Applied Geochemistry (v.24, #7)

Arsenic distribution in the environment: The effects of scale by Clemens Reimann; Jörg Matschullat; Manfred Birke; Reijo Salminen (1147-1167).
The distribution of As in a variety of sample materials was studied at different scales, from continental to local, combining published data sets with the aim of delineating the impact and relative importance of geogenic vs. anthropogenic As sources. Geochemical mapping of As with a variety of sample materials demonstrates that variation is high at all scales (local to continental) – from sample densities of 400 sites per km2 to 1 site per 5000 km2. Different processes govern the As distribution at different scales. A high sample density is needed to reliably detect mineralisation or contamination in soil samples. In both cases the impact on the much larger geochemical background variation is limited to a local scale. Distribution patterns in geochemical maps on the sub-continental to continental scale are dominated by natural variation. Given that the geochemical background is characterised by a high variation at all scales, it appears impossible to establish a reliable single value for “good soil quality” or a “natural background concentration” for As for any sizeable area, e.g., for Europe. For such a differentiation, geochemical maps at a variety of scales are needed.Different sample materials can pick up dissimilar geochemical sources and processes, even when collected from the same survey area. Weathering (soil formation) leads to an As increase in soils when compared to rocks. Soils and stream sediments return very comparable median As concentrations. Plants are usually well protected against As uptake. There are, however, plant species that can accumulate unusually high As concentrations. The marine environment is generally enriched in As. Several geochemical mapping projects deliver indications for crustal As degassing as an important process leading to As enrichment in the surface environment.
Keywords: Geochemical mapping; Exploration; Contamination; Background; Soil; Sediment; Biogeochemistry;

Size distributions of hydrocarbons in suspended particles from the Yellow River by Longjun Zhang; Jiao Zhang; Minna Gong (1168-1174).
Suspended particle samples from the Yellow River estuary were sorted into five grain size fractions to explore the effect of grain size distribution on organic matter content and composition. The n-alkanes and PAHs were determined for each size fraction. PAHs and n-alkanes were more abundant in the finer fractions and the loading decreases steadily with increasing of grain size. However, the total n-alkanes or PAHs normalized to organic C were lower in the smaller size fractions than those in the larger size fractions, suggesting n-alkanes or PAHs may be diluted by the addition of organic matter or gradually decreased by degradation in the smaller size fractions. The particulate n-alkanes in the Yellow River estuary consist of a mixture of compounds from terrigenous and riverine biogenic n-alkanes and more biogenic n-alkanes accumulate in finer particles. Particulate PAHs are related to combustion/pyrolysis processes of coal/wood, and the relative contribution of petrogenic PAHs increase with increasing grain size. The total particulate n-alkane and PAH discharges passing the Lijin Station are about 3.94 t d−1 and 0.52 t d−1, respectively. Fine particles (<32 μm) play a significant role in organic matter transfer.

