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.