Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin

Sediments (568) and suspended particulate matter (SPM. 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (A], Fe. K), trace metals (Mn. Pb). and (206)Pb, (207)Pb ratios using XRF, ICP-OES, ICP-MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e,g. Mn: Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected) <recent tidal flat sediments (RTF) <Helgoland Island M Lid hole sediments (MH) <nearshore SPM (SPM concentration > 5 mg l(-1)) < offshore SPM (<5 mg l(-1)). Besides pollution, RTF contain elevated amounts Of natural Pb-rich materials (K-feldspars and heavy minerals due to a man-made high-energy environment (dike building) in comparison to HTF. (206)Pb/(207)Pb ratios of RTF (1.192 +/- 0.019) are similar to the local geogenic background. determined from HTF (1.207 +/- 0.008). In contrast, Pb isotope ratios of nearshore SPM (1.172 +/- 0.007) and offshore SPM (1.166 +/- 0.012) show a distinct shift towards the anthropogenic/atmospheric signal of 1.11-1.14. This difference between RTF and SPM supports the assumption of low deposition rates OF fine material in the intertidal systems. As the (206)Pb/(207)Pb ratios of SPM do not reach the pure anthropogenic signal. the adsorbed Pb fraction was examined (leaching). However, the leachates also contained large amounts of geogenic Pb (SPM approximate to40%. recent sediments approximate to60%). The authors assume that the uptake of natural Pb occurs in nearshore waters, presumably in the turbid intertidal systems, Possible sources for dissolved Pb are mobilisation during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input. Comparatively small parts of SPM leave the coastal water mass and reach the open North Sea. This process therefore leads to a decontamination of the tidal flat sediments. Due to more pronounced atmospheric input, the offshore SPM becomes enriched in amhropogenic Pb as indicated by decreasing (206)Pb/(207)Pb ratios with increasing distance from the coast. (C) 2002 Elsevier Science Ltd. All rights reserved.