Identifying Acid Lakes and Associated Rock Exposure in Glacial Retreat Zones in the Peruvian Andes using Landsat 8 Imagery

Abstract This study presents an empirical method for identifying and monitoring acidic lakes impacted by acid rock drainage (ARD) processes in the Cordillera Blanca, Peru, using Landsat 8 images. ARD poses a threat to water security for downstream populations and has been associated with glacier retreat in various river catchments in the region. Previous research has linked water source acidification to the progressive exposure of the sulphide-rich Chicama Formation (Js-Chic), previously covered by perennial ice. However, traditional identification methods rely on labor-intensive ground-based field campaigns, limiting the scope of studies and hindering future predictions. Upon correlating the Landsat 8-derived pH proxies with the Js-Chic map, a higher Js-Chic exposure was observed near highly acidic lakes. Notably, Js-Chic exposure areas exceeding 60 hectares coincided with highly acidic lakes (pH < 4). We also analyzed glacial retreat in the study basins through a time series of the Normalized Difference Snow Index (NDSI) from 1986 to 2019, finding greater glacial retreat in basins with acidic lakes. We also compared vegetation quality in these basins over the same period using the average Normalized Difference Vegetation Index (NDVI) and observed higher vegetation quality in acidic-lake basins. We then developed a methodology to assess lake acidity based on the spectral signatures of 28 lakes in the Cordillera Blanca. These results suggest that a greater Js-Chic exposure area near a lake may indicate acidity. We applied this methodology on 267 lakes in the Cordillera Blanca and found 60 lakes to be highly acidic with a pH < 4 (22.5%), and 207 lakes (77.5%) with a pH > 4. However, these findings should be supplemented with more complex analyses, as there is currently limited in situ monitoring data on lake pH.