Mining, metallurgy and the historical origin of mercury pollution in lakes and watercourses in Central Sweden.

In Central Sweden an estimated 80% of the lakes contain fish exceeding health guidelines for mercury. This area overlaps extensively with the Bergslagen ore region, where intensive mining of iron ores and massive sulfide ores occurred over the past millennium. Although only a few mines still operate today, thousands of mineral occurrences and mining sites are documented in the region. Here, we present data on long-term mercury pollution in 16 sediment records from 15 lakes, which indicate that direct release of mercury to lakes and watercourses was already significant prior to industrialization (<AD 1800). Thirteen of our lakes show increases in mercury from 3-fold-equivalent to the enrichment factor in many remote lakes today-to as much as 60-fold already during the period AD 1500-1800, with the highest values in the three lakes most closely connected to major mines. Although the timing and magnitude of the historical increases in mercury are heterogeneous among lakes, the data provide unambiguous evidence for an incidental release of mercury along with other mining metals to lakes and watercourses, which suggests that the present-day problem of elevated mercury concentrations in the Bergslagen region can trace its roots back to historical mining.