Relationship between the loading rate of inorganic mercury to aquatic ecosystems and dissolved gaseous mercury production and evasion

The purpose of our study was to test the hypothesis that dissolved gaseous mercury (DGM) production and evasion is directly proportional to the loading rate of inorganic mercury [Hg(II)] to aquatic ecosystems. We simulated different rates of atmospheric mercury deposition in 10-m diameter mesocosms in a boreal lake by adding multiple additions of Hg(II) enriched with a stable mercury isotope (²⁰²Hg). We measured DGM concentrations in surface waters and estimated evasion rates using the thin-film gas exchange model and mass transfer coefficients derived from sulfur hexafluoride (SF₆) additions. The additions of Hg(II) stimulated DGM production, indicating that newly added Hg(II) was highly reactive. Concentrations of DGM derived from the experimental Hg(II) additions (“spike DGM”) were directly proportional to the rate of Hg(II) loading to the mesocosms. Spike DGM concentrations averaged 0.15, 0.48 and 0.94 ng l⁻¹ in mesocosms loaded at 7.1, 14.2, and 35.5 μg Hg m⁻² yr⁻¹, respectively. The evasion rates of spike DGM from these mesocosms averaged 4.2, 17.2, and 22.3 ng m⁻² h⁻¹, respectively. The percentage of Hg(II) added to the mesocosms that was lost to the atmosphere was substantial (33–59% over 8 weeks) and was unrelated to the rate of Hg(II) loading. We conclude that changes in atmospheric mercury deposition to aquatic ecosystems will not change the relative proportion of mercury recycled to the atmosphere.     

Long-term mercury dynamics in UK soils

A model assuming first-order losses by evasion and leaching was used to evaluate Hg dynamics in UK soils since 1850. Temporal deposition patterns of Hg were constructed from literature information. Inverse modelling indicated that 30% of 898 rural sites receive Hg only from the global circulation, while in 51% of cases local deposition exceeds global. Average estimated deposition is 16 μg Hg m⁻² a⁻¹ to rural soils, 19 μg Hg m⁻² a⁻¹ to rural and non-rural soils combined. UK soils currently hold 2490 tonnes of reactive Hg, of which 2140 tonnes are due to anthropogenic deposition, mostly local in origin. Topsoil currently releases 5.1 tonnes of Hg⁰ per annum to the atmosphere, about 50% more than the anthropogenic flux. Sorptive retention of Hg in the lower soil exerts a strong control on surface water Hg concentrations. Following decreases in inputs, soil Hg concentrations are predicted to decline over hundreds of years.