction accuracy for Group II (Ni, Cr, Cu and Hg) was higher than that for Group I (Pb, Zn and As). This finding was consistent with the fact that the correlation coefficients between Group II and Fe were higher than that between Group I and Fe. It was concluded that element-sorption by spectrally active Fe oxides was the major mechanism by which to predict spectrally featureless contaminant elements. This conclusion was strengthened by the fact that the PLSR regression coefficients, which revealed the most important wavelengths for prediction, were attributed to absorption features of Fe oxides. Future study with real remote sensing data and field measurements are strongly recommended.

Volcanic degassing at Somma–Vesuvio (Italy) inferred by chemical and isotopic signatures of groundwater by S. Caliro; G. Chiodini; R. Avino; C. Cardellini; F. Frondini (1060-1076).
A geochemical model is proposed for water evolution at Somma–Vesuvio, based on the chemical and isotopic composition of groundwaters, submarine gas emission and chemical composition of the dissolved gases. The active degassing processes, present in the highest part of the volcano edifice, strongly influence the groundwater evolution. The geological–volcanological setting of the volcano forces the waters infiltrating at Somma–Vesuvio caldera, enriched in volcanic gases, to flow towards the southern sector to an area of high pCO2 groundwaters. Reaction path modelling applied to this conceptual model, involving gas–water–rock interaction, highlights an intense degassing process in the aquifer controlling the chemical and isotopic composition of dissolved gases, total dissolved inorganic C (TDIC) and submarine gas emission. Mapping of TDIC shows a unique area of high values situated SSE of Vesuvio volcano with an average TDIC value of 0.039 mol/L, i.e., one order of magnitude higher than groundwaters from other sectors of the volcano. On the basis of TDIC values, the amount of CO2 transported by Vesuvio groundwaters was estimated at about 150 t/d. This estimate does not take into account the fraction of gas loss by degassing, however, it represents a relevant part of the CO2 emitted in this quiescent period by the Vesuvio volcanic system, being of the same order of magnitude as the CO2 diffusely degassed from the crater area.

Carbon-14 concentrations between 0.83 and 11.79 pmC of formation water from the Activo Samaria–Sitio Grande petroleum reservoir in SE-Mexico, extracted from 3500 to 4500 m.b.s.l., indicate a common infiltration event of surface water during the late Pleistocene period. Mixing of two components – meteoric water and seawater, previously evaporated at the surface – explain the widespread mineralization (TDI = 15–257 g/L) of Na–Cl and Na–Ca–Cl type reservoir water. Statistical discrimination by clustering and a heterogeneous chemical–isotopic fluid composition indicate the existence of 4 different water types as part of local aquifer systems, which are separated by normal and thrust faults. Tectonic horst and graben structures show an ambiguous, individual hydraulic behaviour – as permeable conduits and/or as impermeable barriers, causing the local limitation of aquifer extent. The recent increase of water production in petroleum wells is not related to the injection of surface water, but the long-term extraction of oil reserves is modifying the original position and flow direction of the reservoir aquifers. The rise of the initial groundwater level reflects the final stage of an exhausted petroleum reservoir with coning effects of underlying aquifer systems. The flexible change towards superior production intervals could represent a feasible technique to avoid the abrupt closure of invaded production wells.

Fluorite (U–Th)/He thermochronology: Constraints on the low temperature history of Yucca Mountain, Nevada by N.J. Evans; N.S.F. Wilson; J.S. Cline; B.I.A. McInnes; J. Byrne (1099-1105).
Fluorite is one of the secondary minerals precipitated in pore spaces at the future nuclear waste repository site at Yucca Mountain, Nevada. The authors have conducted (U–Th)/He dating of this fluorite in an attempt to constrain the temperature and timing of paleo-fluid flux into the site. Repeated analysis of colourless fluorite yielded a weighted average age of 9.7 ± 0.15 Ma (2σ), younger than previously determined sanidine 40Ar/39 Ar ages (12.8 Ma) for deposition of the tuff.Laboratory He-diffusion experiments conducted on the Yucca fluorite yield a preliminary He closure temperature (T c) of 90 ± 10 °C (cooling rate of 10 °C/Ma) and previous studies have determined that the fluorite precipitated from warm fluids (65–80 °C) at depths of <400 m. However, minerals can experience partial He loss at temperatures well below the T c and therefore the (U–Th)/He age of 9.7 Ma is interpreted to be a cooling age. This result implies that the last period of elevated temperature fluid circulation through the Yucca site was approximately 9.7 Ma ago.It was observed that the purple coloured outer portion of the fluorite nodule yielded non-reproducible and invariably older ages than colourless fluorite. Several possible reasons are suggested.

