Concentrations and spatial distribution of trace metals in mangrove sediments from the Brisbane River, Australia

Fifty surface sediment samples were collected from 12 transects through a mangrove woodland near the mouth of the Brisbane River, Australia. Samples were analyzed for 10 trace metals; measured concentrations (microg g(-1)) were: Cu 3.1-34.1; Pb 7.7-84.7; Zn 40.8-144.0; Ag nd-2.8; Cr 7.6-116.8; V 16.4-57.9; Mo 7.6-82.6; Ni 2.4-57.6; As nd-13.0; and Cd nd-2.0. Coefficients of variation ranged from 22-153%. Sediments were moderately contaminated, with Mo being enriched by up to thirty times background levels. Spatial variation was considerable, and canonical trend analysis showed concentrations of all metals but Ag and Cr tended to increase downshore, suggesting tidal deposition was important in determining metal concentrations. Silver and Cr concentrations appeared to be a consequence of contamination from landward point sources. An investigation of different random sampling intensities showed the spatial variation required that many samples be taken for a satisfactory assessment of metal burdens. Even so, mean values were not a good indication of the possible environmental effects of the metal contamination. Since metal concentrations increase downshore in inter-tidal environments, a more appropriate sampling strategy would be to take a downshore series of samples normal to the shoreline.