Passive treatment systems for mine drainage use no energy other than gravity, but they require greater area than active treatment systems. Researchers are considering “hybrid” systems that have passive and active components for increased efficiency, especially where space limitations render passive-only technology ineffective. Flow-through reactor field experiments were conducted at two large net-alkaline anthracite mine discharges in central Pennsylvania. Assuming an Fe removal rate of 20 g m−2  day−1 and Fe loading from field data, 3.6 × 103 and 3.0 × 104  m2 oxidation ponds would be required for the passive treatment of Site 21 and Packer 5 discharges, respectively. However, only a small area is available at each site. This paper demonstrates aeration to drive off CO2, increase pH, and increase Fe(II) oxidation rates, enabling treatment within a small area compared to passive treatment methods, and introduces a geochemical model to accurately predict these rates as well as semi-passive treatment system sizing parameters. Both net-alkaline discharges were suboxic with a pH of ≈5.7, Fe(II) concentration of ≈16 mg L−1, and low Mn and Al concentrations. Flow rates were ≈4000 L min−1 at Site 21 and 15,000 L min−1 at Packer 5. Three-h aeration experiments with flow rates scaled to a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases from 12 to 22 °C, dissolved O2 increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases from 16 to <0.05 mg L−1. A 17,000-L pilot-scale reactor at Site 21 produced similar results although aeration was not as complete as in the smaller reactor. Two non-aerated experiments at Site 21 with 13 and 25-h run times resulted in pH changes of ⩽0.2 and Fe(II) concentration decreases of less than 3 mg L−1.An Fe(II) oxidation model written in a differential equation solver matched the field experiments very well using field-measured pH, temperature, dissolved O2, and initial Fe(II) concentration. The maximum oxidation rate was 1.3 × 10−4  mol L−1  s−1. The model was modified to predict alkalinity, P CO2, dissolved O2, and pH changes based on initial conditions and aeration rate. This more complex model also fits the data well, is more predictive than the first model, and should serve as a tool for predicting pond size needed for aerated Fe(II) oxidation at the field scale without the need for field pilot studies. Iron(II) oxidation modeling of actively aerated systems predicted that a 1-m deep pond with 10 times less area than estimated for passive treatment would lower Fe(II) concentrations to less than 1 mg L−1 at summer and winter temperatures for both sites. The use of active aeration for treatment of CO2-rich, net-alkaline discharges (including partially treated effluent from anoxic limestone drains) can result in considerably reduced treatment area for oxidation and may lower treatment costs, but settling of Fe hydroxides was not considered in this study. The reduced capital cost for earthmoving will need to be compared to energy and maintenance costs for aeration.

Controls on chemistry and diagenesis of naturally occurring iron-oxide phases by Nancy W. Hinman; J. Michelle Kotler; Beizhan Yan; Aaron Tenesch; Richard V. Morris; Andrew Tveter; Daphne L. Stoner; Jill R. Scott (1185-1197).
The purposes of this study were to (i) document chemical and mineralogical compositions in two naturally acidic drainages over a 1 m soil profile, (ii) document organic and inorganic signatures representative of past chemical or biological processes in the soils, and (iii) determine whether mineralogical and chemical differences are a consequence of differences in original composition, depositional conditions, or diagenesis. Two sites were studied: Paymaster Creek in the Heddleston Mining District near Lincoln, Montana and the New World Mining District near Cooke City, Montana. The oldest deposits at both naturally acidic sites pre-date human mining activity by several thousand years, although there is recent human activity at both sites. Both sites have streams with high dissolved Fe and moderately low pH and actively accumulate schwertmannite on streambeds. Soil deposits away from the streambed at Paymaster Creek contained goethite with adsorbed sulfate, but no schwertmannite, suggesting either that the original conditions precluded schwertmannite precipitation or that diagenesis occurred rapidly converting the schwertmannite to goethite. The New World Mining District site showed the expected profile, which is a gradual transition from schwertmannite- and goethite-bearing soils to goethite-only soils. Concentrations of Cr, As and other trace elements shown to retard diagenesis were higher at the New World site than at the Paymaster site.

Twenty years of groundwater evolution in the Triassic sandstone aquifer of Lorraine: Impacts on baseline water quality by Hélène Celle-Jeanton; Frédéric Huneau; Yves Travi; W.M. Edmunds (1198-1213).
The Lorraine Triassic Sandstone Aquifer (LTSA), which has already been the subject of a chemical and radioisotopic study (1979), is used to investigate the impacts of 20 a of large scale pumping on baseline water quality. In parallel, new sampling of the aquifer (2001) provides new inorganic geochemical data (including trace elements) that allow improving the knowledge of baseline conditions and hydrochemical functioning of a major sandstone aquifer. The good correlation between 14C activities, temperature and depth along the main flow line indicate regular downgradient trends and possible water stratification. Unreactive tracers, mainly stable isotope ratios 18O and 2H, as well as C isotopes are used to define a timescale for the aquifer, showing two groups of groundwater, namely of modern and Holocene age, and late Pleistocene age, with a mixing zone. Baseline quality is then represented by a wide range of concentrations, mainly the result of time-dependent water–rock interaction, as already observed elsewhere in Triassic sandstone aquifers. Some trace elements such as Li, Rb, Cs, which are not limited by solubility constraints, show linear trends. During saturated flow downgradient, the chemistry is also specifically characterised by a regular increase in Na and Cl (and locally SO4) as a result of evaporite dissolution related to overlying or basement limits. The aquifer is mostly oxidising with a redox boundary marked by U decrease, some 40 km from outcrop.Groundwater abstraction since the 1970s has created a strong lowering (10–150 m) of the water table, especially from 1970 to 1980. Based on nine boreholes, previously sampled in 1979, a decreasing evolution in radiocarbon content of the TDIC, together with significant evolution of 18O content, indicate that old groundwater has moved upgradient. The major difference in terms of baseline evolution is observed using Cl and Na concentration and, locally, SO4, indicating an increasing influence of water circulation involving overlying or basement formations, or of mixing with Permian waters. From the point of view of aquifer management, the perceptible NO3 increase could provide information on the progress of any contamination under the aerobic conditions. In addition, the few key elements, indicators of disequilibrium, related to overlying or deep waters, should be included in regular monitoring programmes.

