Mercury–carbon relationships in environmental media near artisanal gold mining sites in Guyana

Elevated atmospheric mercury (Hg) concentrations from artisanal gold mining increase Hg exposure in adjacent forest canopies. Although pools and fluxes of Hg and carbon (C) within environmental media are connected, it is unclear how these Hg/C patterns may differ between areas exposed to high Hg levels and regions remote from emission sources. Here, we compare Hg/C patterns in environmental media (foliage, leaf litter, soil, water, sediment) near artisanal gold mining sites in Guyana with 25 studies worldwide to better understand Hg movement within environmental media. Patterns of Hg/C ratios in environmental media were similar between Guyana and globally synthesized studies, but Hg/C values were much higher near gold mines than the global average. Adjacent to a gold mine in Guyana, Hg/C ratios were lowest in foliage (9.8 ± 1.6 ng mg-1) but increased in litter (26.6 ± 3.9 ng mg-1) due to continued accumulation from the atmosphere. Mineral soil Hg/C ratios (5.1 ± 0.2 ng mg-1) are lower than litter and surface water (13.2 ± 2.5 ng mg-1), and stream sediment (7.3 ± 1.1 ng mg-1) exhibits different ratios likely reflecting a mixture of inputs from dominant flow paths (surface soils and wetlands) and erosional losses, with higher Hg/C ratios in water compared with sediment. At the global scale, peat Hg/C ratios (0.2 ± 0.2 ng mg-1) were nearly identical to a global synthesis of surface water (0.3 ± 0.2 ng mg-1), suggesting that transport of C associated with Hg from wetlands due to possible Hg accumulation from the atmosphere, is one of the primary sources of Hg to surface waters globally.