Radon in soil gas: How to identify anomalies caused by earthquakes by B. Zmazek; M. Živčić; L. Todorovski; S. Džeroski; J. Vaupotič; I. Kobal (1106-1119).
Anomalies have been observed in Rn content in soil gas from 3 boreholes at the Orlica fault in the Krško basin, Slovenia. To distinguish the anomalies caused by environmental parameters (air and soil temperature, barometric pressure, rainfall) from those resulting solely from seismic activity, the following approaches have been used: (i) deviation of Rn concentration from the seasonal average, (ii) correlation between time gradients of Rn concentration and barometric pressure, and (iii) regression trees within a machine learning program. Approach (i) is much less successful in predicting anomalies caused by seismic events than approaches (ii) and (iii) if ±2σ criterion is used and is equally successful if ±1σ is used. Approaches (ii) and (iii) did not fail to observe an anomaly preceding an earthquake, but show false seismic anomalies, the number of which is much lower with (iii) than with (ii). Model trees are shown to outperform other approaches. A model has been built which, in the seismically non-active periods when Rn is presumably influenced only by environmental parameters, predicts the concentration with a correlation of 0.8. This correlation is reduced significantly in the seismically active periods.

This paper presents the results of a study on the geochemistry of Fe in a minerotrophic wetland in Central New York. Fine-scale geochemical trends in the peat and peat porewater were investigated to evaluate detailed vertical profiles of Fe in solution. Two sites within the wetland were examined: one site adjacent to an agricultural field receiving nutrient inputs and another (pristine) site in the middle of the wetland. Results revealed that Fe(II) was produced in situ in the wetland, most probably as a result of microbial Fe(III) reduction. Iron(II) concentration profiles suggested the existence of Fe(III) reduction in the zone adjacent to the agricultural field, whereas no significant evidence for Fe(III) reduction occurred in the pristine zone. Theoretical equilibrium modeling predicted that the difference in soluble Fe concentrations between the two zones was probably caused by abiotic reactions, such as pyrite precipitation. Geochemically, a correlation between Fe(II) and bulk density showed the importance of Fe reduction in the decomposition of organic matter in the organic rich peat and demonstrated the importance of the mineralogical composition for the reduction of Fe(III). Finally, a unique reversal of Fe and SO 4 2 - reduction was discovered in the zone adjacent to the agricultural field.