B and Li isotopes as intrinsic tracers for injection tests in aquifer storage and recovery systems by Wolfram Kloppmann; Haim Chikurel; Géraldine Picot; Joseph Guttman; Marie Pettenati; Avi Aharoni; Catherine Guerrot; Romain Millot; Irina Gaus; Thomas Wintgens (1214-1223).
Boron and Li isotopes have been tested as environmental tracers of treated sewage injected into a sandy aquifer (Shafdan reclamation project, Israel). During a 38 days injection test in a newly dug injection well, a conservative artificial tracer (Br) was monitored together with δ 11B and δ 7Li in the injectate, in the unsaturated soil zone (porous cup) and an observation well in the aquifer. In spite of B and Li concentrations in the injectate close to background values, significant shifts of the isotope signatures could be observed over the duration of the injection test. Boron isotope ratios show a breakthrough curve delayed with respect to Br breakthrough due to some reversible sorption on the aquifer material. No isotope fractionation was observed in the unsaturated or the saturated zone so that B isotopes can be considered as conservative in the investigated part of the aquifer system. Lithium isotopes are strongly fractionated, probably due to sorption processes. Lithium concentrations point to a Li sink in the system, δ 7Li values vary strongly with a tendency of 7Li depletion in the liquid phase over the duration of the experiment. This is opposite to the expected preferential sorption of 6Li onto clay minerals. Boron isotopes reveals a valuable tracer of artificial recharge of freshwaters derived from treated sewage, both for short term tracer tests and for long-term monitoring of artificial recharge, even if in aquifers with higher clay contents, sorption-linked isotope fractionation cannot be excluded. More data are needed on Li isotope fractionation in natural groundwater systems to assess the potential of this tracer as monitoring tool.

Redistribution of potentially harmful metals and As was studied based on selective extractions in two active sulphide mine tailings impoundments in Finland. The Hitura tailings area contains residue from Ni ore processing, while the Luikonlahti site includes tailings from the processing of Cu–Co–Zn–Ni and talc ores. To characterize the element solid-phase speciation with respect to sulphide oxidation intensity and the water saturation level of the tailings, drill cores were collected from border zones and mid-impoundment locations. The mobility and solid-phase fractionation of Ni, Cu, Co, Zn, Cr, Fe, Ca, Al, As, and S were analysed using a 5-step non-sequential (parallel) selective extraction procedure. The results indicated that metal redistribution and sulphide oxidation intensity were largely controlled by the disposal history and strategy of the tailings (sorting, exposure of sulphides due to delayed burial), impoundment structure and water table, and reactivity of the tailings. Metal redistribution suggested sulphide weathering in the tailings surface, but also in unsaturated proximal areas beside the earthen dams, and in water-saturated bottom layers, where O2-rich infiltration is possible. Sulphide oxidation released trace metals from sulphide minerals at both locations. In the Hitura tailings, with sufficient buffering capacity, pH remained neutral and the mobilized metals were retained by secondary Fe precipitates deeper in the oxidized zone. In contrast, sulphide oxidation-induced acidity and rise in the water table after oxidation apparently remobilized the previously retained metals in Luikonlahti. In general, continuous disposal of tailings decreased the sulphide oxidation intensity in active tailings, unless there was a delay in burial and the reactive tailings were unsaturated after deposition.

