Factors influencing the achievement of steady state in mercury contamination among lakes and catchments of south-central Ontario

Abstract

Mercury (Hg) concentrations in recent (0.5–1 cm) and preindustrial (>30 cm) sediments were examined across lakes in south-central and eastern Ontario, Canada (45.53°N, 82.41°W to 44.15°N, 76.25°W), to determine whether Hg exported from watersheds is at steady state with atmospheric deposition. An examination of headwater lakes revealed that Hg enrichment was not uniform among watersheds but that the enrichment factor (EF = [Hg] present day /[Hg] preindustrial , standardized for organic matter) decreased as a function of drainage ratio (A d /A o , watershed area /lake area ). Furthermore, the model fit was improved after accounting for differences in sulfate concentrations and pH among lakes: EF = (A d /A o ) –15.96 – 0.07(SO 4 2– ) – (3.55(pH >8.3 )) (R 2 = 0.458, p = 0.0001). Hg concentrations in preindustrial sediments of headwater lakes showed a positive linear relationship with drainage ratio (partial t = 4.83, p < 0.0001, n = 66) that was strengthened following an adjustment for mean annual runoff (MAR) ([Hg] preindustrial = 0.011 ± 0.002(A d /A o ) + 0.0008 ± 0.0003(MAR) (R 2 = 0.108, F [1,66] = 8.01, p = 0.006)). Our results suggest that Hg export from watersheds may be currently lagging behind atmospheric Hg deposition, in which case, Hg export would increase into the future, even as Hg deposition from the atmosphere stabilizes.