The double threat of climate change for estuaries polluted by industry

Ancient sediments from mining and heavy industry are coming to the surface as floods become more frequent. This is an unexpected effect of global climate change.

This new problem has been investigated in Spain by a team led by Jon Gardoki, from the Department of Geology at the University of the Basque Country (UPV/EHU).

The results of the study carried out by Gardoki and his colleagues clearly show the danger that in some places sediments from old toxic wastes are stirred up and brought to the surface by floods, which are becoming more common in many regions of the world due to global climate change.

The study focused on the specific case of the Nalón River estuary in Asturias, Spain, as this enclave is particularly sensitive due to its history linked to coal and mercury mining.

“One of the novelties of the work is the reconstruction of the historical transformation of a highly polluted estuary. These actions, in a context of global climate change and widespread sea level rise, are very important since they help to know and understand the future environmental trajectories of historically polluted coastal areas and to propose coastal management policies based on scientific criteria,” says Gardoki, lead author of the work together with the Harea-Coastal Geology research group of the Faculty of Science and Technology.

According to Gardoki, “climate change is altering the frequency and magnitude of hydrological processes, including floods.” Floods cause sediments containing pollutants to rise to the surface and move. “In the case of the Nalón estuary, the marshes that have acted as ‘stores’ for very high concentrations of mercury are eroding, which could cause this highly toxic element, which was previously inert, to be mobilized and become integrated into the ecosystem, with the negative consequences that this entails,” reveals the researcher.

The study has made it possible to identify the potential environmental risk that the incipient erosion of the estuary’s marshes may pose: “we have analysed both the natural processes (historical floods and intense river action) and those caused by human activity (discharges linked to coal and mercury mining), which have defined the area since the end of the 19th century and which have conditioned its environmental, morphological and geological evolution over the last 150 years. The environmental transformation of this estuary has been recorded in its sediments and from a geological perspective we can read them as if they were chapters in a book and reconstruct their history in detail,” explains the geologist.

Gardoki insists on the need to carry out research like this to recognise the vulnerability of highly polluted coastal ecosystems: “This type of research helps us to understand how these ecosystems of high ecological value can react to climate change. In addition, it allows us to develop adaptation strategies that are of special interest for historically polluted estuaries, such as Bilbao or Pasaia.”

Jon Gardoki investigating. (Photo: UPV/EHU)

This research began in 2019 as a project to study the environmental transformations of the coastline of northern Spain and Portugal. Among the selected sites is the Nalón Estuary, due to its particular mining history since the mid-19th century. “Currently, we continue to investigate other highly polluted estuaries in the north of the peninsula such as the Vigo or Bilbao estuaries, which the group monitors every three years, with a project funded by the Basque Government.”

The study is entitled “Recent environmental and morphosedimentary evolution of the mining-impacted Nalón Estuary (Asturias, N Spain): Disentangling natural and anthropogenic processes”. It has been published in the academic journal Science of the Total Environment. (Source: UPV/EHU)