The Weyburn Oil Field is a carbonate reservoir in south central Saskatchewan, Canada and is the site of a large CO2 injection project for purposes of enhanced oil recovery. The Weyburn Field, in the Mississippian Midale Formation, was discovered in 1954 and was under primary production until secondary recovery by water flood began in 1964. The reservoir comprises two units, the Vuggy and the Marly, and primary and secondary recovery are thought to only have significantly depleted the Vuggy zone, leaving the Marly with higher oil saturations. In 2000, PanCanadian Resources (now EnCana), the operator of the field, began tertiary recovery by injection of CO2 and water, primarily into the Marly. The advent of this project was an opportunity to study the potential for geological storage of CO2.Using 43 Baseline samples collected in August 2000, before CO2 injection at Weyburn, and 44 monitoring samples collected in March 2001, changes in the fluid chemistry and isotope composition have been tracked. The initial fluid distribution showed water from discovery through water flood in the Midale Formation with Cl ranging from 25,000 to 60,000 mg/L, from the NW to the SE across the Phase 1A area. By the time of Baseline sampling the produced water had been diluted to Cl of 25,000–50,000 mg/L as a result of the addition of make up water from the low TDS Blairmore Formation, but the pattern of distribution was still present. The Cl distribution is mimicked by the distribution of other dissolved ions and variables, with Ca (1250–1500 mg/L) and NH3 (aq) increasing from NW to SE, and alkalinity (700–300 mg/L), resistivity, and H2S (300–100 mg/L) decreasing. Based on chemical and isotopic data, the H2S is interpreted to result from bacterial SO4 reduction. After 6 months of injection of CO2, the general patterns are changed very little, except that the pH has decreased by 0.5 units and alkalinity has increased, with values over 1400 mg/L in the NW, decreasing to 500 mg/L in the SE. Calcium has increased to range from 1250 to 1750 mg/L, but the pattern of NW–SE distribution is altered. Chemical and isotopic data suggest this change in distribution is caused by the dissolution of calcite due to water–rock reactions driven by CO2. The Baseline samples varied from −22 to −12‰ δ 13C (V-PDB) for CO2 gas. The injected CO2 has an isotope ratio of −20‰. The Monitor-1 samples of produced CO2 ranged from −18 to −13‰, requiring a heavy source of C, most easily attributed to dissolution of carbonate minerals. Field measured pH had increased and alkalinity had decreased by the second monitoring trip (July 2001) to near Baseline values, suggesting continued reaction with reservoir minerals.Addition of CO2 to water–rock mixtures comprising carbonate minerals causes dissolution of carbonates and production of alkalinity. Geochemical modeling suggests dissolution is taking place, however more detail on water–oil–gas ratios needs to be gathered to obtain more accurate estimates of pH at the formation level. Geological storage of CO2 relies on the potential that, over the longer term, silicate minerals will buffer the pH, causing any added CO2 to be precipitated as calcite. Some initial modeling of water–rock reactions suggests that silica sources are available to the water resident in the Midale Formation, and that clay minerals may well be capable of acting as pH buffers, allowing injected CO2 to be stored as carbonate minerals. Further work is underway to document the mineralogy of the Midale Formation and associated units so as to define more accurately the potential for geological storage.

Differences in properties and Cm(III) complexation behavior of isolated humic and fulvic acid derived from Opalinus clay and Callovo-Oxfordian argillite by Francis Claret; Thorsten Schäfer; Thomas Rabung; Manfred Wolf; Andreas Bauer; Gunnar Buckau (1158-1168).
Fulvic acids from deep clay formations have been isolated by the International Humic Substances Society (IHSS) standard protocol and analyzed. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy reveals, that the basic structural features relating to the origin of the clay organic matter (i.e., terrestrial or marine) are preserved even after the around 150 Ma since deposition (Jurassic sediment). Analysis by asymmetrical flow field flow fractionation (AFFFF) shows the size distribution peak found for typical fulvic acids. In addition, a second larger size peak is found for the fulvic acids influenced by marine deposition. These fulvic acids also have a considerable content of organic material which does not absorb in the visible range. The Cm(III) complexation behavior has been studied by time-resolved laser fluorescence spectroscopy (TRLFS). Despite considerable fluorescence quenching, the complexation constant is shown to be in the same range as published values found for different typical fulvic acids.

Element release from seriously contaminated sediments (especially with Cd, Zn and As) originating from two rivers in northern Belgium was investigated. Oxidation experiments that simulated the deposition of dredged sediments on land or the resuspension of riverbed sediments were performed in the laboratory. The oxidation of anoxic sediments during pHstat tests and the comparison of pHstat leaching in fresh (anoxic) and air-dried (oxic) sediments provided additional information on the changes in heavy metal and As mobility and speciation during oxidation. One of the sediments (GB(rs)) was characterised by a pH-decrease from pH 7.2 to pH 5.9 upon resuspension, while the pH of the other sediment (GL(rs)) did not decrease significantly. Although the release of Cd and Zn was influenced by pH, redox potential was also an important parameter in determining Zn- and Cd-release upon oxidation. In sediment (GB(rs)), Cd and Zn were only released after 48 h of resuspension, when the pH decreased to a value of 5.9. Even when the sediment/water suspension was titrated to pH 4 or 6 during oxidation, it took more than 12 h to have a substantial release of Cd. The apparent correlation in the release of Cd , Zn and SO 4 2 - indicates the association of Zn and Cd with sulphides. From an environmental point of view, especially Cd seems of concern because of the rather elevated concentrations that are released into the porewater and into the suspension upon oxidation. Contrary to other elements such as Zn and As, Cd is not removed from the suspension by coprecipitation and/or readsorption with precipitating Fe-(hydr)oxides within the time span of the observations.

