Inhomogeneous distribution of polycyclic aromatic hydrocarbons in different size and density fractions of contaminated sediment from Auckland Harbour, New Zealand: an opportunity for mitigation

Polycyclic aromatic hydrocarbons (PAHs) in sediment from Auckland Harbour (New Zealand) are not distributed evenly throughout bulk sediment, but highly concentrated in coarser, low-density fractions. Concentrations of 24 PAHs, measured in sediment that was separated into six size fractions that were furthermore separated into two density sub-fractions by flotation in sodium-polytungstate solution (ρ=2.15 g cm−3), varied between 4–103 μg g−1 dw among grain size fractions and 2–998 μg g−1 dw for density sub-fractions. Highest PAH concentrations were measured in the low density, 125–250 μm fraction. All sediment fractions had a similar relative PAH composition, dominated by >3-ring PAHs, suggesting a common pyrogenic origin. Low density material had 10–200 times higher PAH concentrations and 10–100 times higher organic carbon (OC) content, yet differences in OC content only partially accounted for variations in PAH concentration. Low density particles contributed more than 75% of the ΣPAH, while comprising only 3% of bulk sediment dry weight. This may have significant utility for contaminant mitigation efforts in Auckland Harbour.