Monitoring Rock Quarrying using Multi-temporal Sentinel-2 data in Google Earth Engine

The southwestern part of the Indian peninsula, which is part of the Western Ghats, is underlain by Precambrian crystalline rocks that have been extensively mined for construction for decades, seriously affecting environmentally sensitive areas in a densely populated state like Kerala, India. To assess the extent of environmental damage, a detailed field survey carried out in 2021 revealed that there are 1646 quarries across the three districts of Pathanamthitta, Kollam, and Thiruvananthapuram in the state of Kerala. In the present study, spatiotemporal monitoring of quarrying activities in the selected quarries, about 81 of which fall under the category of “active quarries,” was carried out using Sentinel-2 satellite data on the Google Earth Engine (GEE) cloud platform. Quarrying removes vegetation cover and exposes bare rock and soil, which can be accurately identified using the Normalised Difference Vegetation Index (NDVI) and the Bare Soil Index (BSI) calculated using high-resolution Sentinel data. After the field visit, the polygon layer covering the quarry area was digitised from Google Earth imagery for the year 2021. Its corresponding NDVI and BSI were computed from the temporal Sentinel-2 data between 2016 and 2025. Year-wise means were used to analyse changes in vegetation cover resulting from active quarrying. A clear inverse relationship between NDVI and BSI was observed during the study period. The year-wise mean NDVI decreased from 0.38 to 0.23 by 2020 and subsequently increased to 0.33 by 2025, while the year-wise mean BSI remained relatively high during the initial years, indicating exposure of bare rock and soil, then decreased from 0.12 to 0.05. The overall trend indicates a slight, progressive increase in vegetation within the polygons after 2020, suggesting reduced quarrying activity on average across all 81 quarries. But an examination of the individual quarry polygons suggests that many are still under active quarrying activities. However, the trend supports the “Active” status of the 81 quarries during 2021. This study provides baseline information to policymakers and the mining industry for long-term annual monitoring of quarries, enabling the necessary measures for eco-friendly, safe, and sustainable mining in the future.