Historical variations in the stable isotope composition of mercury in a sediment core from a riverine lake: Effects of dams, pulp and paper mill wastes, and mercury from a chlor-alkali plant

• A core from a lake polluted by Hg and organic wastes was analysed for Hg isotopes.
• Hg from a chlor-alkali plant was depleted in lighter isotopes owing to evaporation.
• Mass-independent fractionation caused enrichment in 199Hg but depletion in 201Hg.
• Hg input led to a rise in the δ201Hg/δ199Hg ratio due to an increase in CH3Hg+.
• Earlier variations in the ratio imply effects of pulp mill effluents and a dam.

The Wabigoon River (Ontario, Canada) was affected by dams starting in 1898 and was polluted with pulp and paper mill wastes starting in 1913 and mercury from a chlor-alkali plant from 1962 to 1975. A dated sediment core from a riverine lake was analysed to investigate resultant changes in the biogeochemistry of mercury as revealed by variations in mercury isotope ratios and sediment chemistry. A total mercury maximum formed by the mercury pollution coincided with minimums in the δ-values of the 198Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, and 201Hg/202Hg ratios, and the δ-values decreased in the order δ201Hg > δ200Hg > δ199Hg > δ198Hg. Thus, mass-dependent fractionation caused depletion in lighter isotopes, implying evaporation of Hg(0) and pollution of the atmosphere as well as the river-lake system. Concurrently, mass-independent fractionation caused 199Hg enrichment, possibly reflecting an independently documented upsurge in methylmercury production, and 201Hg depletion, suggesting removal of methylmercury with anomalously high 201Hg/199Hg ratios by aquatic organisms and accumulation of 201Hg-depleted inorganic Hg(II) in sediments. The δ201Hg/δ199Hg ratio rose abruptly when mercury pollution began, reflecting the resultant increase in methylmercury production, and remained high but gradually declined as the pollution abated, paralleling trends shown by methylmercury in aquatic organisms. The δ201Hg/δ199Hg ratio of pre-1962 background mercury increased ca. 1898 and ca. 1913–1929, suggesting accelerated methylmercury production due to stimulation of microbial activities by the damming of the river and the input of pulp and paper mill wastes, respectively. Other variations were linked to economic and technological factors that affected pulp and paper manufacture.