亚热带河口咸淡水混合过程中汞的额外添加与消除
Addition and Removal Pattern of Mercury in a Subtropical Estuarine Water under the Mixing Process of Seawater and Freshwater
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摘要: 河口是海陆界面能量和物质交换的重要场所,研究汞在此界面的迁移特征及影响因素对认识汞的生物地球化学行为具有重要意义。分别于九龙江河口区枯水期和丰水期采集不同潮位的孔隙水样品,测定其总汞(THg)含量和汞同位素组成,并结合表层水及沉积物的总汞含量和汞同位素组成,分析它们与盐度间的关系,探究河水径流量和咸淡水混合作用对汞在河口区分布特征的影响。结果表明,除发生降水事件航次的部分站位,孔隙水THg含量[(38.28±26.80)ng·L-1]总体上高于表层水[(9.81±10.21)ng·L-1];汞同位素结果显示,咸淡水混合过程中河口区水体中汞很可能是依照表层水-孔隙水-沉积物的方向迁移的。此外,受悬浮颗粒物的再悬浮以及絮凝沉降作用影响,丰水期河口区咸淡水混合过程会向表层水中额外添加THg,而枯水期时表层水中THg在混合过程中被额外消除。当咸淡水在河口区中部河道较浅处交锋时,其引发的THg额外添加相较于海水已涌入整个河口区会更加强烈。表层水汞同位素组成与盐度之间的关系显示,当无偶然降水事件发生时,丰水期和枯水期咸淡水混合过程中表层水的δ202Hg和Δ199Hg值均上升,表明THg的额外添加和消除可能均不会直接引起表层水汞同位素组成的改变。而受偶然降水事件影响的枯水期其表层水的δ202Hg值下降、Δ199Hg值基本不变,提示降水事件对河口区中汞迁移特征存在影响。Abstract: Estuaries play a critical role in exchanging of energy and materials between land and ocean. To better understand biogeochemical cycle flux of mercury, it’s important to reveal the migration pattern of mercury and related influencing factors. Porewater samples were collected from the Jiulong River estuary during the dry and wet seasons. The total mercury (THg) concentrations and mercury isotopic compositions of surface water and sediment were also analyzed as well as their relationships with salinity, respectively. The results showed that THg concentrations of porewater[(38.28±26.80)ng·L-1]was generally higher than of surface water[(9.81±10.21)ng·L-1], but the THg concentration of porewater at some stations was lower than that of surface water during the cruise impacted by heavy rain, indicating that influence of precipitation events on mercury migration in this estuary; mercury isotope composition of porewater also indicated that mercury in the estuary may migrate along the direction of surface water-pore water-sediment during the mixing process of seawater and freshwater. During the mixing process of seawater and freshwater, the additional mercury was added to the surface water in the wet season (Hg addition pattern) and mercury was additionally removed from the surface water (Hg removal pattern) in the dry season, which was related to the resuspension and flocculation of suspended particulate matters. In addition, the Hg addition pattern was more intense when the current converged at the shallow part of the channel in the mid estuary compared to when the seawater has already flooded into the whole estuary. The relationship between the mercury isotopic composition and salinity suggested that the δ202Hg and Δ199Hg values of surface water increased during the mixing process in the wet and dry seasons when there was no occasional precipitation event, indicating that the Hg removal or addition pattern may not directly cause the change of mercury isotopic composition in surface water. However, the δ202Hg values of surface water decreased and Δ199Hg values remained unchanged in the dry season affected by occasional precipitation events, suggesting that precipitation events had an effect on the mercury migration in the estuary.
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Key words:
- Jiulong River estuary /
- salinity /
- total mercury concentration /
- Hg isotopic composition /
- porewater
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