Monitoring of chemical and ecological status in Lassedalsbekken: A seasonal assessment of possible influences from former mining activity

Summary

Lassedalen mines in Kongsberg municipality was mined on a small scale for fluorspar (CaF2) by Norsk Hydro during World War II. After 1950, there has been no active mining in the area. Tertiary Minerals plc is considering purchasing land from Norsk Hydro, to extract fluorspar. In connection with the possible new mining activities in Lassedalen, Tertiary Minerals plc contacted NIVA for an assessment of the chemical and ecological status of Lassedalsbekken – a small stream draining the area. We sampled Lassedalsbekken from April through August for priority substances (heavy metals), river basin specific pollutants, basic physico-chemical variables, and fluoride to assess the possible influence of former mining activity on the water quality. Moreover, we sampled the benthic macroinvertebrate fauna to determine ecological status and possible effects of environmental stressors on the taxonomic composition. Compared to the reference station upstream of Lassedalen mines, the water downstream was enriched in fluoride, Pb, Cd, Cu, and Zn. Lassedalsbekken therefore showed signs of influence from Lassedalen mines, but likely also from the old Gott Vermag’s mine on the hill north of Lassedalen. None of the priority substances (Pb, Cd, and Ni) were detected in concentrations higher than the Environmental Quality Standards (EQS) defined in the EU Water Framework Directive (WFD), which means that Lassedalsbekken has a good chemical status. Ecological status is determined by the biological quality elements (here, benthic macroinvertebrates) and the concentrations of river basin specific pollutants. The ASPT-index for benthic invertebrates showed good or better status at both sampling stations. Concentrations of all river basin specific pollutants were below the EQS, except for zinc (Zn), which exceeded the annual average EQS slightly at the downstream station. This caused moderate ecological status downstream of the mines. Fluoride concentrations downstream of Lassedalen mines were high compared to what is typically found in Norwegian waters, but below the limit for what is allowed in drinking water. Interestingly, fluoride concentrations were highest in summer, and concentrations declined after events of heavy rain. This dilution effect indicates that fluoride mostly is fed into the stream through groundwater from the mining area. Concentrations of Pb, Cd and Zn, on the contrary, increased after heavy rain, indicating that these substances are transported into the stream through surface runoff. We found no signs of negative influence from fluoride nor heavy metals on the taxonomic composition of benthic macroinvertebrates. Downstream of the mines, the community of benthic invertebrates was in a close to natural state, but in the upper parts of the stream, the macroinvertebrate community was likely influenced by acidification, as acid-sensitive species were absent. We observed episodes of low pH in the stream in connection with high rainfall, and low pH during snowmelt is also likely. The creek has low calcium content and buffer capacity, especially in the upper parts, which could make the biota sensitive to acidification.