Applied Geochemistry (v.20, #11)

Naturally occurring arsenic: Mobilization at a landfill in Maine and implications for remediation by Alison R. Keimowitz; H. James Simpson; Martin Stute; Saugata Datta; Steven N. Chillrud; James Ross; Monique Tsang (1985-2002).
Elevated levels of dissolved arsenic (∼300 μg L−1) have been detected beneath and in groundwater plumes extending away from a closed landfill in southern Maine. This study sought to determine the source of arsenic to the aquifer, the processes responsible for arsenic mobilization, and to evaluate the effectiveness of remediation efforts that have occurred at this site. The As appears to originate in the natural (glacial) aquifer solids, which contain ∼5 mg kg−1 As on a dry weight basis. This conclusion is supported by the relatively uniform distribution of As in sediment samples, results of laboratory batch incubation experiments, and comparisons with groundwaters in nearby wetlands, which also have high levels of dissolved As that do not appear to originate within the landfill. The As is mobilized in the subsurface by strongly reducing conditions beneath the landfill and in nearby wetlands. In the aquifer beneath the landfill, the average oxidation–reduction potential (ORP) is −95 mV (Eh + 105 mV), and these reducing conditions were primarily induced by landfill leachate. Remediation efforts at this site have included installation of a low permeability clay cap; groundwater extraction, oxidation, and re-injection; and subsurface oxidation by injection of magnesium peroxide. The natural source of arsenic within the aquifer solids, coupled with widespread reducing conditions, has severely limited the effectiveness of these interventions on groundwater arsenic concentrations.

The kinetics of O2(aq) reduction by structural ferrous iron in naturally occurring ferrous silicate minerals by Jennifer Rivas Perez; Steven A. Banwart; Ignasi Puigdomenech (2003-2016).
Batch reactor experiments were developed to measure the kinetic reactivity and the reduction capacity of fracture-filling solids collected from the Hard Rock Laboratory (HRL) at Äspö, Sweden. These properties were experimentally studied using wet-chemical methods in order to assess the ability of these fracture fillings to consume O2(aq) and thus provide a redox buffer against oxidising disturbances to a geological repository for spent nuclear fuel.Experimental data was described by a second-order rate law of the form Rate = k [ O 2 ( aq ) ] [ Fe ( II ) s ] , where Fe(II)s refers to reactive structural Fe(II) sites hosted at the surface of fracture-filling minerals. Parameter values for the second-order rate constant (k, L mol−1  s−1) were estimated from time series data for [O2(aq)], decrease, and for existing data from previously published studies.The majority of the values of k were found to fall between those for the Fe mono-hydroxo (Fe(OH)+, log  k  = 1.4) and the Fe di-hydro (Fe(OH)2, log  k  = 6.9) complexes in solution. An existing free energy relationship for outer-sphere electron transfer kinetics for Fe(II) species oxidation in aqueous solution indicates that the corresponding redox potentials for structural Fe(II) sites are similar to values previously determined from thermodynamic considerations.

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH4, thermogenic C2+ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH4 is bacteriogenic (Calculated δ 13CC1–C  = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH4. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ 13CC1–C  = −35.80‰; Hamitabat Field) and Group-3 (δ 13C1–C  = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C2+ gases. The Group-2 C2+ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ 13C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C2+ gases, produced from one field, were generated from isotopically light marine kerogen (δ 13C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C2+ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ 13C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R eq  = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.
Keywords: Thrace basin; Turkey; Natural gas; Condensate; Oil; GCIRMS; Carbon isotope; Source; Correlation;

