Mining liabilities as a source of toxic metals and physicochemical contaminants in tropical rivers

Global mining activities have led to widespread river pollution, jeopardizing aquatic ecosystems and human well-being by releasing harmful contaminants. The mining liabilities left by a company after ending its activity in 1990 have resulted in the environmental release of metals in surrounding rivers in Líbano de Tilarán, Guanacaste, northern Costa Rica. We examined whether the metal concentrations in water and sediment affected by these mining liabilities are exposure sources for benthic macroinvertebrates in the San José and Cañas rivers. The metal concentrations in these organisms were related to the spatial distribution of exposure concentrations. We also assessed spatial changes in the physicochemical water characteristics due to the direct influence of mining liabilities. Arsenic, cadmium and lead concentrations in water were below the limits of quantification (2-4 μg/L), but in sediments high concentrations were found which were related to seasonal variations in river discharge. The high concentrations of arsenic and lead in the sediments of the San José River explained the low number of macroinvertebrate families, with smaller individuals (<0.4 mg dry weight) of the Leptohyphidae family showing higher average concentrations of arsenic (10.6 μg/g), cadmium (1.25 μg/g) and lead (1.30 μg/g) than larger individuals (>0.4 mg, dry weight) of the same family. The enrichment of sediments with metals, together with their heterogeneous spatial distribution, makes the benthic macroinvertebrates more exposed especially to arsenic, which may reduce their abundance, survival, and growth. The strong correlation between the spatial distribution of metal concentrations in sediments and their accumulation in benthic organisms, alongside changes in water quality, demonstrates the impact of mining liabilities on the San José and Cañas rivers.