Variability of organic contaminants in inter-tidal sandflat sediments from Manukau Harbour, New Zealand

Abstract

Surficial sediments from five stations on inter-tidal sandflats within Manukau Harbour, New Zealand were sampled using a systematic sampling design to give three representative bulked replicates per station. Particle size analysis showed the stations to be similar, with poorly sorted fine to medium sand and mud contents of 21–26%. Sediment readily oxidizable carbon contents were low (0.22–0.73%). These parameters showed similar variability (CVs of 11–50%), both within and between stations, confirming their visual similarity. Organochlorine insecticides, PCBs (23 congeners) and polynuclear hydrocarbons (PAHs, 10 fluorescent compounds) were extracted from the sediments and determined to detection limits of 0.02–0.05 ng/g. An extraction procedure using a ternary solvent system (acetone/hexane/water) gave equivalent results to a standard method but was more rapid, used less solvent, and gave lower blanks. Levels of chlorinated organics were low. Highest levels for all contaminants occurred at one station (total PCBs 1.6 ng/g, DDT + metabolites 18 ng/g, dieldrin 0.38 ng/g lindane 0.34 ng/g, cis- + trans-chlordane 0.38 ng/g, total PAHs 3,740 ng/g). PAH levels were substantially lower at other stations (40–90 ng/g). Differences for other contaminants were not great, although there was a significant gradient of chlordane concentration between the stations. Variability in concentrations of contaminants was large both within and between stations (CVs of 2–126%). Normalizing the concentrations to organic carbon did not markedly alter the above trends, but demonstrated significant chlordane and PCB gradients, and increased the variability within many of the stations. The results were interpreted as indicating multiple contaminant sources, with major contributions from the treated sewage outfall and a more heavily contaminated industrialised inlet at the head of the harbour. The contaminants were not strongly associated with any particular sediment characteristic, with only PCBs strongly correlated with carbon levels.