Migration and natural fate of a coal tar creosote plume

Abstract

A volume of sand containing coal tar creosote was emplaced below the water table at CFB Borden to investigate natural attenuation processes for complex biodegradable mixtures. Coal tar creosote is a mixture of more than 200 polycyclic aromatic hydrocarbons, heterocyclic compounds and phenolic compounds. A representative group of seven compounds was selected for detailed study: phenol, m-xylene, naphthalene, phenanthrene, 1-methylnapththalene, dibenzofuran and carbazole. Movement of groundwater through the source led to the development of a dissolved organic plume, which was studied over a four year period.

Qualitative plume observations and mass balance calculations indicated two key conclusions: 1) compounds from the same source can display distinctly different patterns of plume development and 2) mass transformation was a major influence on plume behaviour for all observed compounds. After being completely leached from the source early in the study, phenol migrated as a discrete slug plume and almost completely disappeared after two years. the m-xylene plume migrated outward to a maximum distance at approximately two years, and then receded back towards the source as the rate of mass flux out of the source decreased to below the overall rate of plume transformation. Carbazole showed similar behaviour, although the reversal in plume development occurred more slowly.

The dibenzofuran plume remained relatively constant in extent and mass over the last two years of monitoring, despite that constant source input over this period. This indicated that the rate of mass input was approximately equal to the rate of plume transformation and is the first conclusive documentation of a "steady state" plume, of which the author is aware. Meanwhile, the naphthalene and 1-methylnapththalene plume continued to advance over the observation period, although decreases in the rate of mass input from the source to the plume were noted for both. The phenanthrene plume was also subject to transformation, although measurement of the rate was less conclusive due the higher proportion of sorbed mass for this compound.

Multiple lines of evidence were used to evaluate whether the observed plume mass loss was due to microbial biodegradation. Comparison of sterile and active laboratory microcosm using aquifer material indicated that aquifer microbes were able to metabolize plume compounds. Measurement of redox-sensitive parameters in the vicinity of the plume showed the types of changes that would be expected to occur due to plume biodegradation: dissolved oxygen and SO4^2- decreased in groundwater within the plume while significant increases were noted for Fe^2+, Mn^2- and methane. Further evidence that plume mass loss was microbially-mediated was provided by the accumulation of aromatic acids within the plume. Measurements of phospholipid fatty acids (PLFA) in aquifer material indicated that microbial biomass and turnover rate were greater within the plume: also consistent with biodegradation.

Computer modelling confirmed that mass transformation was a strong influence on plume behaviour. It also indicated that the observed mass loss was far greater than would be expected due to aerobic respiration leading to complete mineralization. A mass balance of the change in electron acceptor against organics mass loss showed that SO4^2- was almost as important as dissolved oxygen, in terms of utilization for plume transformation. However, the total change in electron acceptors was still substantially smaller than that required for complete mineralization of plume compounds. This discrepancy was qualitatively explained by the measured occurrence of organic metabolites. With the production of these organic intermediates, transformation of plume organics may require an unexpectedly small quantity of electron acceptor. This points to a major weakness in modelling biodegradation with a stoichiometric approach that assumes complete mineralization.