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A field investigation into the fate and transport of naphthalene in a tidally forced anaerobic aquifer Bianchin, Mario

Abstract

The objective of this research is to investigate the fate and transport of naphthalene in a tidally influenced aquifer. Biodegradation, sorption, dispersion, dilution and physical removal of sediments by scour may all contribute to naphthalene loss in a sub-fluvial aquifer. A multiple-line of evidence approach was pursued to investigate processes responsible for natural attenuation of naphthalene. This research included the following studies: (1) bathymetric survey (2) contaminant plume characterization offshore using the Waterloo Drive Point Profiler (3) onshore tracer test and, (4) reactive tracer test offshore using 14C-naphthalene. Over the course of a year, the riverbed appears to be in quasi-equilibrium. Sediment transport occurs continuously as migrating dunes and secularly as scour and deposition during a freshet-flood wave event. Any appreciable amount of sediment scoured away is "new" sediment deposited temporarily during the freshet-flood wave. The study was limited in that it did not address interannual variability of riverbed changes. Also, surveys were too infrequent to conclusively state if contaminated sediments of the aquifer are scoured away to any appreciable extent. Results of the onshore tracer study suggest that dispersion in the upland portion of the aquifer does not appear enhanced by tidal forcing. Further, bromide tracer plume transport offshore suggests that dispersion and or dilution did very little to reduce bromide concentrations over the 230 day study period. The immobility of the offshore bromide plume confirmed the presence of a "low flow" zone associated with onshore pumping operating since 1995. Profiling of the contaminant plume offshore confirmed the presence of a large anaerobic plume consisting mainly of naphthalene. The plume was at least 200m in width and could be as wide as 300m, consistent with profiling conducted onshore in 1997. During both the 1997 and 1999 profiling events the offshore portion of the plume existed in a zone of low groundwater flow due to hydraulic containment. This suggests that in this zone offshore a state of quasi-equilibrium may exist between competing fate and transport processes, namely, degradation, advective transport from the source area and desorption of naphthalene from sediments. Results of the offshore reactive tracer study confirmed that anaerobic degradation is occurring with the mineralization of 14C-naphthalene to 14C-CO2. Regression analyses of the 14C-CO2 data yielded a rate of CO2 production of 3E-9 mmol 14C/L•day. The distribution of radionuclides among CH4 and CO2 fractions was considered to estimate half-life values for naphthalene ranging from 0.72 to 2.9 years. Assuming no fractionation of radionuclides, a half-life for naphthalene under the site conditions was 1.72 years. Migration of the 14C-naphthalene plume with respect to that of the bromide plume revealed that retardation of naphthalene is fairly low at R = 1.7. In summary, results of this research indicate that anaerobic degradation plays a significant role in the fate and transport of naphthalene in this portion of the sub-fluvial aquifer. [Scientific formulae used in this abstract could not be reproduced.]

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