title: Exploring the Dynamics and Distributions of Mercury and Organomercury Species in Soils: Microcosm experiments and Field Studies
Creators: name: Gfeller, Lorenz
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 910 Geography &amp; travel

description: Mercury (Hg) is a pollutant of global concern due to its ubiquitous presence in the environment, ongoing anthropogenic emissions, and its toxic effects on the human nervous, digestive, and immune systems. The toxicity of Hg and its uptake in biota depend on the chemical speciation, with methylmercury (MeHg) representing the most relevant species for bioaccumulation and biomagnification. One significant anthropogenic source of Hg in the environment are historical emissions from chlor-alkali and acetaldehyde-producing chemical plants. Often, their legacy sites exhibit exceptionally high concentrations of Hg but are poorly characterized regarding MeHg and natural mercury methylation. One reason for this is the existing challenges in MeHg analysis in highly polluted substrates. However, analyzing such soils is critical to evaluating potential environmental risks. Polluted soils with high levels of Hg are potential point sources to downstream ecosystems. Repeated flooding (e.g., redox cycling) and agricultural activities (e.g., organic matter addition) may influence the fate and speciation of Hg in a soil system. The formation and aggregation of colloids and particles affect both Hg mobility and its bioavailability to MeHg-forming microbes.
This thesis aimed i.) to assess the differences in regional distribution of Hg and MeHg among landuse types in a alpine mountain valley ii.) to improve and test existing methods for MeHg extraction and analyses of highly contaminated soils and iii.) to assess the influence of anthropogenically induced disturbances such as flooding, and manure application on net-methylation and mobility of Hg in in agriculturally used soils. We addressed these questions by studying soils from agricultural fields and grove sites in the region of Visp, Switzerland. This model site hosts both contaminated and uncontaminated fields and areas that are regularly subjected to flooding. 
i.) We observed significant correlations between Hg and soil organic carbon (OC) in the topsoil (R2 = 0.73, p < 0.05). Furthermore, the Hg enrichment factors in the topsoils (0 to 10 cm) were highest in both grassland (EFHg/Al: -36.2 to 1429; median = 323; n = 38) and tree groves (EFHg/Al: 599 to 3676; median = 1163; n = 10). The correlation between Hg and OC, along with the EFHg/Al profiles, suggests atmospheric deposition as the primary pathway. The absence of a spatial pattern may be attributed to varying wind directions and seasonal temperature inversions in the valley. While the topsoils of grasslands and tree groves exhibit similar levels of Hg, the latter display significantly higher net-methylation potential (MeHg/Hg: 0.28 -19.1%; median = 3.6%; n = 10) compared to grasslands (MeHg/Hg: 0.18 - 2.4%; median = 0.53%; n = 40), indicating a higher input of readily available Hg, greater bioavailability of Hg, and/or enhanced microbial activity. 
ii.) Further, we found that during extraction of MeHg from soil, false positives from artificial methylation may be corrected for by a simple constant correction factor. Methylation factors from iHg spiking were in the range of (0.0075 ± 0.0001%) and were consistent across soils and sediment matrices. Analyzing contaminated soils in the abovementioned area, we suggest that MeHg was anthropogenically deposited and not naturally formed in-situ in two out of three highly contaminated locations. Our line of evidence consists of 1) the concomitant detection of ethyl mercury EtHg, 2) the elevated MeHg concentrations (up to 4.84 μg kg−1), and 3) the absence of hgcA genes at these locations. The combination of Hg speciation and methylation gene (hgcA) abundance analyses proved to be tools suited to assess Hg pollution pathways at Hg legacy sites.
iii.) We conducted a flooding-draining experiment on Hg-polluted floodplain soils from the abovementioned agriculturally used area. The experiment included two 14-day flooding periods and one 14-day draining period, with natural organic matter added to two soils with varying Hg and organic carbon levels. Manure addition resulted in accelerated release of Hg to the soil solution, fast sequestration of Hg, and increased the particulate and colloidal Hg pool bound to dissolved organic matter and HgII bound to inorganic ligands. The experiment showed net MeHg production during the first flooding and draining period, and subsequent decrease in absolute MeHg concentrations. Manure addition did not significantly change net MeHg production. Our results suggest manure addition may promote Hg sequestration by complexation on large organic matter components and formation of inorganic HgS(s) colloids in Hg-polluted fluvisols with low levels of natural organic matter.