Lichen Bioindicators Reveal Potential Source of Mercury from Atmosphere to Reservoirs in a Mercury Mining-impacted Watershed
Abstract
Historic mercury (Hg) mines in California that have been inactive for decades continue to emit gaseous Hg into the atmosphere. Gaseous Hg from mines has the potential to deposit in the local watershed and be a significant source of methylmercury (MeHg) to reservoirs within the watershed. Hg-contaminated sediments have been identified in many mining-impacted areas, but the atmospheric Hg flux and subsequent deposition from local emissions are less understood. Past studies have shown that the total Hg content in epiphytic lichens is a convenient bioindicator of atmospheric mercury contamination in these areas. In this study, we measured total Hg in five species of native lichens across the New Almaden Mining District (NAMD). We found concentrations up to 14 ppm Hg in a former furnace yard dating back to the 1880s. We also quantified Hg stable isotopes and showed that lichens with the highest total Hg concentrations had Δ199Hg values close to 0, and the values tend to get more negative with distance from the source. This trend indicates a low level of photochemical processing in the Hg emitted from mines and verifies that Hg stable isotopes are useful for discriminating between the mining source and the background. Furthermore, we analyzed the structure of Hg in the lichen using Hg L-III HERFD-XANES. Preliminary results suggested mixtures of cinnabar (α-HgS) and meta-cinnabar (β-HgS) in addition to organic Hg-thiol species. This is consistent with our speciated measurements on lichen digests that showed 87% of total Hg as HgII, while the remaining 13% was Hg0. These data suggest that lichens around historic Hg mines readily absorb Hg, and very little of the absorbed Hg reemits to the atmosphere. Given the ubiquity of lichen in this study area, lichen litterfall may significantly contribute to the Hg in the watershed, where the Hg can transport to methylating environments in the downstream reservoirs.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.B15I1233S