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Field sampling and modelling of creosote-derived contamination in a tidally forced aquifer Bieber, Christine

Abstract

Research into the fate and transport of a creosote-derived groundwater contaminant plume found in an aquifer adjacent to and beneath the Fraser River is presented. The site, located in Coquitlam, B.C., has been an active wood preserving facility since the 1920's. In the source zone, creosote has penetrated into the Fraser Sands aquifer. A capture well has been operated since 1996 to contain and capture the contaminant source. Previous research at the site has demonstrated that the primary component of the dissolved phase plume is naphthalene and that biodegradation of naphthalene is taking place. Suspected terminal electron accepting processes are iron reduction and methanogenesis. High naphthalene concentrations sampled in 1999 despite source containment led to the hypothesis that the plume may be at steady state due to buffering of contaminant concentrations by desorption from aquifer sediments. Naphthalene concentrations sampled in this study show that the contaminant plume is not at steady state. Particle tracking results and sorption data show that the continued presence of high concentrations of naphthalene offshore are likely due to incomplete source containment or to slow migration of contaminants from the onshore region of the plume. Although naphthalene has been the focus of all previous investigations of the offshore plume at this site, recent sampling shows that indane and benzothiophene become the dominant components of the aqueous phase plume towards the discharge point. Relative enrichment of benzothiophene and indane along the plume profile despite these compounds greater susceptibility to tidally-enhanced dispersion proves that observed loss of naphthalene over the plume flowpath is not due to a physical process The results of groundwater flow modelling show that the capture well at the site extends the residence times of contaminants in the aquifer, thereby increasing the opportunity for attenuation of contaminants before discharge to the river. This increase in residence times explains the rapid decrease in naphthalene along the plume flowpath observed in this study. Concentrations of iron and dissolved methane are higher throughout the plume axis than in background data collected suggesting that high iron and methane along the plume flowpath are associated with processes specific to the contaminated area. Geochemical modelling shows that degradation of naphthalene at the site may be taking place by iron reduction or a combination of iron reduction and methanogenesis.

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