Aquatic effects and risk assessment of multi-metal leachates from metal mining and acid sulphate soils

Leachates and effluents from acid sulphate soils and metal mining compose a significant part of metal and sulphate loads into aquatic environments in Finland. Acid sulphate soils have deteriorated the water systems of the Ostrobothnian region for several decades. Discharges from metal mining have similar characteristics as leachates from acid sulphate soils. The extent of metal contamination and its effects were surveyed altogether in 10 estuaries in western Finland and in 9 lakes affected by biomining and conventional mining activities. Assessment also included monitoring data and toxicity tests with benthic macroinvertebrates. Metal concentrations in the estuary sediments were elevated and acidity was transported to the estuaries. The most evident acid sulphate soil induced effect was deterioration of the benthic communities in the estuaries. The risk of having negative effects due to acid sulphate soils was elevated in 9 out of 10 estuaries. Effects observed in the mining-affected lakes were comparable to effects induced by acid sulphate soil leachates. In both mining sites, concentrations of the ore metals were elevated especially in the lake sediments close to the mine. Sequential extractions of the sediments suggested that the partitioning of the ore metals Ni and Cu in the studied sediments were of anthropogenic origin. The laboratory toxicity tests with the standard test species Lumbriculus variegatus indicated reduced growth and reproduction and altered behavioural responses, with growth and reproduction being more sensitive endpoints. However, behavioural responses seemed to be sensitive enough for quick screening-level assessments. The field-collected lake sediments did not sustain their original properties, such as pH, which made their ecotoxicity assessment challenging. Thus, there remains a need to develop methods for comprehensive risk assessment of challenging sediments.