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Assessing natural attenuation of petroleum hydrocarbons using reactive transport modelling with aqueous and solid phase data Petersmeyer, Chad William

Abstract

Degradation of petroleum hydrocarbon contamination in groundwater was investigated at a site located in southeast Alberta, Canada. Condensate had been introduced into the unconfined aquifer approximately thirty years prior to the collection of aqueous data. The existing dataset consisted of five recent sampling events of moderate spatial resolution, spanning a timeframe of about one year. This study was undertaken to identify and quantify the dominant processes controlling natural attenuation at the site and to determine the value of supplementing a typical industry dataset with solid-phase data. To achieve these objectives, cores were collected and another round of water sampling was conducted in October 2004. Sequential iron and sulphide extractions were carried out on core intervals to quantify mineral dissolution and precipitation as a result of microbially-mediated degradation and SEM imaging was utilized to qualitatively assess the presence of relevant mineral phases. Reactive transport modelling was used as a data interpretation tool to simulate geochemical processes and a comparison was made between the degradation rates estimated using only the aqueous data and after including the solid-phase data. Sulphate and iron reduction were discovered to be the dominant processes contributing to biodegradation at the site, although iron and sulphate trends were not useful for determining historical electron acceptor utilization rates. Degradation rates estimated from dissolved BTEX concentrations were up to two orders of magnitude lower than those determined when integrating major ion chemistry with the solid-phase data. Using trends in BTEX concentrations, the maximum reaction rate for sulphate reduction and iron-oxide reductive dissolution were estimated to be 1.23 x 10⁻¹² mol L⁻¹s⁻¹ and 2.89 x 10⁻¹² mol L⁻¹ s⁻¹, respectively. When integrating major ion chemistry with the solid-phase data, these rates were found to be 1.19 x 10⁻¹⁰ mol L⁻¹s⁻¹ and 2.34 x 10⁻¹⁰ mol L⁻¹s⁻¹ and it was found that the two processes accounted for 78% and 21% of contaminant degradation, respectively, equivalent to a total mass of seven tonnes of hydrocarbons (as BTEX) degraded at the site. The differences between these estimates are likely due to source depletion and shrinking of the plume, which cannot be observed using aqueous data only.

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