Assessment of Heavy Metals Pollution in Selected Alpine Wetlands of Lesotho

The catchments that contain ecologically critical wetlands supplying the Mohale and Polihali dams under the Lesotho Highlands Water Project (LHWP) are increasingly threatened by expanding agriculture, mining activities, and uncontrolled livestock grazing. Hence, the present study was conducted to assess heavy metal contamination and wetland health across the three higher-altitude sub-catchments in Lesotho: Senqunyane, Khubelu, and Sani. A total of 24 water samples were collected from six wetlands in March 2025 to determine concentrations of copper, iron, manganese, lead, and zinc in accordance with APHA standards. Pollution Load Index (PLI) and Heavy Metal Pollution Index (HPI) were calculated to evaluate water quality. All sites exceeded the HPI safety threshold of 100, with Sani Top showing the highest PLI (5.54), indicating severe contamination primarily driven by manganese and lead. Lead emerged as the dominant pollutant due to its low permissible limits, exacerbating HPI scores across wetlands. Heavy metal concentrations generally declined with increasing altitude, with lead and copper displaying the steepest decreases, while manganese peaked at mid-altitudes (2750 m), potentially linked to local geochemical processes and organic matter decomposition. Principal Component Analysis (PCA) explained 40.6% of total data variance, revealing tight clustering at higher altitudes (3000 m), reflecting uniform, geogenically controlled water quality, whereas lower elevations displayed more variable and anthropogenically influenced patterns. Despite high-altitude sites appearing chemically stable, they carry higher dissolved ions, suggesting treatment needs for water hardness. In contrast, low-to-mid elevation wetlands showed more variable and hazardous metal loads, necessitating targeted management strategies including buffer zones, liming, and pollution source tracing. Findings highlight land use and hydrology in wetland water quality. Continuous monitoring of Lesotho’s alpine wetlands is vital to address heavy metal pollution, guide evidence-based policy, and support prioritized monitoring, mitigation, and restoration for sustainable downstream water management.