Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, School of Marine Sciences, Sun Yat-sen University, Zhuhai 519082, China.
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
Water Res. 2023 Oct 1;244:120455. doi: 10.1016/j.watres.2023.120455. Epub 2023 Aug 6.
Coastal ecosystem is an important zone of mercury (Hg) storage and hotspot of neurotoxic methylmercury (MMHg) production and bioaccumulation. The releases of Hg from coastal aquifer or subterranean estuary (STE) via submarine groundwater discharge (SGD) to coastal waters provide an important source of Hg from land to seas. However, the transport and biogeochemical transformation of Hg in STEs are less quantified. In this study, we documented total Hg (THg) and MMHg dynamics in two distinct STEs (a sandflat at an open coast versus a mudflat at a bay) during two different seasons (December versus August) in the temperate coast of northern China, and further applied the salinity-based conservative mixing model (CMM) to quantify the coupling effect of hydrological and biogeochemical processes on STE Hg cycle. Our field data presented large variations of THg and MMHg concentrations and%MMHg/THg of groundwater and sediment in both STEs over time and space. The CMM results clearly displayed substantial divergences of dissolved THg and MMHg from salinity in groundwater between sites and seasons, and the concentration and percent deviations in the Hg-rich mudflat were significantly higher than those in the Hg-poor sandflat. Our findings indicate the non-conservative mixing behaviors of Hg along the groundwater flow paths of both STEs, and the Hg-rich intertidal zone could be hotspot for the production and source of dissolved THg and MMHg to coastal waters via SGD. Our results provide field evidence to highlight that the hydrological shifts and biogeochemical processes collectively drive complex transport and biogeochemical transformation of Hg in STEs. The non-conservative mixing behaviors of Hg in STEs also highlight that, for more accurately calculating SGD-derived Hg fluxes to coastal seas, we need to carefully select the groundwater zonation of STE to better represent the output endmember. Our findings also address that human activities and climate change will profoundly alter the Hg biogeochemical cycle and toxicology in global coastal aquifers.
沿海生态系统是汞（Hg）储存的重要区域，也是产生和生物累积神经毒性甲基汞（MMHg）的热点区域。通过海底地下水排放（SGD）从沿海含水层或地下河口（STE）向沿海水域释放的 Hg 为 Hg 从陆地向海洋输送提供了一个重要的来源。然而，STE 中 Hg 的传输和生物地球化学转化尚未得到充分量化。在本研究中，我们记录了中国北方温带海岸两个不同 STE（开阔海岸的沙滩与海湾的泥滩）在两个不同季节（12 月与 8 月）的总汞（THg）和 MMHg 动态，并进一步应用基于盐度的保守混合模型（CMM）来量化水文和生物地球化学过程对 STE Hg 循环的耦合效应。我们的现场数据表明，随着时间和空间的推移，两个 STE 中的地下水和沉积物中的 THg 和 MMHg 浓度以及%MMHg/THg 均有很大变化。CMM 结果清楚地显示了在不同地点和季节之间，溶解的 THg 和 MMHg 与地下水盐度之间存在明显的差异，Hg 丰富的泥滩的浓度和百分比偏差明显高于 Hg 贫的沙滩。我们的研究结果表明，Hg 在两个 STE 的地下水流路径上表现出非保守混合行为，富 Hg 的潮间带可能是通过 SGD 向沿海水域产生和释放溶解的 THg 和 MMHg 的热点区域。我们的研究结果提供了现场证据，突出了水文变化和生物地球化学过程共同驱动 STE 中 Hg 的复杂传输和生物地球化学转化。STE 中 Hg 的非保守混合行为也强调，为了更准确地计算 SGD 衍生的 Hg 通量到沿海海域，我们需要仔细选择 STE 的地下水分区，以更好地代表输出端元。我们的研究结果还表明，人类活动和气候变化将深刻改变全球沿海含水层中的 Hg 生物地球化学循环和毒理学。
