Field Measurements and Modeling of Ebullition-Facilitated Flux of Heavy Metals and Polycyclic Aromatic Hydrocarbons from Sediments to the Water Column

Abstract

Gas ebullition-facilitated transport of metals and polycyclic aromatic hydrocarbons (PAHs) from sediment was investigated in 14 urban waterway locations. Gas ebullition varied widely over four seasons (range 2–450 mmol m^–2 d^–1, mean 140 ± 90 mmol m^–2 d^–1) and was highly temperature dependent. Ebullition-facilitated metal fluxes were large: 50 ± 13 mg m^–2 d^–1 (Fe), 2.6 ± 0.71 mg m^–2 d^–1 (Zn), 1.5 ± 0.28 mg m^–2 d^–1 (Pb), and 0.19 ± 0.06 mg m^–2 d^–1 (Cr). Ebullition-facilitated PAH fluxes were also large: 0.61 ± 0.27 mg m^–2 d^–1 for anthracene, 0.65 ± 0.28 mg m^–2 d^–1 for benzo­[a]­pyrene, 0.72 ± 0.28 mg m^–2 d^–1 for chrysene, 3.51 ± 1.23 mg m^–2 d^–1 for fluoranthene, 0.23 ± 0.08 mg m^–2 d^–1 for naphthalene, 3.84 ± 1.47 mg m^–2 d^–1 for phenanthrene, and 2.46 ± 0.86 mg m^–2 d^–1 for pyrene. The magnitude of these fluxes indicates that gas ebullition is an important pathway for release of both PAHs and heavy metals from buried sediments. Multivariate regression analysis of the in situ gas ebullition flux and ebullition-facilitated contaminant flux suggests that metal transport likely is due to sediment particle resuspension, whereas PAH transport is due to both contaminant partitioning to gas bubbles and to sediment resuspension. These results indicate that assumptions regarding the natural recovery potential of ebullition-active sediments should be made with caution