Natural attenuation of dissolved petroleum fuel constituents in a fractured Chalk aquifer: Contaminant mass balance with probabilistic analysis

A plume-scale mass balance is developed to assess the natural attenuation (NA) of dissolved organic contaminants in fractured, dual porosity aquifers. This methodology can be used to evaluate contaminant distribution within the aquifer, plume source term, contaminant biodegradation and plume status. The approach is illustrated for a site on the UK Upper Chalk aquifer impacted by petroleum fuel containing MTBE and TAME. Variability in site investigation data and uncertainty in the mass balance was assessed using probabilistic analysis. The analysis shows that BTEX compounds are biodegraded primarily by denitrification and sulphate reduction in the aquifer, with an equivalent plume-scale first-order biodegradation rate of 0.49 year-1. Other biodegradation processes are less important. Sorption contributes to hydrocarbon attenuation in the aquifer but is less important for MTBE and TAME. Uncertainty in the plume source term and site hydrogeological parameters had the greatest effect on the mass balance. The probabilistic analysis enabled the most likely long-term composition of the plume source term to be deduced and provided a site-specific estimate of contaminant mass flux for the prediction of plume development. The mass balance methodology provides a novel approach to improve NA assessments for petroleum hydrocarbons and other organic contaminants in these aquifer settings

Biogeochemical processes in the active layer and permafrost of a high Arctic fjord valley

Warming of ground is causing microbial decomposition of previously frozen sedimentary organic carbon in Arctic permafrost. However, the heterogeneity of the permafrost landscape and its hydrological processes result in different biogeochemical processes across relatively small scales, with implications for predicting the timing and magnitude of permafrost carbon emissions. The biogeochemical processes of iron- and sulfate-reduction produce carbon dioxide and suppress methanogenesis. Hence, in this study, the biogeochemical processes occurring in the active layer and permafrost of a high Arctic fjord valley in Svalbard are identified from the geochemical and stable isotope analysis of aqueous and particulate fractions in sediment cores collected from ice-wedge polygons with contrasting water content. In the drier polygons, only a small concentration of organic carbon (<5.40 dry weight%) has accumulated. Sediment cores from these drier polygons have aqueous and solid phase chemistries that imply sulfide oxidation coupled to carbonate and silicate dissolution, leading to high concentrations of aqueous iron and sulfate in the pore water profiles. These results are corroborated by δ34S and δ18O values of sulfate in active layer pore waters, which indicate the oxidative weathering of sedimentary pyrite utilising either oxygen or ferric iron as oxidising agents. Conversely, in the sediments of the consistently water-saturated polygons, which contain a high content of organic carbon (up to 45 dry weight%), the formation of pyrite and siderite occurred via the reduction of iron and sulfate. δ34S and δ18O values of sulfate in active layer pore waters from these water-saturated polygons display a strong positive correlation (R2 = 0.98), supporting the importance of sulfate reduction in removing sulfate from the pore water. The significant contrast in the dominant biogeochemical processes between the water-saturated and drier polygons indicates that small-scale hydrological variability between polygons induces large differences in the concentration of organic carbon and in the cycling of iron and sulfur, with ramifications for the decomposition pathway of organic carbon in permafrost environments

The origin of the gold mineralization of the Dolgellau district, North Wales The chemistry and role of fluids

2 vols.; For material accompanying this thesis please apply direct to the issuing universityAvailable from British Library Document Supply Centre- DSC:D69690/86 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Diagenetic controls on the pyritization of graptolites

Observations on pyritized graptolites have revealed a variety of pyrite morphotypes, several of which often coexist within the same fossil steinkern. The commonest forms of pyrite are those consisting of size-sorted microcrysts, which show degrees of ordering ranging from well-ordered framboids to a more homogeneous, unordered groundmass. Larger, euhedral pyrite crystals may be scattered throughout the microcrystalline groundmass, or be the dominant pyrite form in themselves. ‘Floriform’ pyrite frequently overgrows earlier framboids and may act to mould the inner periderm microstructure of the graptolite. Overpyrite occurs either as a primary ‘overspill’ of internal pyrite, or as a separate, later phase. All of the diagenetic pyrite has light sulphur isotope compositions, indicating formation during early diagenesis; isotopic evidence of the relative timing of pyrite generations matches the morphological paragenesis. Under most conditions, available iron appears not to be a limiting factor in pyrite formation, with reactive organic matter only being limiting at the lowest concentrations encountered in these sediments. The availability of organic matter does, however, exert a control on the timing of fossil pyritization. At one locality, a change in diagenetic conditions has allowed for the formation of a morphologically late generation of pyrite with isotopically light and therefore apparently early diagenetic signatures

Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone

Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, contain lobsters and burrows preserved in diagenetic phosphate concretions. Isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritized lobsters and similarly preserved burrows, allows a detailed comparison of these microenvironments with the host sediments. Host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions (mean sulphide δ34S = –48·3 ± 3‰ (1σ, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cretaceous seawater sulphate (mean sulphate δ34S = +8·7 ± 3·2‰ (1σ, n = 19)). These isotopic values can be explained by the action of bioturbating macrofauna; the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite, resulting in porewater sulphate that is enriched in 32S. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite, whilst residual porewater sulphate apparently remains relatively enriched in the 32S isotope. Sulphur isotopic values for the burrows are very different; sulphide isotopic compositions average –34·4 ± 0·4‰ (1σ, n = 3) and sulphate isotopic compositions average –14·4 ± 6·6‰ (1σ, n = 4). These isotopic compositions are the result of rapid development of closed system conditions in burrows, resulting in the build‐up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate (formed by oxidation of isotopically light sulphide surrounding the burrows). Lobster shell isotopic compositions fall between these two groups. On the basis of their isotopic compositions, some lobsters appear to have died close to the sediment–water interface, whilst others appear to have been buried (in burrows) prior to death

Estudio textural e isotópico de los sulfuros diseminados en los sedimentos de la Cuenca de Cameros (La Rioja, España)

Los materiales lutíticos de la Cuenca de Cameros presentan en su composición mineralógica muy pequeñas cantidades de sulfuros diseminados (pirita y pirrotita). La pirita sedimentaria aparece fundamentalmente formando framboides, cristales euhedrales y sustituyendo la concha de fósiles. En ocasiones, se encuentra pirrotita junto a la pirita, producida por un proceso de rotura térmica de la pirita sedimentaria durante el metamorfismo. El valor de 834S de la pirrotita es siempre mayor que el de la pirita en la misma muestra, con un rango de diferencias que oscila entre 4,5 y 57,7 %0. Las piritas sedimentarias se generaron por reducción bacteriana de sulfatos sedimentarios. Su composición isotópica varía en función del sector de la cuenca considerado. En la zona oriental, las composiciones isotópicas son positivas y muy variables, con un rango que oscila entre 2,6 y 19,7 %0. En el sector occidental las composiciones isotópicas son siempre negativas y muy constantes (-18 %0). Estas diferencias están relacionadas con dos factores: a) variaciones en la tasa de subsidencia entre ambos sectores y b) transformación de la pirrotita en pirita durante el retrometamorfism

Denitrification and phenol degradation in a contaminated aquifer

A natural groundwater system modified by pollutant phenols and agricultural nitrate has been modelled in the laboratory by a series of sacrificial microcosm experiments. Samples of aquifer sediment and groundwater from the margin of the phenol plume were used to inoculate anaerobic microcosms enriched in nitrate and pollutant phenols. Rapid degradation of phenol and p-cresol was observed over a 35-day period leading to the generation of inorganic carbon and a number of transient intermediates. O- cresol proved to be recalcitrant on the experimental time- scale. A mass balance calculation shows that. during degradation, carbon was conserved in the aqueous phase. Groundwater-sediment interactions were monitored using carbon stable isotope data. A mass balance for solution TIC indicates that p-cresol degradation stimulated the dissolution of sedimentary carbonate phases due to the formation of carbonic acid. Coin pound-specific carbon isotope analysis (GC-IRMS) was used to search for C-13 enrichment in residual p-cresol. A slight enrichment trend (epsilon = - 2.5 parts per thousand) was tentatively identified. The potential of this fractionation effect for obtaining in situ degradation rates is discussed. Results from the microcosm experiments help to explain the observed distribution of nitrate and phenols within the polluted aquifer