Ferric iron in sediments as a novel CO2 mineral trap: CO2–SO2 reaction with hematite by James L. Palandri; Robert J. Rosenbauer; Yousif K. Kharaka (2038-2048).
Thermodynamic simulations of reactions among SO2-bearing CO2-dominated gas, water and mineral phases predict that FeIII in sediments should be converted almost entirely to dissolved FeII and siderite (FeCO3), and that SO2 should simultaneously be oxidized to dissolved sulfate. The reactions are however, subject to kinetic constraints which may result in deviation from equilibrium and the precipitation of other metastable mineral phases. To test the prediction, a laboratory experiment was carried out in a well stirred hydrothermal reactor at 150 °C and 300 bar with hematite, 1.0 m NaCl, 0.5 m NaOH, SO2 in quantity sufficient to reduce much of the iron, and excess CO2. The experiment produced stable siderite and metastable pyrite and elemental S. Changes in total dissolved Fe are consistent with nucleation of pyrite at ∼17 h, and nucleation of siderite at ∼600 h. Dissolution features present on elemental S at the conclusion of the experiment suggest nucleation early in the experiment. The experiment did not reach equilibrium after ∼1400 h, as indicated by coexistence of hematite with metastable pyrite and elemental sulfur. However, the results confirm that FeIII can be used to trap CO2 in siderite if partly oxidized S, as SO2, is present to reduce the Fe with CO2 in the gas phase.

Light hydrocarbons as redox and temperature indicators in the geothermal field of El Tatio (northern Chile) by F. Tassi; C. Martinez; O. Vaselli; B. Capaccioni; J. Viramonte (2049-2062).
El Tatio (northern Chile), one of the largest geothermal fields of South America, is presently undergoing a new program of geothermal exploration, after the failure of the first exploration phase in the early 1970s. The geochemical features of the fluid discharges characterizing this system mainly consist of boiling pools and fumaroles, and represent the result of a complex mixing process involving 3 main components: (i) hydrothermal; (ii) atmospheric; (iii) magmatic. Chemical reactions involving light hydrocarbons equilibrate at higher temperature than those directly measured in the geothermal wells and calculated on the basis of the composition of the inorganic gas species. This suggests that in the deeper parts of the hydrothermal system temperatures higher than 300 °C may be achieved. Such results can have a strong impact for the evaluation of the potential resources of this geothermal system. Moreover, the chemical characteristics of the organic gas fraction allow the assessment of the chemical–physical conditions governing the geochemical processes acting on geothermal fluids at depth.

This paper deals with chemical and isotope analyses of 21 springs, which were monitored 3 times in the course of 2001; the monitoring program was focused on the groundwater of the Gran Sasso carbonate karst aquifer (Central Italy), typical of the mountainous Mediterranean area.Based on the hydrogeological setting of the study area, 6 groups of springs with different groundwater circulation patterns were distinguished. The hydrogeochemistry of their main components provided additional information about groundwater flowpaths, confirming the proposed classification. The spatial distribution of their ion concentrations validated the assumptions underlying the hydrogeological conceptual model, showing diverging groundwater flowpaths from the core to the boundaries of the aquifer. Geochemical modelling and saturation index computation elucidated water–carbonate rock interaction, contribution by alluvial aquifers at the karst aquifer boundaries, as well as impacts of human activities.The analysis of 18O/16O and 2H/H values and their spatial distribution in the aquifer substantiated the hydrogeology-based classification of 6 groups of springs, making it possible to trace back groundwater recharge areas based on mean isotope elevations; the latter were calculated by using two rain monitoring stations. 87Sr/86Sr analyses showed seasonal changes in many springs: in winter–spring, the changes are due to inflow of new recharge water, infiltrating into younger rocks and thus increasing 87Sr/86Sr values; in summer–autumn, when there is no recharge and spring discharge declines, changes are due to base flow groundwater circulating in more ancient rocks, with a subsequent drop in 87Sr/86Sr values.The results of this study stress the contribution that spatio-temporal isotope monitoring can give to the definition of groundwater flowpaths and hydrodynamics in fissured and karst aquifers, taking into account their hydrogeological and hydrogeochemical setting.

