Applied Geochemistry (v.19, #6)

Soil volatile mercury, boron and ammonium distribution at Cañadas caldera, Tenerife, Canary Islands, Spain by Pedro A Hernández; Nemesio M Pérez; José M.L Salazar; Ray Ferrell; Carlos E Álvarez (819-834).
Spatial distribution of soil Hg, B and NH4 was investigated in the soils of Cañadas caldera, Canary Islands, in the summer of 1992. Soil Hg, B and NH4 were also studied over several transects intersecting structural features. Soil Hg concentration ranged from 10.83 to 45,000 μg kg−1 whereas those of B and NH4 ranged from 8.45 to 4512 μg kg−1 and from 0.31 to 181 mg kg−1, respectively. Probability plot analysis identified 3 geochemical populations for Hg and B whereas only 2 were detected for NH4. Multivariate analysis (cluster, factor and multiple regression analysis) was used to identify subtle geochemical characteristics of the groups and factors and to determine the relationship between Hg, B and NH4 and the soil secondary parameters. High soil Hg, B and NH4 concentrations are interpreted as being related to areas where a convection system has developed, coinciding with the most recent volcanic centers located along the basaltic rift zones and with the main features inside Cañadas caldera: Teide volcano and Roques de Garcı́a. Background levels of soil volatiles are present in locations where no indication of subsurface thermal activity occurs. Generally, the secondary controls on Hg, B and NH4 are subtle and are overwhelmed in areas characterized by subsurface geothermal activity.

Dissolution kinetics of galena in acid NaCl solutions at 25—75 °C by Sheng Zhang; Jianping Li; Yurong Wang; Guangqian Hu (835-841).
Experiments on dissolution kinetics of galena were performed in 1 mol l−1 NaCl solutions at pH 0.43–2.45 and 25–75 °C. When the dissolution reaction is far from equilibrium, a linear relation exits between the dissolution rate, r, and the H+ ion activity, [H+]. The rate law for galena dissolution is given by the following equation: r=k[H+]. With respect to H+, the dissolution reaction is in the first order. The apparent rate constant, k, has values of 2.34×10−7 mol m−2 s−1 at 25 °C, 1.38×10−6 mol m−2 s−1 at 50 °C, and 7.08×10−6 mol m−2 s−1 at 75 °C. The activation energy of dissolution reaction is 43.54 kJ mol−1. The mechanism of dissolution is suggested to be surface chemical reaction, and the rate determining step is the dissociation of the Pb–S bond of the surface complex, which releases Pb2+ into the solution.

The contribution of Chinese sources to the acid deposition in neighbouring countries in Far East Asia has been disputed. This study is to examine the isotopic composition of the S in the natural waters in the Chuncheon area to see if the isotopic composition can be an indicator of the pollution sources in the area. Meteoric water sampled between September 2000 and July 2001 and surface water sampled in December 2000 and April 2001 in the area were collected to examine their chemical and isotopic compositions. The pH values of the meteoric water lie between 4.02 and 6.89, but mostly lower than 5.6, indicating considerable acid deposition. The pH of the surface water is generally higher than that of the atmospheric water. Factor analysis on the concentrations of the dissolved components suggests that the surface water chemistry depends principally on the chemistry of the atmospheric water. The dissolution of carbonate minerals and silicate-water interaction plays a rather minor role in affecting the water chemistry. δ18O and δD of all the natural water samples align fairly well along the global meteoric water line. The δ34S of the dissolved SO4 in the meteoric water has values ranging from 3.4 to 8.2‰, showing little seasonal difference. The contribution from sea-salt to the total atmospheric S is estimated to be less than 10%. The δ34S of the anthropogenic S in the Chuncheon atmosphere is calculated to be 2.5 to 7.2‰, which partly overlaps the reported values of S in Chinese coal, Chinese rainfall, and Japanese fallout. Appropriate tracking and quantitative estimation of the contribution from possible pollution sources to the local S concentrations requires more information on the isotopic compositions of the potential pollution sources. The δ34S of the dissolved SO4 in the surface water has values ranging from 3.2 to 6.2‰, which is a little narrower than that of the meteoric water. The δ34S and the concentration range of the dissolved SO4 in the stream water, the results from factor analysis on the compositional variables, and observations from geological field work suggest that most SO4 comes from the meteoric input, and that the δ34S of the surface water may be utilized for the investigation of tracing and identifying the sources of the atmospheric pollutant S in the study area.