Multiphase flow and reactive transport model in vadose tailings by Patricia Acero; Carlos Ayora; Jesús Carrera; Maarten W. Saaltink; Sebastiá Olivella (1238-1250).
Weathering processes affecting pyritic wastes may generate huge amounts of acid waters with high concentrations of potentially toxic contaminants (acid mine drainage). Acid mine drainage is mainly produced in the vadose zone. In the present study, a coupled non-isothermal multiphase flow and reactive transport model of the vadose zone of sulfide mine tailings was developed. The geochemical model included kinetically controlled reactions for Fe(II)-oxidation and for the dissolution of sulfide and aluminosilicate phases and the Pitzer ion-interaction model to describe the behavior of the pore-water solutions. Model results were compared with experimental observations in unsaturated column experiments under strongly evaporative conditions similar to arid or semiarid climates. Evolution trends for temperature, water saturation, evaporation rates, pore-water hydrochemistry and mineral phases observed during the drying experiment were adequately reproduced. The coupled model reproduced the increase of solute concentrations in the column top and the precipitation of a crust of secondary mineral phases. This crust became a barrier for water vapour diffusion to the atmosphere and modified the thermohydraulic behavior of the tailings. Enhanced downward migration of water vapour and its condensation in this colder end of the column were correctly taken into account by the model, which reproduced the dilution observed in the lower part of the column during the experiments.

This paper presents the results of a comprehensive investigation of the interaction of layered silicates with Ca(OH)2 in hydrothermal conditions. The study is intended to evaluate the stability of the clay buffer in radioactive waste repositories, at the intermediate stages of concrete leaching, when the pH is controlled by the dissolution of portlandite. The influence of layer nature, octahedral occupation, presence of tetrahedral Al and degree of crystallinity will be assessed by analysing the behaviour of a set of well-selected phyllosilicates and using the combined capabilities of 29Si and 27Al MAS-NMR spectroscopy, powder X-ray diffraction and SEM/EDX. The results show that the main factor affecting the stability of the clay is the octahedral occupation, so that trioctahedral phyllosilicates are much more stable than dioctahedral ones. The nature and expandability of the layer does not seem to much influence the stability of the clay, so that a 2:1 expandable phyllosilicate shows the same stability as a chemically analogous 1:1 non-expandable phyllosilicate. However other factors like the poor crystallinity of the starting material or the presence of Al in the tetrahedral sheet of trioctahedral phyllosilicates weaken the clay structure in alkaline conditions and favour the transformation towards other phases.

Road-deposited sediment (RDS) is an accumulation of particulates upon street surfaces in urban centres. It is commonly highly contaminated and has major potential impacts upon surface water quality and human health, as well as becoming a waste material upon street sweeping. Although significant research has been undertaken upon the fine fraction within these systems, there is a lack of detailed, high-resolution grain-specific mineralogical and chemical data for the coarser fractions, which contain the bulk of the contamination by mass. The study reported here utilizes backscatter electron microscopic, electron microprobe and Raman spectroscopic analysis to provide novel data on RDS material from Manchester, UK, with significant implications for the improved understanding of source discrimination and contaminant metal speciation. The RDS samples studied are highly heterogeneous and the abundant nature of anthropogenic grains is particularly apparent. The RDS material has been found to be composed of a number of grain types: (i) silicate and alumino-silicate grains derived from a wide range of sources; (ii) iron oxide grains derived from the corrosion of galvanized steel; (iii) iron-rich glass grains derived as slag material from metal and waste processing activities and (iv) spherical Fe oxide and Fe-rich glass grains derived from high temperature combustion processes. Elemental metallic grains (Fe, Cu, Pb) are also present in minor amounts.Electron microprobe analysis shows that the Fe oxide and Fe-rich glass grains act as the major hosts for contaminant metal elements within these sediments. The recognition of Fe oxides being an important host for metals is consistent with previous inferences made from bulk chemical sequential analysis. However, the presence of glass slag phases as major hosts of metals is not recognised in these sequential extraction schemes, leading to erroneous inferences and inputs into risk assessments. It is clear that high resolution grain-specific chemical analysis, as reported here, is the key to fully understanding these urban contaminated sediment systems.

