Estuarine saltmarsh sediments as a repository of trace metals from long-term mining activities in Asturias (Northern Spain)

The Nalón estuary is located in the central-southwest part of the Asturias coast line (North of Spain). This area acts as a coastal outlet for waters and sediments from the main hydrological network of the North of Spain, the Nalón watershed. In the watershed, an intense mining activity has been developed from the middle of the 19th century to the current days (e.g. coal, mercury, gold). As a consequence, significant amounts of trace metals were introduced in the river channels and transported to the estuary area. A 6 km long and meandering narrow channel characterize this estuary. Several salt-marsh areas have developed at both estuary banks. Two of these salt-marshes, not affected by land uses, were selected for this study. The aim of the present work is a preliminary assessment of the metal enrichment in the salt- marshes sediments. For this purpose, several cores were collected in the non-active parts of the salt-marshes surface. Each core was subsamples and analyzed for grain size by laser equipment, whereas the fine muddy component (<63 μm) was subjected to X-ray fluorescence analysis (XRF). Results show that the fine particles (mainly silt) are the predominant grain-size fractions in the sediments. In addition, grain-size variability along the cores displays differentiated deposition rates comparing the sampling sites near the water line and those located inland; this fact can be attributed to flood events, and this is reflected in the highest content of coarse fractions in the cores near the water line. Significant amounts of As (up to 119 μg g-1), Pb (up to 1066 μg g-1) and Zn (up to 239 μg g-1) in the upper core section (0-40 cm), suggest important anthropogenic contributions. In the same section, concentration variability for Cr, Cu, Fe and Mn is low, and this is interpreted as due to lithogenic origin. In the lower core section (> 40 cm), metals(oids) concentration remains fairly constant, thus suggesting to be suitable baseline values for the area. Considering these baseline valuesand using Titanium concentrations as normalized element, the calculated Enrichment Factor (EF) for As (up to 3.3 ), Pb (up to 116.9) and Zn (up to 3) showed that long term mining activities have significantly contributed to the metal load of the Nalón estuarine salt-marsh sediments.