Temporal analysis of land cover change in Ayapel's alluvial gold mining zones, Colombia

Alluvial gold mining has been a longstanding economic activity in the municipality of Ayapel, Córdoba, dating back to the colonial era, as noted by historians. However, most of this mining has occurred outside legal frameworks, often facilitated by the presence of lawless groups. This illegality complicates efforts to monitor and implement ecological restoration plans in the area. Additionally, the municipality of Ayapel is home to a significant natural asset: La Ciénaga de Ayapel. Designated as a RAMSAR site since 2018, this wetland holds international recognition for its ecological importance. However, it faces the threat of contamination from the nearby mining activities. Therefore, monitoring this mining activity becomes a crucial point to ensure, among other components, the care of La Ciénaga de Ayapel. The objective of this study is to analyze land cover changes in areas affected by alluvial mining between 1985 and 2020, using Landsat images, digital image processing techniques, unsupervised classification, and sequential pattern mining for the multitemporal analysis. Regarding the analysis conducted for the study area, encompassing the Quebradona microbasin in the municipality of Ayapel, for the years 1985 to 2020, there was an increase in mining activity of 503 ha, equivalent to 12 times that of 1985, with a peak in 2010, 54 times the 1985 levels. Bare soil, on the other hand, increased by 96% by 2020. As for vegetation cover, dense or high vegetation remained relatively constant over time, going from 2434 ha in 1985 to 2446 ha in 2020. Low vegetation, typically corresponding to grasslands, increased from occupying 3855 ha in 1985 to 5859 ha by 2020. Medium vegetation decreased by 25% compared to 1985, going from 7588 ha to 5674 ha by 2020. Finally, compared to 2010, there was a 14% increase or recovery in medium vegetation, and high vegetation saw a 5% recovery. It is important to highlight that the images from 1989 and 2013 exhibited a significant presence of cloud cover, which led to an underestimation of the area classified for each type of land cover.