Image analysis
The satellite imagery used in this study was of Landsat 5, Landsat 7, and Landsat 8 satellites (30x30 resolution) obtained from Google Earth Engine (GEE) website. Different satellites were necessary to obtain images from different years, so Landsat 5 was used for 1996, 1999, and 2002 maps, Landsat 7 for 2005, 2008, and 2011 maps, and Landsat 8 for 2013, 2016, 2019, and 2022 maps (Chandler et al., 2009). In recent times Google Earth Engine has become an important database of earth observation data and land cover classification in many mining-related studies and other applications such as vegetation mapping and monitoring, agricultural application, disaster management, and earth science (Gbedzi et al., 2022). In addition to these satellite images, information obtained from the remnants of Pampean grassland identified and delineated with Google Earth Imagery was added.
Images from 27 October to 27 May in each year of study were used to analyze the different land uses. This date was chosen due the spring/summer season is when the vegetation associated with the quarry is in bloom. NDVI index (Normalized Difference Vegetation Index) in GEE was applied, paying special attention to the threat posed by quarries in the Tandilia Mountains. The NDVI index has been used to monitor characteristics of vegetation cover and health status and detect changes in an ecosystem over time (Paruelo, 2008; Tong et al., 2016). Calculation of NDVI for a given pixel always results in a number that ranges from minus one (-1) to plus one (+1): bare soils (quarries in that case), giving a value close to zero, and very dense green vegetation have values close to +1 (Musa & Jiya, 2011). In general, land covers are often mixtures of several types, so even fine-resolution remote sensing data do not measure pure spectra but mixed reflectance of vegetation and non-vegetation, making it challenging to identify exposed bedrock (Yue et al., 2012). For this reason, the satellite images were then processed with QGIS software.
The image was processed using QGIS 3.24.2, a Geographic Information Systems (GIS) software. QGIS allows the input, manipulation, analysis, and presentation of data and information related to a place on the earth’s surface and therefore works with geo-referenced points (Ershad, 2020). This tool could locate and characterize quarries (dimensions and associated threats). To do this, a visual interpretation of land use was made: quarries with a lot of vegetation or water are considered inactive, and quarries with bare soil are active. The resulting maps show the evolution of the quarries during the years of the study. Some characteristics of the quarries, such as the state of their activity, only were reliable in the current year (2022). The quarrying activity was confirmed from the information obtained in the Mining Cadastre of the province of Buenos Aires (Subsecretaría de Minería, 2023). Moreover, the area of each remnant and the total amount of Highland grassland were calculated using the QGIS software.