Increased Hg Methylation Risks in Management-Induced Terrain Depressions in Forests with Organic-Matter-Rich Soils

Mercury (Hg) is a toxic contaminant that bioaccumulates in trophic chains in its organic form—methylmercury (MeHg). Hg methylation is driven by microorganisms in favourable conditions, stagnant water pools being among potential methylation hotspots. In the present study, we estimated the total Hg and MeHg concentrations in the sediments of water-filled management-induced terrain depressions (ruts, mounding pits and a partly functional drainage ditch) and in nearby undisturbed soil in six hemiboreal forest sites with organic-matter-rich soils in Latvia. Environmental samples were taken in the spring, summer and autumn of 2022. Furthermore, we evaluated the risks of element leaching from the depressions using high-resolution digital terrain models (DTM) and meteorological data. The results suggested a possible leaching of Hg in the past as THg concentrations in the sediments of the depressions were significantly lower than in the surrounding soil. Furthermore, significantly higher MeHg and %MeHg concentrations were found in the sediments than in the surrounding soil identifying the management-induced depressions as Hg methylation hotspots. Spatial analysis of the DTMs pointed to a very likely periodical leaching of elements from the depressions during high precipitation episodes as well as during snowmelts. Moreover, it was observed that ruts left by heavy machinery often channel surface runoff.