The Biogeochemical Cycle Of Mercury:from Tibetan Plateau Soil To Artisanal Small-scale Gold Mining

Mercury(Hg)is a neurotoxic heavy metal that is associated with cardiovascular effects in adults and neurocognitive defects in fetuses.It can be exchanged between the atmosphere,terrestrial ecosystems,and aquatic ecosystems through long distance transportation in the atmosphere.The anthropogenic emissions of mercury may have an adverse effect on the ecological environment and will eventually affect the biosphere,causing adverse effects on human health through the way of food methylmercury exposure.This study focuses on the biogeochemical cycle of mercury.On one hand,we explore the driving factors of mercury storage in the Tibetan grassland soils underlain by permafrost,mainly focusing on the soil-air exchange of mercury.On the other hand,we evaluate the ecological environmental impact and human health risk of mercury caused by the artisanal small-scale gold mining(ASGM),focusing on the transpoart and exchange of mercury between the atmosphere,hydrosphere,lithosphere,and biosphere,especially the health risk and economic loss on the biosphere.Soils,especially permafrost in the Arctic and the Tibetan Plateau,are one of the largest reservoirs of Hg in the global environment.The Hg concentration in the grassland soils over the Tibetan Plateau and its driving factors have been less studied.This study analyzes soil total mercury(STHg)concentrations and its vertical distribution in grassland soil samples collected from the Tibetan Plateau.We adopt a nested-grid high-resolution GEOS-Chem model to simulate atmospheric Hg deposition.The relationship between STHg and soil organic carbon(SOC),as well as atmospheric deposition,are explored.Our results show that the STHg concentrations in the Tibetan Plateau are 19.8±12.2 ng/g.The concentrations are higher in the south and lower in the north in the Tibetan Plateau,consistent with the previous results.Our model shows that the average deposition flux of Hg is 3.3 ?g m-2 yr-1,with 57% contributed by dry deposition of elemental mercury(Hg0),followed by dry(19%)and wet(24%)deposition of divalent mercury.We calculate the Hg to carbon ratio(RHg:C)as 5.6±6.5?g Hg/g C,and the estimated STHg is 86.6±101.2 Gg in alpine grasslands in the Tibetan Plateau.We find a positive relationship between STHg and SOC in the Tibetan Plateau(r2 = 0.36)and a similar positive relationship between STHg and atmospheric total Hg deposition(r2 = 0.24).A multiple linear regression involving both variables better model the observed STHg(r2 = 0.42).Through the analysis of literature data,we extend the results to the whole country.In general,SOC and atmospheric deposition influence STHg simultaneously,but the dominant factors at different scales are different.The data provides information to quantify the size of the soil Hg pool in the Tibetan Plateau further,which has important implications for the Hg cycles in the permafrost regions as well as on the global scale.The Hg emitted by the ASGM activities has become the largest anthropogenic source at present,but the current health assessment of th ASGM activities is mainly focused on the occupational population,the indigenous population,and the coastal population.The impact of Me Hg exposure and risk assessment on a global scale are still relatively limited.This study applies a comprehensive atmosphere-land-oceanecosystem for Hg and simulates the environmental concentration of ASGM emissions in the atmosphere,soils and ocean.We calculate the food Me Hg exposure of freshwater fish,seafood and rice based on food intake inventory and food Me Hg concentrations for individual countries at a global scale.We estimate the health risk and economic loss of Me Hg exposure from the aspects of the per-fetus IQ decrement and fatal heart attack(FHA)deaths based on the dose-response relationship.Our results show that atmospheric deposition and marine plankton methylmercury concentration are highly sensitive to mercury released by ASGM activities,while STHg concentration is relatively insensitive to this source.In 2012,the global average annual mercury atmospheric deposition was 1.6 ?g m-2 yr-1,the average annual marine plankton methylmercury concentration was 0.59 f M,and the STHg was 1630.76 g,the proportions of the base year are 12.2%,24.5%,and 0.63%,respectively.The mercury emitted by ASGM is mainly ingested by people from freshwater fish(Finland,229.1?g/yr)and seafood(Maldives,718.9 ?g/yr),and the potential importance of eating rice is not obvious.That is,dietary preference are the important factors that affect the level of food methylmercury exposure.In 2012,We calculated that the global health risks associated with Me Hg exposure of ASGM are 5.8x105 points of IQ decrements(4.3x10-3 point per-fetus)and 1430.2 FHA deaths.We also estimate that the global economic loss is $7.13 billion(2020 USD),with 52.6% contributed by IQ decrements($3.75billion),and 47.4% contributed by FHA deaths($3.38 billion).From 1970-2012,the total economic loss due to ASGM activities is 154.2 billion,while total economic benefits are 319.1 billion.The total economic loss accounts for 48.3% of the total economic benefits(Discount rate: 3%).In general,ASGM activities has an impact on the environment and human health,and there is a situation where the main body of risk and the main body of income are misaligned.People all over the world should share the responsibility of reducing ASGM emissions.This study has applied an atmosphereland-ocean-ecosystem and an exposure-risk model for Hg.This study devotes the effort to modeling the mercury pollution in the environment and estimating the health risk and economic loss associated with Me Hg exposure based on food intake inventory and food Me Hg concentrations for individual countries at a global scale.It provides scientific support for governments of various countries to formulate control policies.