Study of the precipitation equilibria of arsenate anion with calcium and magnesium in sodium perchlorate at 25 °C by Juan C. Raposo; Olatz Zuloaga; Marı́a A. Olazabal; Juan M. Madariaga (855-862).
The stabilities of solid Ca and Mg arsenates were established by solubility experiments from the pH and total aqueous Ca, Mg and As concentrations. The experimental data were consistent with the following solid phases defined as a function of the pH range: Ca3(AsO4)2 .10H2O (7<pH <10), CaNaAsO4 .7.5H2O (10<pH <12), MgHAsO4 . 4H2O (6.5<pH <7.1) and Mg3(AsO4)2 .8H2O (7.2<pH <9). All the experimentation was performed at constant ionic strength in aqueous NaClO4 solutions and at 25 °C, where the stoichiometric solubility products of the different solids were calculated. The corresponding thermodynamic solubility products of all the precipitates were computed by means of the Modified Bromley’s Methodology (MBM) that considers the variation of the activity coefficients of the ions involved in the precipitation equilibria with the medium and ionic strength. Moreover, all the solid arsenates obtained were characterised by FT-IR and FT-Raman spectroscopies and the hydration of each precipitate was confirmed by X-Ray Diffraction data.

Large scale redox processes were investigated in a river recharged aquifer in the Oderbruch polder alongside the river Oder in north-eastern Germany. Major hydraulic and hydrochemical processes were identified qualitatively. As a result of intensive drainage activities in the past 250 a, the groundwater level within the polder is situated below the river water level and a levee prevents flooding of the lowland. As a consequence, river water permanently infiltrates into the shallow confined aquifer. A sequence of redox reactions, driven by organic matter degradation, can be observed during infiltration of oxic river water into the groundwater. Up to 3 km from the river, reduction processes from O2 respiration to SO2− 4 reduction dominate the groundwater chemistry. While reduction of Fe- and Mn(hydr)oxides is the source of the high amounts of dissolved Fe2+ and Mn2+, carbonate dissolution/precipitation reactions control the actual groundwater concentration of Mn2+. The first order rate constant for SO2− 4 reduction was found to be −0.0169 a−1. Fe2+ is released into the groundwater at a rate of 0.0033 mmol l−1 a−1. The groundwater chemistry is strongly linked to the hydraulic conditions. Near the river, the groundwater is confined and recharged by bank-filtration only. In contrast, in the central polder the groundwater is unconfined and percolation of rainwater through the dried loam is possible because of texture changes such as shrinkage fissures. Geogenic pyrite present within the alluvial loam is oxidised and large amounts of SO2− 4 are released into the groundwater.

Hazard assessment on arsenic and lead in soils of Castromil gold mining area, Portugal by Eduardo Ferreira da Silva; Chaosheng Zhang; Luı́s Serrano Pinto; Carla Patinha; Paula Reis (887-898).
Castromil is one of the Au mining areas in Portugal that has been abandoned since 1940. Due to the lack of regulations and environmental education, Castromil is now a residential area suffering from the considerable consequences of poorly regulated mining activities; tailings, shafts and adits are present. Geochemical data related to environmental studies in old mining areas frequently show extremely high values and very skewed distributions that need to be properly addressed. Agricultural soils from this region have high concentrations of As and Pb. In this study, the Box–Cox transformation and geostatistics were applied to study heavy element (As and Pb) concentrations in soils in order to characterize the hazard posed by them in the area. This provides a decision support tool to define the areas where remedial action is needed in light of the risks to humans and ecosystems and for contaminant migration. The results discussed here take into account the hazard-based standards for soils as target and intervention values.

A 7 step sequential extraction procedure has been conducted on a podzolic soil profile from the Vosges Mountains in order to determine the ability of several elements to be released to the environment. Very little Si, K and Al were extractable (<10% of the total soil concentration) but larger proportions (> 10% of the total soil concentration) of Ca, P, metals (Fe, Pb), REE and actinides (Th, U) could be leached. For each element, preferential binding sites can be recognized. High recovery of P and Ca in the acid soluble fraction (AS) suggests that phosphate minerals are highly involved in this step of the extraction. Organic matter appears to control the adsorption of Ca, Fe, Th, U and REE, even at depths in the soil profile where organic matter content is particularly low (0.5%). Weak acid leaching experiments (with HCl and acetic acid 0.001 N) were also performed in order to characterize the origin of the insoluble material in this soil profile. The leachable REE distributions indicate that a large part of the labile REE in the surface horizon has an atmospheric origin whereas at greater depth phosphate mineral (apatite) alteration is the main factor controlling REE release in the leachate. The study further suggests that adsorbed material holding actinides and REE are not strictly the same. So, caution should be taken when using REE as analogues for actinides in soils systems.

