Mercury-bearing black shales and human Hg intake in eastern Finland: impact and mechanisms

Migration of Hg from black shales to the aquatic ecosystems and local residents was studied comprehensively with samples of black shales (627 samples), surface waters (22), organic stream sediments (40), organic lake sediments (172), noble crayfish (72) and human hair (225). The black shales exhibited elevated C, S, Ni, Cu, Zn, Cd and Hg concentrations. Gneiss granite, mica schist, and quartzite with very low heavy metal contents are encountered just next to the black shales, offering good reference areas for comparison. There are no local anthropogenic Hg sources in the study areas. Black shale areas were reflected as elevated Ni, Cu, Zn and Cd and low Ca in glacial till, surface waters, organic stream sediments and lake sediments. Mercury behaved differently in surface waters than the other heavy metals: there was no correlation between the Hg concentration of the surface waters and the Hg concentration of the underlying bedrock. Median values for total Hg and methyl-Hg in stream water were 1.6 and 0.2 ng/l, respectively, in the black shale area, and 2.0 and 0.3 ng/l in the adjacent reference area with Hg-poor bedrock. Even though Hg concentrations in surface waters were not elevated in the black shale areas relative to the reference areas, mean Hg concentrations of the muscle of noble crayfish (Astacus astacus) were higher in black shale area lakes than in lakes in the reference areas. The explanation of this correlation probably is not Hg flux from the bedrock to watercourses. As a more easily eroded rock than granites, quartzites, and mica schists, black shale tends to occur in topographic depressions in glaciated terrain. Lakes in the black shale areas are typically surrounded by peatlands, and the catchment areas, from which they collect humic material rich in Hg, may be extensive. The total Hg concentrations in hair ranged from 0.04 to 12.1 microgram. The distribution of hair Hg content did not appear to correlate with the distribution of black shales. In the present study, the main route of Hg to humans was via fish. Even though Hg contents were higher in crayfish and fish in the black-shale-area lakes, people consumed fish not only from the lakes next to their residence, but also from lakes situated in the reference areas. Among the non-fish-eaters in one black shale area, the use of dug well water, as well as the consumption of local potatoes and mushrooms, slightly increased the Hg in hair. This contribution may reflect the heavy metal load of the natural black shale environment.