Identifying acid lakes and associated rock exposure in glacial retreat zones in the Peruvian Andes using Landsat 8 imagery

This study offers a novel satellite-based methodology for identifying and monitoring acidic lakes in the Cordillera Blanca, Peru, focusing on the environmental impact of acid rock drainage (ARD) due to glacier retreat. We utilized Landsat 8 imagery and field-collected pH data from 28 glacial lakes, provided by the Glaciology and Water Resources Unit of the Peruvian National Water Authority (ANA, Peru). Integrating this with geological mapping of the sulfide-rich Chicama Formation (Js-Chic), we identified a correlation between exposed Js-Chic areas and lake acidity. Our approach involves the Spectral Angle Mapper (SAM) method to map exposed Js-Chic areas and an empirical spectral equation to infer lake pH. We found a distinct pattern in highly acidic lakes (pH < 4), with a greater Js-Chic exposure (> 60 hectares) correlating with higher acidity. These lakes, predominantly in the southern Cordillera Blanca, also showed distinct spectral signatures in the first three Landsat 8 bands, forming the basis of our pH estimation method. Further, we observed that lakes with higher acidity are associated with greater glacial retreat and higher vegetation quality in their basins, as indicated by the Normalized Difference Vegetation Index (NDVI). Our findings highlight the potential of remote sensing for ARD monitoring in mountainous regions and suggest a link between glacial retreat, Js-Chic exposure, and lake acidification, which could have significant implications for water security.