Numerical simulation of CO2 disposal by mineral trapping in deep aquifers by Tianfu Xu; John A Apps; Karsten Pruess (917-936).
Carbon dioxide disposal into deep aquifers is a potential means whereby atmospheric emissions of greenhouse gases may be reduced. However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO2 disposal must be refined if this technology is to be implemented safely, efficiently, and predictably. As a prelude to a fully coupled treatment of physical and chemical effects of CO2 injection, the authors have analyzed the impact of CO2 immobilization through carbonate mineral precipitation. Batch reaction modeling of the geochemical evolution of 3 different aquifer mineral compositions in the presence of CO2 at high pressure were performed. The modeling considered the following important factors affecting CO2 sequestration: (1) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, (2) CO2 solubility dependence on pressure, temperature and salinity of the system, and (3) redox processes that could be important in deep subsurface environments. The geochemical evolution under CO2 injection conditions was evaluated. In addition, changes in porosity were monitored during the simulations. Results indicate that CO2 sequestration by matrix minerals varies considerably with rock type. Under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO2 dissolution in pore waters. The precipitation of ankerite and siderite is sensitive to the rate of reduction of Fe(III) mineral precursors such as goethite or glauconite. The accumulation of carbonates in the rock matrix leads to a considerable decrease in porosity. This in turn adversely affects permeability and fluid flow in the aquifer. The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters.

Analysis of stable isotopes and major ions in groundwater and surface waters in Belize, Central America was carried out to identify processes that may affect drinking water quality. Belize has a subtropical rainforest/savannah climate with a varied landscape composed predominantly of carbonate rocks and clastic sediments. Stable oxygen (δ18O) and hydrogen (δD) isotope ratios for surface and groundwater have a similar range and show high d-excess (10–40.8‰). The high d-excess in water samples suggest secondary continental vapor flux mixing with incoming vapor from the Caribbean Sea. Model calculations indicate that moisture derived from continental evaporation contributes 13% to overhead vapor load. In surface and groundwater, concentrations of dissolved inorganic carbon (DIC) ranged from 5.4 to 112.9 mg C/l and δ13CDIC ranged from −7.4 to −17.4‰. SO4 2, Ca2+ and Mg2+ in the water samples ranged from 2–163, 2–6593 and 2–90 mg/l, respectively. The DIC and δ13CDIC indicate both open and closed system carbonate evolution. Combined δ13CDIC and Ca2+, Mg2+, and SO4 2− suggest additional groundwater evolution by gypsum dissolution and calcite precipitation. The high SO4 2−content of some water samples indicates regional geologic control on water quality. Similarity in the range of δ18O, δD and δ13CDIC for surface waters and groundwater used for drinking water supply is probably due to high hydraulic conductivities of the karstic aquifers. The results of this study indicate rapid recharge of groundwater aquifers, groundwater influence on surface water chemistry and the potential of surface water to impact groundwater quality and vise versa.

Metal extraction from road-deposited sediments using nine partial decomposition procedures by Ross A. Sutherland; Filip M.G. Tack; Alan D. Ziegler; Joseph O. Bussen (947-955).
Nine partial decomposition procedures and a total digestion treatment were applied to road-deposited sediments. The objective was to define a parsimonious, time-efficient decomposition procedure that (1) has limited impact on the alumnio-silicate matrix and/or refractory-associated fractions, (2) has metal recoveries independent of CaCO3 content, and (3) produces high anthropogenic signals for known contaminants (e.g., Cu, Pb and Zn). The 9 digestions varied from weak single reagents (0.11 M acetic acid) to strong multi-step procedures (BCR 3-step plus aqua regia). Eight metals were examined: Al, Co, Cu, Fe, Mn, Ni, Pb, and Zn. Cold (room temperature) 0.5 M HCl shaken over a 1-h period with a solid-to-solution ratio of 1 g:20 ml, was judged superior based on the defined criteria. This simple, rapid treatment had limited impact on the residual matrix (mean and 95% confidence interval for Al recovery was 6±1%); recoveries of all elements examined were independent of CaCO3 content; the treatment produced high mean extraction efficiencies for Cu (58±9%), Pb (84±5%), and Zn (73±7%), and produced high anthropogenic signals. Thus, dilute HCl can be widely recommended as an optimal partial decomposition procedure for assessing non-residual fractions of complex solid media.

