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Geochemical Conditions (geochemical + condition)
Selected AbstractsArsenic in Glacial Aquifers: Sources and Geochemical ControlsGROUND WATER, Issue 4 2005Walton R. Kelly A total of 176 wells in sand-and-gravel glacial aquifers in central Illinois were sampled for arsenic (As) and other chemical parameters. The results were combined with archived and published data from several hundred well samples to determine potential sources of As and the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the As concentrations. High concentrations were confined to areas smaller than 1 km in diameter. Arsenic and well depth were uncorrelated. Arsenic solubility appeared to be controlled by oxidation-reduction (redox) conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing water does As accumulate in solution. In wells in which total organic carbon (TOC) was below 2 mg/L and sulfate (SO42,) was present, As concentrations were low or below the detection limit (0.5 ,g/L). Arsenic concentrations >10 ,g/L were almost always found in wells where TOC was >2 mg/L and SO42, was absent or at low concentrations, indicating post,SO42,reducing conditions. Iron (Fe) is common in the aquifer sediments, and Fe oxide reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as Fe oxide is reduced. [source] Microbial response to salinity change in Lake Chaka, a hypersaline lake on Tibetan plateauENVIRONMENTAL MICROBIOLOGY, Issue 10 2007Hongchen Jiang Summary Previous investigations of the salinity effects on the microbial community composition have largely been limited to dynamic estuaries and coastal solar salterns. In this study, the effects of salinity and mineralogy on microbial community composition was studied by using a 900-cm sediment core collected from a stable, inland hypersaline lake, Lake Chaka, on the Tibetan Plateau, north-western China. This core, spanning a time of 17 000 years, was unique in that it possessed an entire range of salinity from freshwater clays and silty sands at the bottom to gypsum and glauberite in the middle, to halite at the top. Bacterial and archaeal communities were studied along the length of this core using an integrated approach combining mineralogy and geochemistry, molecular microbiology (16S rRNA gene analysis and quantitative polymerase chain reaction), cultivation and lipid biomarker analyses. Systematic changes in microbial community composition were correlated with the salinity gradient, but not with mineralogy. Bacterial community was dominated by the Firmicutes -related environmental sequences and known species (including sulfate-reducing bacteria) in the freshwater sediments at the bottom, but by halophilic and halotolerant Betaproteobacteria and Bacteroidetes in the hypersaline sediments at the top. Succession of proteobacterial groups along the salinity gradient, typically observed in free-living bacterial communities, was not observed in the sediment-associated community. Among Archaea, the Crenarchaeota were predominant in the bottom freshwater sediments, but the halophilic Halobacteriales of the Euryarchaeota was the most important group in the hypersaline sediments. Multiple isolates were obtained along the whole length of the core, and their salinity tolerance was consistent with the geochemical conditions. Iron-reducing bacteria were isolated in the freshwater sediments, which were capable of reducing structural Fe(III) in the Fe(III)-rich clay minerals predominant in the source sediment. These data have important implications for understanding how microorganisms respond to increased salinity in stable, inland water bodies. [source] Conceptual models for burrow-related, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, CanadaGEOBIOLOGY, Issue 1 2004Murray K. Gingras ABSTRACT The formation of dolomite is generally explained using models that reflect larger-scale processes that describe the relationship between the supply and transport of Mg, and geochemical conditions that are amenable to the formation of dolomite. However, heterogeneities in the substrate, such as those made by bioturbating infauna, may play a more important role in dolomitization than has been previously considered. Burrow-facilitated dolomitization is evident in the Ordovician Tyndall Stone (Red River Group, Selkirk Formation) of central Canada. The diagenetic fabrics present are attributed to dolomitizing fluids that both flowed through and evolved within burrow networks. Petrographic analysis suggests that two phases of dolomite formation took place. The first formed a fine-grained, fabric-destructive type that probably accompanied early burial; the second is a fine- to medium-grained, locally sucrosic dolomite that is interpreted to have precipitated during later burial. Isotopic analysis supports the proposed paragenetic history: (1) an apparent linking of the stable isotopes 13C and 18O strongly suggests that the micrite matrix formed during