Trends in mercury, lead and cadmium concentrations in 27 European streams and rivers: 2000–2020

Temporal trends for concentrations of mercury (Hg), lead (Pb) and cadmium (Cd) were evaluated from year 2000–2020 in 20 (Hg), 23 (Pb) and 11 (Cd) watercourses in remote forest catchments in Europe. Decreasing trends were observed in 15% (Hg), 39% (Pb) and 45% (Cd) of the watercourses during the period of evaluation. Decreasing trends were mainly observed between 2000 and 2005 for Hg and between 2000 and 2015 for Pb and Cd. For the last five years of the studied time period (2015–2020), more watercourses showed significant increasing, rather than decreasing Hg, Pb and Cd trends. This was interpreted as a legacy effect of metals still retained in catchment soils. The overall negative trends during the earlier part of the study period were likely driven by declining deposition of metals over Europe, especially for Pb and Cd. Other changes related to metal transport and chemistry may have contributed to the observed trends as well, including recovery from acidification and the ongoing browning of surface waters at northern latitudes. Here we found that organic carbon could explain the seasonal variation in Hg and Pb, but was not related the interannual trends. This study highlights the need for long-term monitoring and robust statistical methods that can detect multidirectional, long-term change in water chemistry. Abstract art. Overall trends for Hg, Pb and Cd concentrations in the studied watercourses between 2000 and 2020. Note that trends are not monotonic and that different concentration units have been standardized for comparison purposes. [Display omitted] •Mercury, lead and cadmium trends in 27 European streams and rivers were evaluated.•2000 to 2020, mercury, lead and cadmium concentrations declined in many watercourses.•Trends were not monotonic, since 2015 increasing trends outnumbered decreasing ones.•Declining deposition and recovery from acidification may drive declining trends.•Organic matter explained the seasonal but not long-term trends for mercury and lead.