Application of iron and zinc isotopes to track the sources and mechanisms of metal loading in a mountain watershed by David M. Borrok; Richard B. Wanty; W. Ian Ridley; Paul J. Lamothe; Briant A. Kimball; Philip L. Verplanck; Robert L. Runkel (1270-1277).
Here the hydrogeochemical constraints of a tracer dilution study are combined with Fe and Zn isotopic measurements to pinpoint metal loading sources and attenuation mechanisms in an alpine watershed impacted by acid mine drainage. In the tested mountain catchment, δ 56Fe and δ 66Zn isotopic signatures of filtered stream water samples varied by ∼3.5‰ and 0.4‰, respectively. The inherent differences in the aqueous geochemistry of Fe and Zn provided complimentary isotopic information. For example, variations in δ 56Fe were linked to redox and precipitation reactions occurring in the stream, while changes in δ 66Zn were indicative of conservative mixing of different Zn sources. Fen environments contributed distinctively light dissolved Fe (<−2.0‰) and isotopically heavy suspended Fe precipitates to the watershed, while Zn from the fen was isotopically heavy (>+0.4‰). Acidic drainage from mine wastes contributed heavier dissolved Fe (∼+0.5‰) and lighter Zn (∼+0.2‰) isotopes relative to the fen. Upwelling of Fe-rich groundwater near the mouth of the catchment was the major source of Fe (δ 56Fe ∼ 0‰) leaving the watershed in surface flow, while runoff from mining wastes was the major source of Zn. The results suggest that given a strong framework for interpretation, Fe and Zn isotopes are useful tools for identifying and tracking metal sources and attenuation mechanisms in mountain watersheds.

Arsenic-enriched groundwater has been a pressing human health issue for more than a decade, with tens of millions of people worldwide being at risk of chronic As poisoning through the consumption of As-burdened groundwater. To elucidate the importance of dissolved S on the scale of As concentrations, the composition of groundwater samples from 926 locations spanning over the floodplains of three severely arsenic affected regions in Asia (Bengal-, Mekong-, Red River deltas), were assessed. A binary mixing model based on Cl or B as conservative tracers implies that two types of water may be regarded as end-members with respect to groundwater composition in these deltas, namely surface derived water (approximated by river water) and saline water identical to residual sea water. Six redox zones were distinguished by comparing the model-calculated SO 4 2 - concentrations with the measured values. Only one zone (denoted methanogenic) had very high average As concentrations and they were significantly higher than in the other zones – for all three regions, regardless of applying Cl or B as a tracer in the model. Average As concentrations ± standard error in the methanogenic zone were 182 ± 23 μg L−1 (n  = 50%), 41 ± 6 μg L−1 (n  = 43%), and 61 ± 20 μg L−1 (n  = 24%) in the Mekong, Red River and Bengal delta, respectively. Arsenic levels were significantly lower in the SO4-reducing and the Fe-reducing zones, where averages were 23 ± 7 μg L−1 (n  = 27%, zone I), 14 ± 3 μg L−1 (n  = 48%, zone S) and 26 ± 9 μg L−1 (n  = 64%, zone S). These results suggest that a sufficient supply of SO 4 2 - inhibits the release of As to groundwater and that SO 4 2 - reduction may be as important as Fe reduction in controlling the enrichment of As in groundwater.

Modeling reactive transport in non-ideal aqueous–solid solution system by Haibing Shao; Svitlana V. Dmytrieva; Olaf Kolditz; Dmitrii A. Kulik; Wilfried Pfingsten; Georg Kosakowski (1287-1300).
The numerical simulation of reactive mass transport processes in complex geochemical environments is an important tool for the performance assessment of future waste repositories. A new combination of the multi-component mass transport code GeoSys/RockFlow and the Gibbs Energy Minimization (GEM) equilibrium solver GEM-Selektor is used to calculate the accurate equilibrium of multiple non-ideal solid solutions which are important for the immobilization of radionuclides such as Ra. The coupled code is verified by a widely used benchmark of dissolution–precipitation in a calcite–dolomite system. A more complex application shown in this paper is the transport of Ra in the near-field of a nuclear waste repository. Depending on the initial inventories of Sr, Ba and sulfate, non-ideal sulfate and carbonate solid solutions can fix mobile Ra cations. Due to the complex geochemical interactions, the reactive transport simulations can describe the migration of Ra in a much more realistic way than using the traditional linear KD approach only.