very early diagenesis and was derived from seawater; (2) the initial phase of dolomitization is potentially microbially mediated, as evidenced by the enrichment of 13C; and (3) isotopic values for the second generation of dolomite reflect the mixing of ground water and resorbed early dolomite. This paper conceptualizes the physical and chemical conditions required for the formation of dolomite in association with burrow fabrics. The proposed model reveals a composite of biological and inorganic reactions that demonstrates the interdependence of sediment fabric, organic content and microbial interactions in the development of burrow-mottled dolomitic limestone. It is suggested that where burrow-associated dolomite occurs, it is most likely to develop in two stages: first, the byproducts of the degradation of organic material in burrows locally increase the permeability and porosity around burrow fabrics in shallow diagenetic depositional environments; and, second, the passing of burrowed media into deeper dysaerobic sediment is accompanied by the establishment of fermenting micro-organisms whose byproducts mediate dolomitization. [source] Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironmentsGEOFLUIDS (ELECTRONIC), Issue 2 2002K. A. Campbell Abstract More than a dozen hydrocarbon seep-carbonate occurrences in late Jurassic to late Cretaceous forearc and accretionary prism strata, western California, accumulated in turbidite/fault-hosted or serpentine diapir-related settings. Three sites, Paskenta, Cold Fork of Cottonwood Creek and Wilbur Springs, were analyzed for their petrographic, geochemical and palaeoecological attributes, and each showed a three-stage development that recorded the evolution of fluids through reducing,oxidizing,reducing conditions. The first stage constituted diffusive, reduced fluid seepage (CH4, H2S) through seafloor sediments, as indicated by Fe-rich detrital micrite, corroded surfaces encrusted with framboidal pyrite, anhedral yellow calcite and negative cement stable isotopic signatures (,13C as low as ,35.5, PDB; ,18O as low as ,10.8, PDB). Mega-invertebrates, adapted to reduced conditions and/or bacterial chemosymbiosis, colonized the sites during this earliest period of fluid seepage. A second, early stage of centralized venting at the seafloor followed, which was coincident with hydrocarbon migration, as evidenced by nonluminescent fibrous cements with ,13C values as low as ,43.7, PDB, elevated ,18O (up to +2.3, PDB), petroleum inclusions, marine borings and lack of pyrite. Throughout these early phases of hydrocarbon seepage, microbial sediments were preserved as layered and clotted, nondetrital micrites. A final late-stage of development marked a return to reducing conditions during burial diagenesis, as implied by pore-associated Mn-rich cement phases with bright cathodoluminescent patterns, and negative ,18O signatures (as low as ,14, PDB). These recurring patterns among sites highlight similarities in the hydrogeological evolution of the Mesozoic convergent margin of California, which influenced local geochemical conditions and organism responses. A comparison of stable carbon and oxygen isotopic data for 33 globally distributed seep-carbonates, ranging in age from Devonian to Recent, delineated three groupings that reflect variable fluid input, different tectono-sedimentary regimes and time,temperature-dependent burial diagenesis. [source] Evaluation of TCE and MTBE in situ Biodegradation: Integrating Stable Isotope, Metabolic Intermediate, and Microbial Lines of EvidenceGROUND WATER MONITORING & REMEDIATION, Issue 4 2007Jennifer R. McKelvie Compound specific isotope analysis (CSIA) was used to investigate biodegradation of trichloroethene (TCE) and methyl tert -butyl ether (MTBE) at contaminated field sites in Alaska and New York State, respectively. At both sites, geochemical conditions and the presence of metabolic intermediates (cis -1-2-dichloroethene and tert -butyl alcohol [TBA]) suggested the potential for biodegradation of TCE and MTBE, respectively. Given that in both cases these metabolic intermediates could also have been present as cocontaminants in the source zone, CSIA was undertaken to evaluate the possibility of in situ biodegradation. At the TCE-contaminated field site in Alaska, ,13C values of TCE in ground water determined in this study showed no evidence of biodegradation (mean ,13C of ,27.0 ± 1.0, for nine wells), and quantitative-polymerase chain reaction analyses of ground water from four wells found no evidence of dechlorinator Dehalococcoides sp. at this site. At the MTBE-contaminated field site in New York, TBA was present in the ground water but was not present in gasoline sampled from underground storage tanks (UST) on-site, suggesting that at this site, TBA was potentially a metabolite of MTBE biodegradation rather than a cocontaminant. However, at all sampling times and locations, ,13C and ,2H values of MTBE in ground water were