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  • Publication
    Métadonnées seulement
    Transport behaviour and natural attenuation of organic contaminants at spill sites
    (2004) ;
    Butler, Barbara J
    Organic contaminants pose a significant threat to groundwater resources. These contaminants are often released as nonaqueous phase liquids (NAPLs) during spills of, for example, gasoline, crude oil, creosote, coal tar or chlorinated solvents. Once released, the liquids seep downward and dissolve into the groundwater. In many cases, the impacted groundwater contains a mixture of contaminants, either due to the complexity of the NAPL (e.g., gasoline) or due to co-disposal/co-spillage (e.g., landfill leachates). Many organic contaminants are hazardous to human health and the environment and therefore threaten our potable water resources and natural ecosystems. Active remediation of contaminated groundwater is often very expensive so that cost-effective alternatives have to be found. If natural attenuation is intended to be used as a means of achieving specific remedial objectives at a contaminated site, it will require a sound understanding of the ongoing processes as well as careful control and monitoring ("monitored natural attenuation" (MNA)). Therefore, a major goal of remediation research today is to develop methods to predict the mass fate of multiple organic compounds in heterogeneous aquifers under natural conditions. (C) 2004 Elsevier Ireland Ltd. All rights reserved.
  • Publication
    Métadonnées seulement
    Influence of transient flow on contaminant biodegradation
    (2001) ;
    Durrant, Graham C
    ;
    Molson, John W
    ;
    Frind, Emil O
    The rate of biodegradation in contaminated aquifers depends to a large extent on dispersive mixing processes that are now generally accepted to result from spatial variations in the velocity field. It has been shown, however, that transient flow fields can also contribute to dispersive mixing. The influence of transient flow on biodegrading contaminants is particularly important since it can enhance mixing with electron acceptors, further promoting the reactive process. Using numerical simulations, the effect of transient flow on the behavior of a biodegradable contaminant is evaluated here both with respect to the development of apparently large horizontal transverse dispersion and also,vith respect to enhanced mixing between the substrate (electron donor) and electron acceptor. The numerical model BIO3D, which solves for advective-dispersive transport coupled with Monod-type biodegradation of substrates in the presence of an electron acceptor, was used for the simulations. The model was applied in a two-dimensional plan view mode considering a single substrate. Transient flow fields were found to yield larger apparent transverse dispersion because the longitudinal dispersivity also acts transverse to the mean flow direction. In the reactive case, the transient now field increases substrate-oxygen mixing, which in turn enhances the overall rate of biodegradation. The results suggest that in the case of moderate changes of flow directions, a steady-state flow field can be justified, thereby avoiding the higher computational costs of a fully transient simulation. The use of a higher transverse horizontal dispersivity in a steady flow field can, under these conditions, adequately forecast plume development.
  • Publication
    Métadonnées seulement
    A study of long-term MTBE attenuation in the borden aquifer, Ontario, Canada
    (1998) ;
    Barker, James F
    In 1988 and 1989, a natural gradient tracer test was performed in the shallow aerobic sand aquifer at Canadian Forces Base (CFB) Borden. A mixture of ground water containing dissolved oxygenated gasoline was injected below the water table along with chloride (Cl-) as a conservative tracer. The migration of BTEX, MTBE, and Cl- was monitored in detail for 16 months. The mass of BTEX compounds in the plume diminished significantly with time due to intrinsic aerobic biodegradation, while MTBE showed only a small decrease in mass over the 16-month period. In 1995/96, a comprehensive ground water sampling program was undertaken to define the mass of MTBE still present in the aquifer. Since the plume had migrated into an unmonitored section of the Borden Aquifer, numerical modeling and geostatistical methods were applied to define an optimal sampling grid and to improve the level of confidence in the results. A drive point profiling system was used to obtain ground water samples. Numerical modeling with no consideration of degradation predicted maximum concentrations in excess of 3000 mu g/L field sampling found maximum concentrations of less than 200 mu g/L. A mass balance for the remaining MTBE mass in the aquifer eight years after injection showed that only 3% of the original mass remained. Sorption, volatilization, abiotic degradation, and plant uptake are not considered significant attenuation processes for the field conditions. Therefore, we suggest that biodegradation may have played a major role in the attenuation of MTBE within the Borden Aquifer.