The Sedimentation And Sources Of Hg In Lake Sediments In Different Areas Of China

Mercury is a rare and highly toxic and non-essential element.With the global industrialization,the intentional use of mercury（such as chlor-alkali industry,plastic industry,electronic and battery industry）and unintentional use（such as mercury mining,coal combustion,cement production,lead and zinc smelting,etc.）,the amount of Hg emission showed an explosive growth trend.Mercury can exchange between atmosphere,soil and water bodies.Lake sediments act as both sources and sinks of mercury and strongly adsorb dissolved substances in water.Mercury deposited in watersheds and lakes is readily methylated to bio-accumulately neurotoxin methyl mercury（MeHg）.As an important reservoir for the transformation of various environmental substances in the basin,the lake sediments can completely record the information of regional environmental changes.In this study,lake sediments were used as a research carrier.Through the collection of sediment samples from six lakes in China’s remote areas in northeastern,northwestern and southern China and the industrialized cities in central China,combined with the determination of total mercury and the dating analysis,the Hg sedimentation rate of sediments were calculated The mercury isotope composition of lake sediments were also measured,that can be used as indicator of sources of Hg in the sediment.The results showed that:（1）According to the vertical distribution of mercury in sediments of various lakes,the peak of mercury pollution in Sayram Lake and the Dalongwan lake occurred after the industrial revolution and showed a gradual increasing trend.Affected by the reform and opening up,a large number of industries and transportations were developed in the coastal areas.Huguangyan sediments also showed minor pollution since 1978;atmospheric mercury emissions from the lead-zinc smelting plants around Shiyan Lake have a significant impact on the mercury increase in sediments.With the mercury pollution control measures strengthened in the lead-zinc smelter since 1990s,sedimentary mercury content stabilized and showed a downward trend;and the sediments of the Guanzhuang reservoir contaminated by Hg due to the local small scale and illegal gold mining activities,and the peak of mercury pollution appeared in 2002 to 2003 at the depth of10 cm,,then mercury content showed a rapid decline.In the early years,the Aha Reservoir was affected by the local coal mine waste water and the mercury pollution peak appeared at depth of26 cm.（2）The mercury pollution in various lake/reservoir sediments were assessed by the method of geo-accumulation index,the results showed that Guanzhuang reservoir was the most polluted,with status of moderate pollution,then followed by Aha Reservoir and Shiyan Lake with mild pollution.The Dalongwan Maar Lake,Sayram Lake and Huguangyan Maar Lake are all relatively clean due to their special geographical location far away from anthropogenic source and the protection environment in recent years.In this study,the organic matter content and the total mercury in the six lake/reservoir sediments have two distinct kind of correlations,that positive correlated with each other in Sayram Lake,Shiyan Lake and Aha Reservoir,this is due to the precipitation and vegetation（litter fall）is the main source of mercury input to sediment.;while,not correlated in two Maar Lakes,probably due to the fact that the sediment composition is mainly volcanic ash.（3）The peak value of mercury concentration in the sediments of Aha reservoir appeared in the middle and upper part of the sediment column with a accumulation factor of 1.19±0.47,which was 2-3 times of mercury concentration in lake sediments in remote areas.Hg in surface sediments of relatively clean lakes of Dalongwan,Sayram Lake,and Huguangyan are about3.2-5.8 times higher than that of the lower sediments.（4）In the sediment profile,the mercury isotope ratioδ²⁰²Hg in Sayram Lake,Huguangyan and Dalongwan maar lake in the clean lake ranged from-2.21‰to-0.29‰,with an average value of-1.03‰±0.40‰.The mercury isotope ratioδ²⁰²Hg of Shiyan lake,Guanzhuang and Aha reservoirs affected by anthropogenic activities ranged from-1.7‰to 0.01‰,with an average of-0.70‰±0.55‰.It can be seen that the mercury isotope ratioδ²⁰²Hg of lakes sediments affected by anthropogenic activities is heavier than that of lakes sediments in relatively clean areas.The more serious mercury pollution in sediments,the more positive the mercury isotopeδ²⁰²Hg.（5）The variation ofδ²⁰²Hg in lake sediments and surrounding soils is significantly affected by precipitation and the input of mercury from the surrounding vegetation and organic soil.From the distribution ofδ²⁰²Hg value andΔ¹⁹⁹Hg value in sediment,the three relatively clean lakes are less affected by surface runoff.The mercury pollution in sediments is mainly affected by precipitation and atmospheric mercury deposition,However,mercury in the three lake sediments in the polluted area is more affected by the activities of local human activities and the input of soil erosion,litter and the like.The mercury isotopes ofΔ¹⁹⁹Hg/Δ²⁰¹Hg in six lake sediments of this study are regular,all located in a linear relationship of 1:1,which is mainly caused by the magnetic isotope effect,that the mass independent fractionation（MIF）is dominated by photoreduction of Hg（II）.