Groundwater Fate of Aromatic Hydrocarbons at Industrial Sites: a Coal Tar Site Case Study

The fate of groundwater contaminants in anthropogenic fill materials was investigated at a coal tar site, Site YYZ. This site was representative of other contaminated sites with a history of industrialization at which wastes from process operations form the local subsurface solids. The solids composing the groundwater-bearing unit at Site YYZ were reactants (oil), byproducts (tar, coke) and wastes (gas purification box waste) used and produced during 100 years of gas manufacture operations at this site. The in situ groundwater transfer and reaction processes acting upon aromatic hydrocarbons in the subsurface at this site were hypothesized by comparing the groundwater fingerprints of individual compound concentrations to measured aqueous concentrations of these hydrocarbons in coal tar-equilibrated water. In general, the groundwater concentrations agreed with tar-equilibrated aqueous concentrations, indicating the source of aromatic hydrocarbons in the groundwater was equilibrium dissolution of the nonaqueous phase liquid tar. The first field evidence of colloid-enhanced solubilization of hydrophobic organic colloids was found at Site YYZ. At some monitoring wells, groundwater polycyclic aromatic hydrocarbon (PAH) concentrations were greater than measured for aqueous equilibrium with tar by a factor which increased with compound hydrophobicity. Two thirds of the PAH mass in excess of dissolved solubility was associated with particles that could be settled from solution over 5 months. The remaining excess PAH mass was associated with 4 mgc/L suspended organic carbon that was stable over 5 months, but could be precipitated at pH1, suggesting that PAHs were associated with humic acid-like molecules. About 5 mgc/L of humic and fulvic acids were present in the groundwater at all monitoring wells sampled. The presence of colloids in the groundwater will increase the off-site flux of hydrophobic PAHs over flux estimates assuming only dissolved equilibrium with coal tar. Evidence of bioattenuated xylene, naphthalene and methylnaphthalene concentrations was found at the shallow monitoring wells. The compound depletion patterns and groundwater ion concentrations were consistent with aromatic hydrocarbon removal by sulphate reducers. Biodegradation acted to decrease the off-site flux of these compounds, relative to tar-water equilibrium at Site YYZ. Solid-water partitioning to carbon-containing anthropogenic fill solids isolated from Site YYZ was also quantified. The overall partition coefficients for fill solids mixtures were described by summing the sorption contributions of the individual materials, using sorbentspecific partition coefficients. Predictions of overall partitioning made on a carbon basis, assuming a natural organic carbon partition coefficient, were up to two orders of magnitude ir~i---. ~-l-.-~ rruluxrri~---~ -~---r^r*--rxr~c . -I-~~I*-L--~LII--^-^X-II-IP~ II___I__I__I .-.-1~--~-~~~~~--~11~1~