Calcite is an important component of many potential host rocks currently under consideration for the disposal of radioactive wastes. Even in the chemically disturbed zone formed around a cementitious repository, this mineral remains largely unaffected by the hyper-alkaline waters migrating out of the near field. Thus, due to its abundance and geochemical stability, calcite could play an important role in the retardation of radionuclides released from a repository for nuclear wastes. Actinides are an important class of elements present in almost all radioactive waste streams, and for this reason, investigations of their retention behaviour under representative chemical conditions are particularly relevant to assessing safe disposal in the long term. Organic ligands originating from the degradation of cellulosic materials in the repository or present as cement additives could possibly reduce the retardation of tri- and tetravalent actinides due to the formation of stable metal–ligand complexes in solution. In this study, isosaccharinic acid (ISA) and gluconic acid (GLU) have been taken as representatives of cellulose degradation products and concrete admixtures, respectively. Batch-type sorption experiments have been conducted to investigate the effect of ISA and GLU on the retardation of 152Eu, 241Am and 228Th by calcite. 152Eu and 241Am are representatives of the trivalent lanthanides and actinides, respectively, and 228Th is a representative of the tetravalent actinides.High ISA and GLU concentrations in solution were found to significantly affect the sorption of the radionuclides. R d values for Eu(III) and Am(III) decreased significantly at ISA concentrations above 10−5  M and at GLU concentrations above 10−7  M. The critical concentration limits were similar for Th(IV), that is 2 × 10−5  M in the case of ISA and 10−6  M in the case of GLU. The effects of ISA and GLU on the immobilisation of Eu(III), Am(III) and Th(IV) were interpreted in terms of complex formation in solution. In the case of Eu(III) and Am(III) in ACW, the metal–ligand complexes were found to have a 1:1 stoichiometry. Complexation constants of these aqueous ISA and GLU complexes with Eu(III) and Am(III) were determined and discussed in connection with the presently unclear situation concerning the stability constant of the Eu ( OH ) 3 0 species. In the case of Th(IV) in ACW, it was assumed that a Th(IV)–ISA–Ca complex and a Th(IV)–GLU–Ca complex are formed, both having a 1:2:1 stoichiometry. The complexation constants of these complexes were determined and compared with the literature data.Assuming maximum concentrations for ISA and GLU in the pore water of the disturbed zone of a cementitious repository based on representative near-field conditions ([ISA]aq  = 3 × 10−6  M, [GLU]aq  = 10−7  M), it is predicted that the formation of aqueous ISA and GLU complexes would not significantly affect Eu(III), Am(III) and Th(IV) sorption on calcite.

Determination of kinetic parameters and simulation of early CO2 production from the Boom Clay kerogen under low thermal stress by I. Deniau; F. Behar; C. Largeau; P. De Cannière; C. Beaucaire; H. Pitsch (2097-2107).
The main purpose of the present study is to evaluate the nature and amount of gaseous compounds that would be generated from the Boom Clay kerogen due to the foreseen thermal stress associated with the geological disposal of high activity nuclear waste. To this end, pyrolysis experiments were carried out on this low maturity, O-rich kerogen with focus on mild conditions, including Rock–Eval and closed pyrolyses using a wide range of temperature/time conditions. The residual kerogen recovered after the closed pyrolyses was re-examined by Rock–Eval and elemental analyses and the components of the gas fractions were identified and quantified by gas chromatography. These experiments showed substantial production of CO2 (corresponding to ca. 1/5 of the total O content of the kerogen) under mild thermal stress. The kinetic parameters (frequency factor and distribution of activation energy) of this early production of CO2 were determined and used to simulate the possible consequences for the deep disposal of highly radioactive waste. Extrapolation to thermal stresses, corresponding to 80 and 100 °C over 1 ka, indicated that this production of CO2 might influence the geochemistry and perhaps therefore the effectiveness of the geological barrier. For example, unless diffusion out of the heated zone counterbalances the effect of CO2 generation, significant acidification and large changes in bicarbonate concentration may take place, in the interstitial water of the clay, at a time scale of only tens to a few hundred years.

