Contaminant pathways in Port Curtis: Final report - OzCoasts

Contaminant pathways in Port Curtis: Final report3: Sediment contaminantsThe lack of 210 Pb excess count in the upper layers of the cores may be attributedto sediment mixing, a phenomenon which has been observed in estuarinesediment cores (Pfitzner et al. 2004). Longer cores would probably have providedbetter estimates of the sedimentation history.Taking the year 1958 as the time when 137 Cs activity above backgroundconcentrations was detected in the southern hemisphere (Pfitzner et al. 2004),it appears that at least 28 cm of sediment was deposited over the past 47 years(core sampled in 2005). This gives an estimated sediment accumulation rate ofat least 0.60 cm/y. This is roughly one-third of the sediment accumulation rate ofabout 1.9 cm/y (also based on 137 Cs activity) for a 1.2 m sediment core fromoffshore Keppel Bay, Central Queensland sampled in 2000 (V. Vicente-Beckett,unpublished data). This difference is not surprising considering the difference inthe hydrodynamics and nature of human activities between the two locations.It is quite certain, however, that the sediments were post-1958 because of thepresence of 137 Cs activity. This would be around the start of industrialisation ofthe Gladstone area.3.3.4 Stable lead isotope ratios (PbIRs)The variety of lead ores used in various industrial applications has led to theintroduction of lead in the environment with distinct relative isotopic abundances.The relative ratios of the four stable lead isotopes 206 Pb (from radioactive decayof 238 U), 207 Pb (from 235 U decay), 208 Pb (from 232 Th decay) and 204 Pb (no knownradioactive parent) depends upon the age and U/Pb and Th/Pb ratios of the orefrom which the lead was derived. Very old ores such as those from Broken Hill,Australia contain small amounts of radiogenic lead isotopes; younger ores derivedfrom high U/Pb sources such as that mined in Missouri have much higherproportions of 206 Pb, 207 Pb and 208 Pb relative to 204 Pb (Chillrud et al. 2003).PbIRs of sediments from Port Curtis estuary (plus a grab from Awoonga Dam)and selected soils/sediments from the lower Fitzroy catchment are plotted inFigure 3.6. The mean PbIRs for all Port Curtis sediments were 208 Pb/ 206 Pb =2.0758 ± 0.0111 and 207 Pb/ 206 Pb = 0.83676 ± 0.0068. The observed mean208 Pb/ 206 Pb ratio is comparable to the value of 2.0635 measured for near-pristineestuarine and marine tropical northern Australia (Munksgaard et al. 2003). Themodelled present-day average crustal values of these ratios was reported byStacey and Kramers (1975) as 208 Pb/ 206 Pb = 2.06058 and 207 Pb/ 206 Pb = 0.83572.This point was included in Figure 3.6, as well as the lead isotope ratios for MountIsa (Queensland) lead deposits and those for oceanic sediments (Atlantic andPacific) (Stacey & Kramers 1975).29