Mercury in Coastal-Marine Environments: A Case Study in the Port of Llanes, Asturias (Northern Spain)

Mercury is one of the most problematic elements in the environment due to its persistence, bioaccumulation, and transfer across different trophic levels of the food chain. One of the environments where this element poses the greatest challenges is the coastal-marine ecosystem, owing to its rich biodiversity. Within these environments, anthropized estuarine areas, such as port zones, present increased issues because of the significant accumulation of sediments that act as reservoirs for this contaminant.

In Asturias (northern Spain), the Port of Llanes, located in a small town in the eastern part of the region, has shown elevated mercury concentrations in sediments over the last 20 years, requiring detailed monitoring and investigation. This study provides an initial approach to the issue, examining mercury concentrations, dispersion, associations, and speciation in sediments collected from the port.

The results revealed that mercury concentrations in the sediments ranged from 0.07 to 2.98 µg g⁻¹, with higher concentrations associated with finer-grained sediments and increased organic matter content. These concentrations exceed contamination thresholds established by Spanish legislation regarding port dredging, and certain samples tested exhibited toxicity levels harmful to organisms. Granulometric fractionation of the samples showed that 70% of the mercury was associated with particle sizes <63 µm, while up to 85% was associated with particles <125 µm.

Thermal speciation analysis was conducted to identify the predominant mercury species and their potential association with the observed toxicity. The results indicated three main mercury species in the sediments. Mercury associated with oxides was the most abundant, followed by mercury bound to organic matter. Finally, mercury associated with sulfur (e.g., cinnabar) was the least abundant of the three.

These findings validated the results of mercury granulometric fractionation, as the identified species were closely linked to finer sediment fractions. Furthermore, they allowed the formulation of a hypothesis regarding the origin of mercury in these sediments. In this context, since there is no identifiable local source of mercury release into the coastal environment, the hypothesis of a natural origin for these mercury concentrations gains relevance. Studies on groundwater in the area have reported significant mercury levels, suggesting that the geological substrate is the most likely source of these anomalous concentrations.

Future research should focus on conducting a more detailed study of the immediate coastal strip surrounding the port to validate these preliminary findings.