Tracing of anthropogenic zinc sources in coastal environments using stable isotope composition

摘要

The use of zinc isotopes to trace anthropogenic sources in coastal areas has been tested in this study. We determined the stable isotopic composition of zinc in sediment cores, suspended particulate matter (SPM) and rocks collected at the Sepetiba Bay (southeastern Brazil), an estuarine lagoon heavily impacted by metallurgic activities. These isotopic signatures were compared with those from willemite ore, which represent the main mineral refined by the major industrial source of zinc. The aim was to test if this tracer system enables to identify sources and sinks of anthropogenic zinc and to reconstruct the temporal and spatial evolution of zinc contamination. The zinc isotopic compositions (expressed using the δ66Zn notation relative to the JMC 3-0749-L solution) showed significant variations in the sediment cores, the SPM, and willemite ore minerals, ranging between − 0.01 and + 1.15‰. Spatial and temporal analysis of sediments samples fit well in a model of mixing involving three main end-members: i) Terrestrial background (δ66ZnJMC = + 0.28 ± 0.12‰, 2σ); ii) marine detrital material (δ66ZnJMC = + 0.45 ± 0.03‰, 2σ); and iii) a major anthropogenic source associated with electroplating wastes released into the bay (δ66ZnJMC = + 0.86 ± 0.15‰, 2σ). Sediment cores collected in the mud flats showed high correlation between δ66Zn and zinc enrichment factors, suggesting good preservation of the isotopic records of natural and anthropogenic sources. The sediment core sampled from a mangrove wetland located in a zone impacted by the metallurgy presented levels of zinc up to 4% (sediment dry weight) and preserved the isotopic signatures of electroplating wastes, despite evidences that post depositional processes slightly changed the isotopic signatures in some layers from this core toward heavier δ66ZnJMC values (above + 1.0‰). A two component mixing model suggests contributions of this major anthropogenic Zn source up to nearly 80% during periods of electroplating activities. Our work suggests that Zn isotope compositions of sediments are reliable tracers of anthropogenic sources and, therefore, can be useful to improve environmental monitoring efforts in coastal systems.