Trace metals, arsenic and lead isotopes in dissolved and particulate phases of North Australian coastal and estuarine seawater

Total dissolved (<0.45 μm) metal concentrations (Cd, Co, Cu, Fe, Pb, Ni and Zn), analysed by an ammonium pyrrolidine dithiocarbamate (APDC) solvent extraction/Hg exchange back-extraction method, arsenic concentrations and lead isotope ratios (208Pb/206Pb and 207Pb/206Pb), were determined on seawater samples from nine tropical north Australian estuarine and coastal areas. The low metal concentrations (e.g. av. Zn 38–154 ng/l and Pb 3.9–13.3 ng/l) and low molar Zn/Cd ratios (av. 17–65) were at near-pristine levels. There were only few strong element/salinity and inter-element correlations, which is attributed mainly to the restricted salinity range sampled (91% of samples had salinity >20). Metal (APDC-extractable) and arsenic concentrations and lead isotope ratios were also determined on unfiltered samples (dissolved plus particulate phases). The APDC-extractable metals method provided a measure of labile metals in the particulate phase. There were strong positive correlations between suspended particulate matter (SPM) and unfiltered metal concentrations, except for Cd. The slopes of the trendlines approximated the average labile metal concentration in the SPM. In the shallow Bynoe and Norman Riversʹ estuaries, which have extensive depositional banks of fine-grained sediments, SPM consisted mainly of re-suspended local bottom sediments. In these estuaries, the ratio of labile metal concentration in SPM to acid extracted total metal concentration in the <63-μm grain size fraction of bottom sediments was in the order Pb>Cu>Zn≈Ni. A more direct measure of metal lability was obtained from the South Alligator and Adelaide Riversʹ estuaries by comparing APDC-extractable concentrations in SPM with total concentrations derived by analysis of acid-digested SPM retained on 0.45 μm membrane filters. The order of lability in these estuaries was Pb>Zn>Cu≈Ni. Distribution coefficients (KD) for metals between the particulate (>0.45 μm) and dissolved (<0.45 μm) phases showed a general order of log KD values for all sites of Fe>Pb>Zn>Ni≈Cd≈Cu. This is in contrast to the order Cu>Cd>Zn>Ni commonly reported in polluted estuaries and rivers. The north Australian data showed strong log KD versus SPM correlations for Fe and Cu. The Pb isotopic composition of both filtered and unfiltered samples was similar to previously recorded values for oceanic seawater (208Pb/206Pb=2.02–2.10). In contrast, the 208Pb/206Pb ratios of Pb–Zn ores in the region range from 2.21 to 2.25 allowing the use of Pb isotope data as sensitive indicator of any future impacts from mining developments. Variations in the Pb isotope ratios of unfiltered samples (dominated by Pb in SPM) between some estuaries were found to reflect different compositions of geological units in their catchments.