冬夏两季太湖潜流带水流形态对其氮素运移转化及交换通量的影响机制

Hyporheic zone, as an important joint and transition region between surface- and ground-water, is an international hotspot of science research over recent two decades. The hyporheic zone in a lake has significant stratification characteristic and gradient environmental variables, with abundant biogeochemical elements. This study will focus on the flow patterns and nitrogen transport in hyporheic zone of Taihu Lake, a large shallow freshwater lake, to better illuminate the relationship of surface sediment as a source or a sink to surface water at different sites in the lake and its seasonal conversion rule. Results from field monitoring, intact sediment core experiments, and mathematical simulation would distinctly elucidate three crucial aspects: (1) the water regimes and varying gradients of internal environment variables in different-site hyporheic zones in Taihu Lake during both wet and dry seasons, and subsequent influence mechanism on nitrogen transport and transformation processes; (2) the vertical distributions of aerobic- and anaerobic-zones and corresponding rates of nitrification and denitrification derived both from various water regimes in hyporheic zone and seasonal temperature of surface water; and (3) the seasonal dynamics of nitrogen exchange fluxes in different-site hyporheic zones with surface water. This will help us to better understand the mechanism of nitrogen transport and transformation processes in the hyporheic zone in a large shallow lake, and the relationship of hyporheic zone as a sink or a source to the lake water, and will also strongly help to propose scientific ideas and measures for comprehensive managements of Taihu Lake, especially for selecting hydrophytes planting areas.

潜流带是地表水-地下水交互作用的重要衔接过渡区，近二十年来其水流形态及物质循环成为国际上科学研究的热点。湖泊潜流带分层特征明显，各种生物地球化学元素丰富，内部环境梯度显著，对其水流形态和氮素运移转化影响较大。为探明湖泊不同区域表层沉积物对地表水体的“源”、“汇”关系及季节性转化规律的科学问题，本申请课题以大型浅水湖泊——太湖的潜流带为对象，通过现场监测、原状柱模拟试验和数值模拟等方法，针对冬夏两季太湖不同区域潜流带内水流形态及其对氮素运移转化的影响机制进行研究，揭示地表水-地下水交互作用下潜流带水流形态、内部环境变化梯度和温度分布季节性变化对潜流带好氧-厌氧区分布范围及氮素硝化反硝化系数的影响，测算太湖各区域潜流带中氮素与地表水体的交换通量及季节变化规律，掌握大型浅水湖泊潜流带中氮素循环特征，为水生植物种植区选择提供参考，也为太湖水体综合治理提供科学依据。

本项目以大型浅水湖泊——太湖的潜流带为对象，现场监测了太湖东岸区、南岸区、西岸区、北岸区（梅梁湾）及湖心区共14个监测点位的柱样分层沉积物样和相应水体分层水样，分析指标包括粒径分布、孔隙度、有机质、pH、ORP、温度、不同形态氮素含量、硫化物、Fe等的垂向分布及上覆水体的相关指标；研究了太湖周边农田氮素下渗进入浅层地下水的过程，初步明确了氮素经历非饱和带向饱和带迁移过程的转化关系，初步证实周边潜在污染源（农田等）通过湖岸带进入潜流带并向湖泊水体迁移的过程；在室内实验室采用采集的原状沉积物柱样，开展了不同水头压力分布和不同温度变化下氮素矿化、硝化、反硝化、厌氧氨氧化、硝酸盐异化还原为铵等转化过程的影响实验研究，以及亚铁氧化耦合硝酸盐还原脱氮机制方面的研究；采用热量传递模型模拟等率定了不同区域不同季节下潜流带水流形态及不同形态氮素的迁移通量历时分布。研究结果显示，随着降水、灌溉活动等变化，太湖周边农田、鱼塘、湿地等污染蓄积体可通过浅层地下水和潜流带与太湖水体交互，东部湖区潜流带水流方向随着季节发生变化，流量分布在-5.49-2.48 cm/day，在南部湖区则主要为下行流，流量分布在-2.7-9.5 cm/day，而在西部和北部湖区则常年为上行流，流量分布为-11.12-2.48和-12.26-0.21 cm/day，中部湖区则平均流量接近于零。根据现场监测资料及流量通量分布，初步估算太湖潜流带年均氮素释放通量约为9.6 g/m2，其中东部、南部、西部、北部和中部点位分别约为1.7、-20.4、34.3、43.7和0.25 g/m2。同时，各个点位特别是东部点位的潜流带水流方向会随着季节发生变化，所有点位的潜流大小也随着季节水位变化而发生变化。本项目结果将有利于进一步提升太湖水环境模拟计算及污染调控方面的精度并考虑到季节和空间差异性。在综合治理太湖水环境的过程中，应该进一步考虑湖泊流域的综合思维，特别是湖泊周边的农田、旱地、湿地、鱼塘等营养物质蓄积体应该统一纳入综合治理范畴，从地表和地下双管齐下，截断或减少通过地下进入太湖水体的营养物质。