Black sand mining can increase the frequency and the magnitude of land subsidence, which makes local communities particularly vulnerable to seasonal typhoons, climate change and sea level rise. The aim of this research is to use remote sensing to measure the scope of black sand mining and associated environmental impacts.
Magnetite is a valuable commodity used in the production of steel. In the Philippines magnetite naturally occurs in black sand in rivers and along much of the coast, and is extracted through sand mining and processing.
The researchers inspected satellite imagery corresponding to areas of suspected mining activity. Using the coordinates of identified mining sites, they analyzed the magnitude of ground displacement as a measure of land subsidence using InSAR (Interferometric Synthetic Aperture Radar) and calculated the approximate time for these sites to be submerged below sea level. Out of the 20 sites surveyed only 12 were not decorrelated (showing coherence), meaning the ground did not change too much too abruptly and the satellites’ acquisitions could be compared through time.
Results and Policy Implications
Out of the 12 sites with coherence, 7 showed any subsidence, ranging from 1.3 cm/yr to up to 4.6 cm/yr. This corresponded to a vertical ground displacement reaching up to 5.5 cm/yr. The projected time for sites to sink below sea level was calculated for five of the seven sites showing subsidence. Sites with 3 cm/yr and 3.9 cm/yr rates of subsidence were projected to be underwater in 66-98 year and 40-80 years, respectively. Sites at 4.3-4.6 cm/yr subsidence rates were projected to be underwater in approximately 30-60 years. These rates took into account the projected 1.3 cm/yr mean rate of sea level rise outside Manila. Furthermore, InSAR revealed subsidence in 6 additional sites not previously identified as mining sites, showing that InSAR is a good complement to other methods of identifying mining sites.