within range of published values for undegraded MTBE in gasoline. While the occurrence of a small extent of in situ MTBE biodegradation cannot be ruled out, the findings suggest that it is more likely that multiple gasoline spills occurred through time, and while present day USTs do not contain TBA as a cocontaminant, gasoline spilled at the site in the past may have. At both contaminated field sites, CSIA, chemical, and microbiological lines of evidence suggest that biodegradation was not a significant attenuation process. The results of these two studies underscore the need for an integrated approach to site assessment that draws on measurements of metabolic intermediates, analysis of stable isotopes, and microbial evidence to give a reliable assessment of in situ biodegradation at contaminated field sites. [source] Simultaneous analyses and applications of multiple fluorobenzoate and halide tracers in hydrologic studiesHYDROLOGICAL PROCESSES, Issue 14 2005Qinhong Hu Abstract An analytical method that employs ion chromatography has been developed to exploit the use of fluorobenzoic acids (FBAs) and halides more fully as hydrologic tracers. In a single run, this reliable, sensitive, and robust method can simultaneously separate and quantify halides (fluoride, chloride, bromide, and iodide) and up to seven FBAs from other common groundwater constituents (e.g. nitrate and sulphate). The usefulness of this analytical method is demonstrated in both field and laboratory tracer experiments. The field study examines the hydrologic response of fractures and the matrix to different flow rates and the contribution of matrix diffusion in chemical transport. Laboratory tracer experiments with eight geologic media from across the USA,mostly from Department of Energy facilities where groundwater contamination is prevalent and where subsurface characterization employing tracers has been ongoing or is in need,reveal several insights about tracer transport behaviour: (1) bromide and FBAs are not always transported conservatively; (2) the delayed transport of these anionic tracers is likely related to geologic media characteristics, such as organic matter, pH, iron oxide content, and clay mineralogy; (3) use of iodine as a hydrologic tracer should take into account the different sorption behaviours of iodide and iodate and the possible conversion of iodine's initial chemical form; (4) the transport behaviour of potential FBA and halide tracers under relevant geochemical conditions should be evaluated before beginning ambitious, large-scale field tracer experiments. Copyright © 2005 John Wiley & Sons, Ltd. [source] Nanotechnology and groundwater remediation: A step forward in technology understandingREMEDIATION, Issue 2 2006Christian Macé Nanotechnology application to contaminated site remediation, and especially the use of nanoscale zero-valent iron particles to treat volatile organic compound (VOC)-impacted groundwater, is now recognized as a promising solution for cost-effective in situ treatment. Results obtained during numerous pilot tests undertaken by Golder Associates between 2003 and 2005 in North America (United States and Canada) and Europe have been used to present a synthetic cross-comparison of technology dynamics. The importance of a comprehensive understanding of the site-specific geological, hydrogeological, and geochemical conditions, the selection of appropriate nanoscale particles, the importance of monitoring geochemical parameters during technology application, and the potential of nanoparticle impact on microbial activity are discussed in this article. The variable technology dynamics obtained during six pilot tests (selected among numerous other tests) are then presented and discussed. © 2006 Wiley Periodicals, Inc. [source] Types of Pyrophyllite Deposits in FoldbeltsRESOURCE GEOLOGY, Issue 4 2005Irina Sinyakovskaya Abstract. Pyrophyllite deposits can be divided into five types on the basis of geology and genesis. The first two types are associated with hydrothermally altered rocks in felsic and intermediate volcanogenic suites. They are characterized by their metasomatites and their subsequent mineralogic transformations under varying volcanic conditions. The third type includes deposits and occurrences of metamorphic-metasomatic genesis, which is caused by transformations of terrigenous-sedimentary interbeds in felsic volcanics under greenschist facies conditions. The fourth type is associated with low and mid-temperature stages of hydrothermal vein formation at the limits of volcanogenic and metamorphic strata. The fifth type comprises pyrophyllite occurrences in weathering crusts on metamorphic strata and metasomatite. The formation conditions and distribution of raw pyrophyllite deposits were influenced by the geodynamic situations and geochemical conditions, such as character of tectonic dislocations, volcanism and chemical composition of hydrothermal solutions. [source] | |||||||||||||