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[en] Sediment analysis using wet extraction techniques and model simulations were carried out to investigate the role of ferric iron during biodegradation of dissolved petroleum hydrocarbon compounds at a field site in Perth, Western Australia. Sediment cores were analysed for iron and sulphur species. The total iron concentrations were found to be low while the fraction of Fe(II) was surprisingly high. Pyrite was detected in the contaminated zone, but no iron monosulphides were found. A reactive multi-component transport model, coupling advective-dispersive transport of organic compounds and inorganic aqueous components with a geochemical equilibrium model and a biodegradation model, was applied to simulate qualitatively the fluxes and reactions involved in the biodegradation of BTEX compounds. Both field investigations and modelling suggest that ferric iron minerals play no important role as electron acceptors while sulphate provides the major part of the oxidation capacity. (Author)
[en] Enhanced reductive dehalogenation is an attractive treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils' natural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron-accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil's buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron (III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition required to control the pH for user-specified operating conditions.