Accurate modeling of changing geochemistry in mine water can be an important tool in post-mining site management. The Pollutant Sources and Sinks in Underground Mines (POSSUM) model and Pollutant Loadings Above Average Pyrite Influenced Geochemistry POSSUM (PLAYING POSSUM) model were developed using object-oriented programming techniques to simulate changing geochemistry in abandoned underground mines over time. The conceptual model was created to avoid significant simplifying assumptions that decrease the accuracy and defensibility of model solutions. POSSUM and PLAYING POSSUM solve for changes in flow rate and depth of flow using a finite difference hydrodynamics model then, subsequently, solve for geochemical changes at distinct points along the flow path. Geochemical changes are modeled based on a suite of 28 kinetically controlled mineral weathering reactions. Additional geochemical transformations due to reversible sorption, dissolution and precipitation of acid generating salts and mineral precipitation are also simulated using simplified expressions. Contaminant transport is simulated using a novel application of the Random-Walk method. By simulating hydrogeochemical changes with a physically and thermodynamically controlled model, the ‘state of the art’ in post-mining management can be advanced.

A series of laboratory column tests on reactive mine tailings was numerically simulated to study the effect of high water saturation on preventing sulfide mineral oxidation and acid mine drainage (AMD). The approach, also known as an elevated water table (EWT), is a promising alternative to full water covers for the management and closure of sulfidic tailings impoundments and for the long term control of acid mine drainage. The instrumented columns contained reactive tailings from the Louvicourt mine, Quebec, and were overlain by a protective sand cover. Over a 13–19 month period, the columns were exposed to atmospheric O2 and flushed approximately every month with demineralized water. A free draining control column with no sand cover was also used. During each cycle, water table elevations were controlled by fixing the pressure at the column base and drainage water was collected and analyzed for pH and Eh, major ions, and dissolved metals (Fe, Zn, Cu, Pb, and Mg). The columns were simulated using the multi-component reactive transport model MIN3P which solves the coupled nonlinear equations for transient water flow, O2 diffusion, advective–dispersive transport and kinetic geochemical reactions. Physical properties and mineralogical compositions for the material layers were obtained from independent laboratory data. The simulated and observed data showed that as the water table elevation increased, the effluent pH became more neutral and SO4 and dissolved metal concentrations decreased by factors on the order of 102–103. It is concluded that water table depths less than or equal to one-half of the air entry value (AEV) can keep mine tailings sufficiently saturated over the long term, thus reducing sulfide oxidation and AMD production.

Total Pb concentrations and isotopic composition were determined in stream-bed sediments and bedrock from 29 small agricultural or forested catchments in the Gascogne area (SW France). The contribution of Pb from various natural and anthropogenic sources was investigated in this rural area which is very weakly impacted by industrial or urban emissions. Environmental parameters in catchments (importance of forest cover, organic matter and oxide content in sediments) were considered. A combination of geochemical (enrichment calculation, sequential extraction) and isotopic investigations was performed to constrain the origin of Pb and the distribution of anthropogenic Pb in sediments. Most of the sediments have low total Pb content compared to other agricultural regions more impacted by industrial or urban emissions. The results indicated a moderate but significant Pb enrichment, particularly for catchments draining forested areas. This enrichment was positively related to organic C content in sediment and catchment forest cover, whereas in entirely cultivated catchments it was related to Fe-oxide content.An average anthropogenic end-member was determined using Pb isotopes, and was supposed to be representative of background atmospheric Pb pollution, with a weak influence of Pb from recent gasoline and local fertiliser spreading. The amount of anthropogenic Pb (Pbanthr) in sediments estimated using a geochemical approach (mean 63.7 ± 20.4%) was higher than that estimated using an isotopic approach (mean 36.6 ± 17.8%), but the same trend was observed among the samples, except for low anthropogenic contributions. The distribution of Pbanthr in sediments from weakly forested catchments indicated a strong affinity for carbonates and Fe-oxides. Amorphous Fe-oxides became preferential trapping compounds as soon as Pb enrichment increased and carbonate content decreased. Finally, in cultivated catchments, organic C was not a main trapping component for Pbanthr in sediments.

