Anthropocene footprint in the Nalón estuarine sediments (northern Spain)

Four boreholes drilled at depths ranging from 9 to 27m and retrieved from the Nalón estuary (Asturias, northern Spain) were investigated to assess the impact of the historical human footprint on the sedimentary sequence. Analytical measurements of grain size, geochemical composition (major and trace elements), and stable Pb isotopic ratios were performed. Two different sections of the sedimentary profile were identified throughout the sedimentary sequence using Pb isotopic ratios and then corroborated with grain size characteristics and As, Hg, Pb and Zn concentrations. The upper sediment layer of the boreholes (variable depth ranging between 3 and 9 m) exhibited an increase in 208Pb/206Pb ratios and a decrease in 206Pb/207Pb ratios, whereas, in the deepest layers, these ratios remained unchanged. These variations reflected the anthropogenic input of Pb into estuarine sediments, which can be attributed to mining activities when compared to Pb isotopic ratios from coal mined in the region. The anthropogenic input can be also noted on the basis of the variability of the trace element concentrations with depth. The enrichment factors of As (max 7.5), Hg (max 45.4), Pb (max 10.3), and Zn (max 8.6), calculated using local geochemical background values helped distinguish the impact of intense anthropogenic activities, which can be attributed to mining exploitation in the Nalón River drainage basin. Despite the closure of several of these mines during the second half of the 20th century, geochemical characteristics have revealed the ongoing input of As, Hg, Pb, and Zn into the coastal zone. Thus, the legacy of contamination from these elements, mainly from leaching slags and tailing dams, is still being recorded in estuarine sediments and requires careful scrutiny on the part of local authorities in terms of its possible effects on the aquatic biota.