Holocene background concentrations and actual enrichment factors of metals in sediments from Ria Formosa, Portugal.

Metal history in a natural system was described since the early Holocene by analysing the preserved subsurface sediment record and present-day surface sediments. Normalized geochemical data from six sediment cores (129 samples) was compared with 28 surface samples. Pre-anthropic sedimentary environments allowed the definition of local natural Background Values. Enrichment Factors were then used for elements discrimination in terms of natural and anthropogenic inputs to the system. While subsurface results displayed a similar behaviour in all cores, pointing to largely undisturbed system, surface sediments show significant contamination, with high enrichment factors for As, Cu, Pb, and Zn. Other metal pollutants have generally low enrichment values, suggesting natural conditions. Surface results were reproduced as metal enrichment maps which evidenced anthropogenic sources in specific locations. This work reveals the importance of combining subsurface and surface geochemical data with mapping techniques in order to better evaluate the environmental quality of a natural system.