Biogeochemical characterization in the San Quintín mining area based on the IDV Index obtained through RPAS technology

Environmental issues resulting from abandoned mining activities currently represent a significant challenge due to their persistence in soils, water bodies, and surrounding ecosystems. The San Quintín Mining District, located in the Valle de Alcudia (Spain), has generated substantial local geochemical anomalies of lead (Pb), zinc (Zn), calcium (Ca), manganese (Mn), iron (Fe), and copper (Cu), among other elements. This mid-20th-century mining operation has left a legacy of contamination affecting not only the soils in the area but also the surrounding vegetation, including agricultural and grazing lands.

Numerous studies have emphasized the critical role of plants as hyperaccumulators, capable of absorbing heavy metals such as Pb, Zn, Ca, Mn, Fe, and Cu from the soil. In this study, a biogeochemical assessment was conducted to determine the concentrations of these metals in soils and plants, using Quercus ilex (holm oak) as the prototype species due to its ubiquitous presence throughout the mining district.

The study employed two key parameters: the Bioaccumulation Factor (BAF), which measures the plant's capacity to accumulate heavy metals, and the Normalized Difference Vegetation Index (NDVI), used to evaluate vegetation health through multispectral imaging. The methodology combined multielement analysis via Energy-Dispersive X-ray Fluorescence (ED-XRF) for determining trace elements (EPTs) and RPAS (Remotely Piloted Aircraft Systems) technology to capture multispectral images for NDVI calculation.

The results reveal significant variability in soil geochemistry, with high levels of Fe₂O₃, Zn, Cu, and Pb, indicating localized contamination sources. Quercus ilex demonstrated varying absorption capacities depending on its specific location, with Mn and Fe showing the highest concentrations, reaching up to 26.2 mg/kg and 11.8 mg/kg, respectively, indicating substantial accumulation in certain areas. Pb and Zn concentrations displayed high coefficients of variation in soils near waste piles and tailings, supporting the hypothesis that these sources contribute to soil contamination and subsequent bioaccumulation in local vegetation.

Additionally, a deficiency in Mn, an essential nutrient for chlorophyll production and photosynthesis, could reduce chlorophyll content, thus lowering the plant's photosynthetic efficiency. This is reflected in higher NDVI values, which coincide with increased Mn concentrations in soil and plants. Finally, a negative correlation was observed between NDVI values and relatively low concentrations of metallic elements, highlighting the complex interactions between soil composition, vegetation health, and environmental contamination.