Biogeochemical controls on the persistence of saturated and aromatic hydrocarbons in urban harbor sediments

Due to concern over the persistence and toxicity of hydrocarbons in harbor sediments, an investigation was initiated to examine rate limitations to the bacterial degradation of sediment-associated saturated and polycyclic aromatic hydrocarbons (PAHs).; The effects of sediment resuspension on the mineralization rate and extent of 14C-phenanthrene were examined in an experiment where sediments were resuspended at different frequencies in a flow through microcosm system. Mineralization rates increased with increasing resuspension and a model based upon the duration and extent of oxygen exposure and aerobic and anaerobic degradation rates determined in separate oxic/anoxic slurries, predicted the results.; The rate and extent of aromatic and saturated hydrocarbon degradation was related to desorption rate and extent in sediments collected from greater New York Harbor and Long Island Sound. Degradation was measured as the fractional disappearance of compound in oxic slurries, and desorption as fractional desorption onto hydrophobic XAD resin in oxic incubations of sediment, water and resin. Geochemical source indicator ratios revealed a mixed source of hydrocarbons at all sites, with two sites having a greater oil-derived signature. Differences were observed in desorption rate and extent, as well as degradation rate and extent, between sites. The extent of degradation generally correlated with the extent of desorption for PAHs, with the exception of one site where the absence of degradation accompanied by nearly complete desorption was attributed to sediment toxicity. In contrast, there was little to no relationship between desorption and degradation of alkanes, which was consistent with the mode of bacterial uptake which (unlike for PAHs) has been shown to occur from the sorbed phase.; Through modeling and further experimentation, desorption-limited PAH degradation was examined in a number of scenarios encountered in laboratory and field studies. A comparison of desorption rates (measured or predicted) and degradation rates showed that for spiked experiments desorption was mostly not limiting, while in field-aged sediments, it was. Comparisons of radial diffusion model predictions of desorption, to porewater-normalized mineralization rates under different conditions such as soot content, sediment-water ratios, particle aggregate size and compound hydrophobicity provided a “first step” toward a comprehensive, time-varying linked desorption-degradation model.