Great Lakes Sediment Surveillance Program: Mercury Concentrations and Mercury Stable Isotopes in Sediments of the Laurentian Great Lakes

As part of the larger Great Lakes Sediment Surveillance Program (GLSSP), the U.S. Geological Survey Mercury Research Laboratory (MRL) partnered with the U.S. Environmental Protection Agency and the University of Minnesota-Duluth to assess contaminant concentrations within surface sediments and sediment cores from the Laurentian Great Lakes. Sediments were assessed for mercury concentrations and mercury stable isotopes to examine spatial trends and sources of mercury within the lakes. In 2021, 30 sites were surveyed in Lake Superior. Mercury concentrations within Lake Superior surface sediment ranged from 1.1 to 161.2 ng/g. Mercury isotopes displayed a range in δ202Hg from -0.12 to -1.74 ‰ with measurable Δ199Hg and Δ200Hg across all Lake Superior sites. In 2022, 32 surface sediment sites were surveyed in Lake Huron. Mercury concentrations within Lake Huron sediment ranged from 0.7-to- 113.7 ng/g across all samples. Mercury isotopes displayed a range in δ202Hg from -0.12 to -1.74 ‰. In 2023, surface sediments were collected at 30 sites across Lake Ontario. Total mercury concentrations in surface sediments ranged from 19 ng/g to 2001.1 ng/g. Three deep sediment cores were collected in each lake. The sediment cores ranged in length from 15 cm to 50 cm. All cores showed a similar pattern for HgT except for cores at H038 and H054. From the top of the core, HgT concentrations increase to a maximum concentration (127.4 - 3035.3 ng/g) that occurs between 5 and 15 cm below the surface and then decreases to concentrations that range from 18.7 ng/g to 40 ng/g. The cores from H038 and H054 show decreasing or no change in HgT concentration in the upper portion of the core and then follow similar trends to the third core (H012) collected in Lake Huron.