Lead isotope signatures of Holocene fluvial sediments from the Loire River valley by Philippe Négrel; Wolfram Kloppmann; Manuel Garcin; Denis Giot (957-972).
The distribution of Mn, V, Th, Pb and isotopes of Pb in the labile fraction of sediments from a channel infill in the Middle Loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10 ka BP. The acid extractable matter (AEM) in the sediment samples is variable in amount and in trace element contents. Iron-Mn oxyhydroxides are the principle trace element carrying phase in the labile fraction and carbonates are a secondary carrier. Vanadium and Pb originate from the weathering of silicates and are used as a silicate erosion rate index in the fluviatile record. Most of the AEM data plot along a general trend between 3 endmembers (basalts, Cretaceous carbonate rocks and granites) in the relationship between 207Pb/206Pb and 206Pb/206Pb. These endmembers have been mixed in various proportions depending on natural Holocene inputs (erosion, volcanic events) or human influences (mining and smelting of ore).

Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations by Alexander Neaman; Flore Mouélé; Fabienne Trolard; Guilhem Bourrié (973-979).
The association of rare earth and other trace elements with Fe and Mn oxides was studied in Fe-Mn-nodules from a lateritic soil from Serra do Navio (Northern Brazil). Two improved methods of selective dissolution by hydroxylamine hydrochloride and acidified hydrogen peroxide along with a classical Na–citrate–bicarbonate–dithionite method were used. The two former reagents were used to dissolve Mn oxides without significant dissolution of Fe oxides, and the latter reagent was used to dissolve both Mn and Fe oxides. Soil nodules and matrix were separated by hand. Inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry after fusion with lithium metaborate, and X-ray diffraction were used to determine the elemental and mineralogical composition of the nodules and soil matrix. The latter was composed of kaolinite, gibbsite, goethite, hematite, and quartz. In the nodules, lithiophorite LiAl2(MnIV 2MnIII)O6(OH)6 was detected in addition to the above-mentioned minerals. The presence of hollandite (BaMn8O16) and/or coronadite (PbMn8O16) in the nodules is also possible. In comparison to the matrix, the nodules were enriched in Mn, Fe, K, and P, and relatively poor in Si, Al, and Ti. The nodules were also enriched in all trace elements determined. Phosphorus, As and Cr were associated mainly with Fe oxides; Cu, Ni, and V were associated with both Fe and Mn oxides; and Ba, Co, and Pb were associated mainly with Mn oxides. Distribution of rare earth elements indicated a strong positive Ce-anomaly in the nodules, compared to the absence of any anomaly in the matrix. Some of Ce was associated with Mn oxides. The improved methods achieved almost complete release of Mn from the sample without decreasing the selectivity of dissolution, i.e., without dissolving significant amounts of Fe oxides and other minerals, and provided reliable information on associations of trace elements with Mn oxides. These methods are thus proposed to be included in sequential extraction schemes for fractionation of trace elements in soils and sediments.

A sequential dissolution technique and a prolonged extraction method were used to investigate aluminium (Al) release from 4 Haplic Acrisols in southeastern China. The results show that the order of acidification of the 4 soils is: Tunxi soil>Yongchun soil>Shengxian soil>Ninghai soil. The amount of exchangeable Al is directly proportional to the extent of soil acidification. Al was released from both organically bound and inorganic Al pools after acid input. During several initial cycles of extraction the release of Al was mainly from the weakly organically bound Al pool. After prolonged extraction, Al release from the inorganic Al pool became more important to the total dissolved Al due to a rapid depletion of the weakly organically bound Al pool. The sizes of readily reactive Al pools in the Ninghai soil and Shengxian soil are larger than in the Yongchun soil and Tunxi soil. Al released from the inorganic Al pool in the Ninghai soil, Shengxian soil and Yongchun soil after 20 cycles of extraction was higher than from the organically bound Al pool, whereas the opposite was the case for the Tunxi soil. A low saturation of Al binding on soil organic matter (SOM) does not necessarily lead to a low Al release as in the case of the Shengxian soil. Also a relatively high saturation does not necessarily ensure a large Al release from the Al pool as in the case of the Tunxi soil and Yongchun soil. Once both organically bound and inorganic Al pools are depleted of readily reactive Al phases due to high soil acidification, Al dissolution would be small even under strong acid input. The high concentration of aqueous Al after rapid Al release from the weakly organically bound Al pool and subsequent depletion of the pool may have significant ecological and environmental effects.