The missing piece: sediment records in remote Mountain lakes confirm glaciers being secondary sources of persistent organic pollutants.

After atmospheric deposition and storage in the ice, glaciers are temporary reservoirs of persistent organic pollutants (POPs). Recently, the hypothesis that melting glaciers represent secondary sources of these pollutants has been introduced by investigations of the historical trend of POPs in a dated sediment core from the proglacial Alpine Lake Oberaar. Here, the hypothesis is further confirmed by the comparison of sediment data gathered from two Alpine lakes with a glaciated and a nonglaciated hydrological catchment. The two lakes (Lake Engstlen and Lake Stein in the Bernese Alps in Switzerland) are situated only 8 km apart at similar altitude and in the same meteorological catchment. In the nonglacial lake sediment of Lake Engstlen, PCBs and DDT (polychlorinated biphenyls and dichlorodiphenyl trichloroethane) levels culminated with the historic usage of these chemicals some 30-50 years ago. In the glacial Lake Stein, this peak was followed by a reincrease in the 1990s, which goes along with the accelerated melting of the adjacent glacier. This study confirms the hypothesis of glaciers being a secondary source of these pollutants and is in accordance with the earlier findings in Lake Oberaar.