Secondary Fe phases, which could attenuate Cu, Zn and Ni, formed in samples of unacidified filtered surface and ground water from two deposits of sulfide mine tailings at Thompson and Leaf Rapids, Manitoba. The precipitates were separated from the solutions by filtration, and analyzed by X-ray diffraction, scanning electron microscopy and electron microprobe. Selective dissolution with 0.2 M ammonium oxalate solution at pH 3 was used to separate phases of different crystallinity.An assemblage of goethite and schwertmannite precipitated from initially anoxic ground water with a pH of 5.7–6.2, whereas jarosite and/or schwertmannite were identified in acidic oxygenated (pH 2.7–3.2) surface water. This difference in mineralogy is attributed to the redox and acidity conditions. In the surface water, activities of SO 4 2 - and H+ control the proportions of minerals in the assemblage and the composition of schwertmannite.More Cu, Zn and Ni were precipitated from the ground water than from surface waters due to the higher initial content of Fe and higher pH of ground water. The concentrations of the metals were not significantly changed in the surface water because of limited sorption of cations at low pH.The affinity of the metals to the precipitate rather than the solution is in the order Cu > Zn > Ni. Sequential extraction indicated that Ni and Zn accumulate in poorly crystalline schwertmannite, while Cu concentrates in crystalline phases (goethite and jarosite). However, schwertmannite, as the principal phase in most of the precipitates, contains a significant proportion of Cu. During the recrystallization of metastable schwertmannite to goethite, the metals could be redistributed and released back into the mine water.

Heavy metal distribution and accumulation in two Spartina sp.-dominated macrotidal salt marshes from the Seine estuary (France) and the Medway estuary (UK) by A.B. Cundy; L. Hopkinson; R. Lafite; K. Spencer; J.A. Taylor; B. Ouddane; C.M. Heppell; P.J. Carey; R. Charman; D. Shell; S. Ullyott (1195-1208).
The upper intertidal zone, and salt marshes in particular, have been shown by numerous authors to be effective medium to long-term storage areas for a range of contaminants discharged or transported into the estuarine environment. A detailed understanding of the specific controls on the trapping and storage of contaminants, however, is absent for many estuarine systems. This paper examines heavy metal distribution and accumulation in two contrasting Spartina sp.-dominated macrotidal salt marsh systems – a rapidly prograding, relatively young marsh system at the Vasiere Nord, near the mouth of the Seine estuary, France, and a more mature, less extensive marsh system in the Medway estuary, UK. The spatial distribution of the heavy metals Zn, Cu, Pb, Ni and Co is assessed and compared in both systems via detailed surface sampling and analysis, while the longer-term accumulation of these metals and its temporal variability is compared via analysis of dated sediment cores. Of the two sites studied, the more extensive marsh system at the Vasiere Nord in the Seine estuary shows a clear differentiation of heavy metals across the marsh and fronting mudflat, with highest metal concentrations found in surface sediments from the more elevated, interior marsh areas. At Horrid Hill in the Medway estuary, the spatial distribution of heavy metals in surface sediments is more irregular, and there is no clear relationship between heavy metal concentration and site elevation, with average concentrations similar in the marsh and fronting mudflats. Sediment core data indicate that the more recent near-surface sediments at Horrid Hill are clearly more contaminated than those at greater depth, with most heavy metal contamination confined to the upper 20 cm of the sediment column (with peak metal input in the late 1960s/early 1970s). In contrast, due to extremely rapid sediment accretion at the mouth of the Seine, heavy metal distribution with depth at the Vasiere Nord site is relatively erratic, with metal concentrations showing a general increase with depth. These sediments provide little information on temporal trends in heavy metal loading to the Seine estuary. Overall, heavy metal concentrations at both sites are within typical ranges reported for other industrialised estuaries in NW Europe.