A combined CDB-MAGIC method for the determination of phosphorus associated with sedimentary iron oxyhydroxides by Miguel Angel Huerta-Diaz; Antonio Tovar-Sánchez; Gabriel Filippelli; Jennifer Latimer; Sergio A. Sañudo-Wilhelmy (2108-2115).
A combined CDB-MAGIC method for the determination of sedimentary Fe-oxyhydroxide-bound P in citrate–dithionite–bicarbonate (CDB) reducing solutions is described. Quantitative removal of P from solution is accomplished through the alkaline precipitation of Mg(OH)2 (brucite) with 10 M NaOH. After subsequent separation by centrifugation and two washings with 10% NH4OH, the precipitate is re-dissolved in 10% HCl. The P concentration in the sample is subsequently measured by the conventional molybdate blue technique using a UV–vis spectrophotometer. The detection limit of the method is 1.2 μM with precision within 1.3–8.1% in the 6–30 μM P concentration range. The results obtained using the CDB-MAGIC protocol were equivalent to those obtained using ICP-AES (r 2  = 0.860; p  ⩽ 0.001; n  = 29; with a linear slope of 1.19 ± 0.09). The CDB-MAGIC method provides then a simple, low cost alternative to the current methods based on ICP quantification.

Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana by Damon A. Pellicori; Christopher H. Gammons; Simon R. Poulson (2116-2137).
Samples of mine water from Butte, Montana were collected for paired geochemical and stable isotopic analysis. The samples included two sets of depth profiles from the acidic Berkeley pit lake, deep groundwater from several mine shafts in the adjacent flooded underground mine workings, and the acidic Horseshoe Bend Spring. Beginning in July-2000, the spring was a major surface water input into the Berkeley pit lake. Vertical trends in major ions and heavy metals in the pit lake show major changes across a chemocline at 10–20 m depth. The chemocline most likely represents the boundary between pre-2000 and post-2000 lake water, with lower salinity, modified Horseshoe Bend Spring water on top of higher salinity lake water below. Based on stable isotope results, the deep pit lake has lost approximately 12% of its initial water to evaporation, while the shallow lake is up to 25% evaporated. The stable isotopic composition of SO4 in the pit lake is similar to that of Horseshoe Bend Spring, but differs markedly from SO4 in the surrounding flooded mine shafts. The latter is heavier in both δ34S and δ18O, which may be due to dissolution of hypogene SO4 minerals (anhydrite, gypsum, barite) in the ore deposit. The isotopic and geochemical evidence suggests that much of the SO4 and dissolved heavy metals in the deep Berkeley pit lake were generated in situ, either by leaching of soluble salts from the weathered pit walls as the lake waters rose, or by subaqueous oxidation of pyrite on the submerged mine walls by dissolved Fe(III). Laboratory experiments were performed to contrast the isotopic composition of SO4 formed by aerobic leaching of weathered wallrock vs. SO4 from anaerobic pyrite oxidation. The results suggest that both processes were likely important in the evolution of the Berkeley pit lake.

Management scenarios for the Jordan River salinity crisis by Efrat Farber; Avner Vengosh; Ittai Gavrieli; Amer Marie; Thomas D. Bullen; Bernhard Mayer; Ran Holtzman; Michal Segal; Uri Shavit (2138-2153).
Recent geochemical and hydrological findings show that the water quality of the base flow of the Lower Jordan River, between the Sea of Galilee and the Dead Sea, is dependent upon the ratio between surface water flow and groundwater discharge. Using water quality data, mass-balance calculations, and actual flow-rate measurements, possible management scenarios for the Lower Jordan River and their potential affects on its salinity are investigated. The predicted scenarios reveal that implementation of some elements of the Israel–Jordan peace treaty will have negative effects on the Jordan River water salinity. It is predicted that removal of sewage effluents dumped into the river (∼13 MCM/a) will significantly reduce the river water’s flow and increase the relative proportion of the saline groundwater flux into the river. Under this scenario, the Cl content of the river at its southern point (Abdalla Bridge) will rise to almost 7000 mg/L during the summer. In contrast, removal of all the saline water (16.5 MCM/a) that is artificially discharged into the Lower Jordan River will significantly reduce its Cl concentration, to levels of 650–2600 and 3000–3500 mg/L in the northern and southern areas of the Lower Jordan River, respectively. However, because the removal of either the sewage effluents or the saline water will decrease the river’s discharge to a level that could potentially cause river desiccation during the summer months, other water sources must be allocated to preserve in-stream flow needs and hence the river’s ecosystem.

by Ian Hutcheon (2154-2155).