GRSG 35th Conference 2024 Abstract
Title: Multi-temporal mapping of iron minerals in mining-affected ramblas: A Sentinel-2 analysis of sedimentation dynamics in the Sierra Minera de Cartagena-La Unión
Author: Inés Pereira Rodríguez
Organisation: University of León
Located in the southeast of the Iberian Peninsula, the Sierra Minera Cartagena-La Unión, has been mined for silver (Ag), lead (Pb), zinc (Zn), copper (Cu) and iron (Fe) since Phoenician and Carthaginian times until its closure in 1991. Over 2,500 years, the region has faced significant environmental impacts, particularly in the latter decades due to the large-scale of the mining facilities targeting Pb, Zn and Fe. As a result, numerous silt ponds and mining tailings are scattered throughout the area, especially in the headwater sectors of the ephemeral watercourses known as “ramblas”.
These dry riverbeds can experience sudden and intense flows during heavy precipitation events, leading to the transport of contaminated sediments and heavy metals downstream.
The eroded sediments and heavy metals are transported into surrounding areas, affecting agricultural lands and posing a risk to the coastal lagoon Mar Menor, a protected area under different national and European figures. Some of these materials coming from the mining tailings are spectrally active, making them detectable and mappable through remote sensing techniques. At the same time, secondary iron oxides, which form as a result of oxidation processes, exhibit distinct spectral signatures, particularly in the Visible and Near-Infrared (VNIR) region
Therefore, this research focuses on the multi-temporal mapping of sedimentary processes on the Rambla del Beal of the Sierra Minera de Cartagena. Sentinel-2 satellite images from years 2017 to 2023 were analyzed using Google Earth Engine (GEE), a cloud-base computing platform, allowing enhanced spatial and temporal analysis of fluvial dynamics and sediment transport. The study aimed to map the spatial distribution of two ferric iron oxides—hematite and goethite—and the iron hydrous sulphate mineral jarosite. All of them are formed from the oxidation of pyrite.
Their formation is influenced by environmental conditions such as sediment concentration, pH, moisture and temperature. To identify and map the spatial distribution of these iron minerals, a band ratio and the Spectral Angle Mapper (SAM) algorithm classification were applied. These maps were compared with the precipitation record of the study area.
The results reveal a distinct seasonal pattern in the distribution of these iron minerals, with variations in mineral prevalence depending on the environmental conditions. Hematite was found to increase its presence during dry periods, likely due to its formation under oxidation conditions. In addition, the transformation of goethite and jarosite into hematite over time following precipitation events further contributed to its dominance during dry periods. This transformation is a well-known process that occurs as oxidation conditions domain, promoting the stability of goethite and jarosite into hematite in arid conditions.
Goethite and jarosite, on the other hand, were more prevalent after rainfall events, due to their need of alternating redox conditions. Goethite was more commonly found after moderate precipitation events, as it forms in environments with sufficient moisture but not excessive water saturation. Its presence reflects conditions where iron remains in a slightly oxidized state. Jarosite, however, reached its highest concentrations after more extreme torrential events, where rapid and significant changes in moisture levels provided the ideal conditions for its formation. Jarosite forms in highly acidic environments with high sulphate content, conditions that can be triggered by intense rainfall and flooding. These extreme events create an environment in which jarosite can rapidly precipitate, especially in areas, like Sierra Minera de Cartagena-La Unión, where sulphide-rich mining residues are exposed to fluctuating redox conditions.
By combining the spectral images from Sentinel-2 with land use maps, the study identified areas most affected by the deposition of mining residues. Agricultural lands were found to be particularly impacted by hematite deposition. In contrast, the rambla channels and surrounding grasslands exhibited higher concentrations of goethite and jarosite, especially after rainfall events.