Gas geochemistry of natural analogues for the studies of geological CO2 sequestration by N. Voltattorni; A. Sciarra; G. Caramanna; D. Cinti; L. Pizzino; F. Quattrocchi (1339-1346).
Geological sequestration of anthropogenic CO2 appears to be a promising method for reducing the amount of greenhouse gases released to the atmosphere. Geochemical modelling of the storage capacity for CO2 in saline aquifers, sandstones and/or carbonates should be based on natural analogues both in situ and in the laboratory. The main focus of this paper has been to study natural gas emissions representing extremely attractive surrogates for the study and prediction of the possible consequences of leakage from geological sequestration sites of anthropogenic CO2 (i.e., the return to surface, potentially causing localised environmental problems). These include a comparison among three different Italian case histories: (i) the Solfatara crater (Phlegraean Fields caldera, southern Italy) is an ancient Roman spa. The area is characterised by intense and diffuse hydrothermal activity, testified by hot acidic mud pools, thermal springs and a large fumarolic field. Soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of CO2 per day; (ii) the Panarea Island (Aeolian Islands, southern Italy) where a huge submarine volcanic-hydrothermal gas burst occurred in November, 2002. The submarine gas emissions chemically modified seawater causing a strong modification of the marine ecosystem. All of the collected gases are CO2-dominant (maximum value: 98.43 vol.%); (iii) the Tor Caldara area (Central Italy), located in a peripheral sector of the quiescent Alban Hills volcano, along the faults of the Ardea Basin transfer structure. The area is characterised by huge CO2 degassing both from water and soil. Although the above mentioned areas do not represent a storage scenario, these sites do provide many opportunities to study near-surface processes and to test monitoring methodologies.

Predicting contaminant fate and transport in sediment caps: Mathematical modelling approaches by Jason Go; David J. Lampert; Julia A. Stegemann; Danny D. Reible (1347-1353).
Sediment capping is a remedial option for managing contaminated sediments that involves the artificial placement of a layer of material over a contaminated area. Sorbent materials such as activated C and coke can be used to amend sand caps to improve cap performance. In this study, analytical and numerical modelling approaches were compared for predicting contaminant fate and transport in sediment caps using several diffusion-controlled and advection-dominated contaminant transport scenarios. An analytical tool was used to predict cap performance at steady-state. These results were compared with the results from the numerical CoReTranS model in which the effective diffusivity and degradation rates were modelled as discontinuous functions at a prescribed bioturbation depth. The numerical approach was also applied to modelling a sorptive cap. It was shown that, while the analytical approach can be used to predict steady-state contaminant transport, the numerical approach is needed to evaluate multiple sediment layers with different transport and sorption characteristics and to examine the transient performance between the time that the single layer transient is applicable (i.e., before penetration of the cap containment layer) and until steady-state in the upper layer. For the 30 cm thick sand cap that was considered in this study, the predicted time to reach steady-state conditions for a diffusion-controlled scenario is 1 ka. For an advection-dominated transport, the time to reach steady-state conditions is reduced to 100 a. The activated C-amended sand cap was more effective in isolating the contaminant within the sorbent layer for a sustained period of time (∼100 a). Results from both modelling approaches showed that capping can effectively reduce contaminant flux to the overlying water with critical variables being cap thickness, groundwater velocity, and sediment sorptivity.

Corrigendum to “Naturally acid waters from Copahue volcano, Argentina” [Applied Geochemistry 24 (2009) 208–220] by J.C. Varekamp; A.P. Ouimette; S.W. Herman; K.S. Flynn; A. Bermudez; D. Delpino (1354).