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

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 delta-values of the Hg-198/Hg-202, Hg-199/Hg-202, Hg-200/Hg-202, and Hg-201/Hg-202 ratios, and the d-values decreased in the order delta Hg-201 > delta Hg-200 > delta Hg-199 > delta Hg-198. 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 Hg-199 enrichment, possibly reflecting an independently documented upsurge in methylmercury production, and Hg-201 depletion, suggesting removal of methylmercury with anomalously high Hg-201/Hg-199 ratios by aquatic organisms and accumulation of Hg-201-depleted inorganic Hg(II) in sediments. The delta Hg-201/delta Hg-199 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 delta Hg-201/delta Hg-199 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. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.