地下水向海排泄过程中无机氮迁移转化及动力学数值模拟

Submarine groundwater discharge(SGD) is a major process operating at the land-sea interface. Quantifying the SGD nutrient loads and the marine/terrestrial controls of this transport may play a significant role in nutrient cycling and primary productivity in the coastal ocean. In most of current works, nutrient concentrations measured in onshore wells were multiplied by offshore groundwater discharge (measured by geochemical tracers, seepage meters or numerical model) to derive fluxes. However, the method has a limitation: it assumes there is no add-in or removal of nutrient on the path from groundwater to seawater. To solve these problems, we need to understand how the nutrient migrate and transform that effect by the redox condition and biogeochemical reactions in the coastal subterranean area. Taking dissolved inorganic nitrogen as example, we propose to combine multiple techniques, including field survey with infrared camera, in-situ observation of hydrogeological features in typical test site, to acquire long-term, persistent, and high frequent measurement of hydrochemical environmental factors. Based on these data, we will construct the coupled reactive transport and groundwater flow numerical model and explore the temporal and spatial variability of ammonia, nitrate and nitrite and their environmental controls. A potential estimation method for quantifying dissolved inorganic nitrogen discharge with groundwater to ocean will be proposed with model results.

海底地下水排泄(SGD)是陆地向海洋输送水量和物质的重要通道，对沿海生态环境的影响不容忽视，准确地刻画SGD携带营养盐通量对理解海陆物质交换机制有着十分重要的作用。目前计算营养盐通量时，主要是以陆源测量的营养盐浓度和SGD排泄量的乘积来表示。此方法假设营养盐含量和形态在迁移过程中是保守的，忽略了营养盐在地下河口复杂生物地球化学环境中的转化及其受水动力条件的影响。针对主要营养盐元素-无机氮，本项目以珠江口邻近海域的大鹏湾为研究区，首先利用红外影像确定地下水主要排泄区，然后选择典型区建立水动力试验场，对水位、理化因子、氮营养盐和有机物等变量进行连续监测，揭示氨氮、硝态氮和亚硝态氮在地下河口淡水、咸水和过渡带的时空变化及迁移转化机理，在此基础上耦合生物化学反应构建反应溶质运移模型，计算氮营养盐通量并分析其主控环境因子，为地下河口氮循环提供新认识，为科学评估SGD对海洋生态环境影响提供依据。

海底地下水排泄(SGD)是陆地向海洋输送水量和物质的重要通道，对沿海生态环境的影响不容忽视，准确地刻画SGD携带营养盐通量对理解海陆物质交换机制有着十分重要的作用。本研究基于地下水动力试验场观测、水化学采样分析和热红外成像等手段，获取高分辨率的滨海水表温度和水文环境要素。采用氡同位素示踪技术与相应的平衡模型计算了SGD携带的氮营养盐入海通量。同时通过16S rRNA基因测序方法解析了SGD过程无机氮迁移转化微生物时空分布特征和无机氮迁移转化规律及主控环境因子。主要结果如下：（1）总体海表水温在近海由沿岸向湾内逐渐增高，地下水在湾内离散点状排泄进入海水，丰水期海水排泄量高于枯水期；（2）特征排泄点位的海底地下水排泄通量的平均值为659.96Bq/（m2d），平均速率0.12m/d。营养盐中溶解无机氮(DIN)通量较大，说明是该区海陆物质交换过程中的一条重要途径；（3）表层沉积物的水化学指标变化趋势总体与下层沉积物中各指标变化趋势相同。表层沉积物中温度、pH值、电导率、总溶解氮、亚硝态氮平均值均低于底层沉积物，表层沉积物和下层沉积物中磷、总溶解磷、铵态氮平均值含量相等，而表层沉积物中的总溶解性固体、盐度、总溶解氧、氧化还原电位、硝态氮和硅含量平均值高于下层沉积物；（4）无机氮迁移转化过程中细菌与古菌均参与其中。Proteobacteria相对丰度最高，其次是Bacteroidetes和Actinobacteria。（门水平）；（5）从潮上带到潮下带，硝化作用、固氮作用、硝酸盐呼吸作用和氮呼吸作用微生物群落均逐渐减少。