Mobilization of actinides by interaction with humic colloids in aquifers is essentially determined by the geochemical conditions. In this study, the pH dependence of the influence of humic acid on metal adsorption on a variety of geological solids (kaolinite, phyllite, diabase, granite, sand) was investigated for Tb(III) as an analogue of trivalent actinides, using 160Tb as a radiotracer. Humic material was radiolabelled with 131I to allow experiments at low DOC concentrations, as encountered in subsurface systems in the far-field of a nuclear waste repository. For all solids, a changeover from mobilization to demobilization is observed on acidification. Except for phyllite, the reversal occurs at slightly acidic pH values, and is thus relevant in respect of risk assessments. A composite distribution model was employed to reproduce the changeover on the basis of the underlying constituent processes. For this purpose, humate complexation of Tb(III) and adsorption of humic acid as a function of pH were investigated as well. Although the ternary systems cannot be constructed quantitatively by combining the binary subsystems, the relevant interdependences are adequately described by the composite approach. For a more general discussion in view of the diversity of natural organic colloids, adsorption isotherms of various humic and fulvic acids on sand were compared.

Arsenic(III) carbonate complexing by Carla S. Neuberger; George R. Helz (1218-1225).
A recent hypothesis that As(III) forms strong complexes with (bi)carbonate is tested by measuring the solubility of As2O3 in concentrated (up to 0.72 m) bicarbonate solutions at near-neutral pH and 25.0 ± 1.8 °C. A small, but statistically significant solubility enhancement is observed in NaHCO3-containing solutions compared to NaCl solutions of essentially the same ionic strength. The effect is well-explained by formation of a single complex: As ( OH ) 3 0 + HCO 3 - = As ( OH ) 2 CO 3 - + H 2 O , K c  = 0.22 ± 0.04 (corrected to infinite dilution). This complex’s small stability constant value is consistent with a published, quantum-based computational prediction. The measurements also yield a value for the solubility of arsenolite that agrees reasonably with recently reported values: 1 / 2 As 2 O 3 ( arsenolite ) + 3 / 2 H 2 O = As ( OH ) 3 0 , K s  = 0.178 ± 0.004 (also corrected to infinite dilution). The new data show that As(III) carbonate complexes will be negligible at carbonate concentrations found in most natural waters, but could be significant, though minor, in extremely carbonate-rich waters, such as found in interior drainage basins.

Arsenate and chromate incorporation in schwertmannite by Simona Regenspurg; Stefan Peiffer (1226-1239).
High concentrations of Cr (up to 812 ppm) and As (up to 6740 ppm) were detected in precipitates of the mineral schwertmannite in areas influenced by acid mine drainage. Schwertmannite may act as well as a natural filter for these elements in water as well as their source by releasing the previously bound elements during its dissolution or mineral-transformation. The mechanisms of uptake and potential release for the species arsenate and chromate were investigated by performing synthesis and stability experiments with schwertmannite.Schwertmannite, synthesized in solutions containing arsenate in addition to sulphate, was enriched by up to 10.3 wt% arsenate without detectable structural changes as demonstrated by powder X-ray diffraction (XRD). In contrast to arsenate, a total substitution of sulphate by chromate was possible in sulphate-free solutions. Thereby, the chromate content in schwertmannite could reach 15.3 wt%.To determine the release of oxyanions from schwertmannite over time, synthetic schwertmannite samples containing varying amounts of sulphate, chromate and arsenate were kept at a stable pH of either 2 or 4 over 1 year in suspension. At several time intervals Fe and the oxyanions were measured in solution and alterations of the solid part were observed by XRD and Fourier-Transform infrared (FT-IR) spectroscopy. At pH 2 schwertmannite partly dissolved and the total release of arsenate (24%) was low in contrast to chromate (35.4–57.5%) and sulphate (67–76%). Accordingly, the ionic activity product (log IAP) of arsenated schwertmannite was lowest (13.5), followed by the log IAP for chromated schwertmannite (16.2–18.5) and the log IAP for regular (=non-substituted) schwertmannite (18). At pH 4 schwertmannite transformed to goethite, an effect which occurred at the fastest rate for regular schwertmannite (=arsenate- and chromate-free), followed by chromate and arsenate containing schwertmannite. Both chromate and more evidently arsenate have a stabilizing effect on the schwertmannite structure, because they retarded the dissolution and transformation reactions.These kinetic investigations as well as crystallographic considerations demonstrated that the strength of the Fe(III) complexes with the anions controls the formation process and the stability of schwertmannite: with increasing affinity of the oxyanions to form complexes with Fe(III), the strength of the resulting binding and thus the stability and substitution preference increases.