Collaborative Research: Surface water-groundwater connectivity in the tidal freshwater zone and the fate of nitrogen in tidal rivers

人类通过食物和能源生产活动极大地改变了氮循环。景观氮负荷的增加往往导致氮输出到海岸，导致藻华，死亡区和渔业下降。河岸带含水层和河床中硝酸盐的去除提供了一种重要的生态系统服务，可减轻海岸带的氮负荷。 然而，绝大多数氮素从未到达海岸，而是保留在流域内或被微生物转化。河岸和河底是这种保留和改造的主要地点，提供了宝贵的生态系统服务。在潮汐淡水区（TFZ），潮汐将河水泵入和泵出河岸和河床，硝酸盐的去除可能特别有效。本研究将确定TFZ的脱氮效率。如果TFZs发挥不成比例的大作用，在流域内的氮去除，管理策略应寻求保护TFZs通过维持河岸缓冲区和限制沉积物来源，可能堵塞河床，降低其去除效率。这项研究将提高我们管理淡水中硝酸盐的能力，并更好地评估潮汐淡水区和河口的生态系统服务。 研究结果将通过两种方式与公众分享：（1）Creek Fest，这是一项向1000多名与会者推广流域管理的外联活动，PI将与当地利益相关者讨论TFZ管理的影响;（2）PI还将开发虚拟实地考察，在内陆教室教育高中和大学生潮汐环境中的生态系统服务。很少监测排放或营养通量，TFZ是河流和河口之间的动态过渡区。在TFZs，半日阶段波动应促进激烈的银行存储和释放。河岸蓄水区可能是去除硝酸盐的有效场所。然而，相关的地下水位波动也可能使浅层地下水通气，并增强硝化作用，使地下水中的硝酸盐增加。这两个潮汐引起的，相反的过程的净效应范围可以从净硝酸盐生产到TFZs内的去除。该建议询问潮汐引起的水动力过程，如银行存储和地下水位波动控制氮转化的TFZs和这些过程如何升级到影响氮通量从陆地到海洋的河岸含水层。假设：1）TFZ水动力促进了河岸带含水层中氮转化的两个热点：土壤-地下水界面处的硝化热点（地下水位的半日波动使浅层地下水发生硝化）和河流-地下水界面处的反硝化热点（地表水通过贫氧沉积物进行交换）; 2）随着在TFZ内的下游方向上潮差的增加，通过硝化作用产生的硝酸盐比通过反硝化作用去除的硝酸盐增加更多。这些假设将使用TFZ内的实地观察，实验室实验和数值模型的组合进行测试。现场部分将描述河流-含水层界面的流体和氮通量，并确定克里斯蒂娜河流域（特拉华州）TFZ河岸含水层的非保守氮迁移。沉积物芯将用于实验室柱实验，以探索地下水位波动，地下水氧化还原条件和氮转化之间的关系。数值模型将被用来升级氮的命运沿着的克里斯蒂娜河流域的TFZ。

Humans have dramatically altered the nitrogen cycle through food and energy production activities. Increased nitrogen loading to landscapes often results in nitrogen export to the coast, leading to algal blooms, dead zones, and declines in fisheries. Nitrate removal in riparian aquifers and riverbeds provides an important ecosystem service that mitigates nitrogen loads to coasts. However a vast majority of nitrogen never reaches the coast, rather it is retained within the watershed or transformed by microbes. Riverbanks and bottoms are prime locations for this retention and transformation, providing a valuable ecosystem service. In tidal freshwater zones (TFZs) where tides pump river water in and out of the riverbanks and riverbed, nitrate removal may be particularly effective. This study will determine the nitrogen removal efficiency of TFZs. If TFZs play a disproportionately large role in nitrogen removal within watersheds, management strategies should seek to protect TFZs by maintaining riparian buffer zones and limiting sediment sources that could clog riverbeds, reducing their removal efficiency. The research should improve our ability to manage nitrate in freshwater and better value the ecosystems services of tidal freshwater zones and estuaries. The results will be shared with the public in two ways: (1) Creek Fest, an outreach event that promotes watershed stewardship to over 1000 attendees, the PIs will will discuss implications for TFZ management with local stakeholders; and (2) the PIs will also develop a virtual field trip to educate high school and college students in land-locked classrooms on ecosystem services in tidal environments. Rarely monitored for discharge or nutrient fluxes, TFZs are dynamic transition zones between rivers and estuaries. In TFZs, semidiurnal stage fluctuations should promote intense bank storage and release. Bank storage zones may be efficient sites of nitrate removal. However, associated water table fluctuations may also aerate shallow groundwater and enhance nitrification, adding nitrate to groundwater. The net effect of these two tidally induced, opposing processes could range from net nitrate production to removal within TFZs. This proposal asks how tidally induced hydrodynamic processes such as bank storage and water table fluctuations control nitrogen transformations within the riparian aquifers of TFZs and how these processes upscale to influence nitrogen fluxes from land to sea. It is hypothesized that: 1) TFZ hydrodynamics promote two hot spots of nitrogen transformation in the riparian aquifer: a nitrification hot spot at the soil-groundwater interface where semidiurnal water table fluctuations oxygenate the shallow groundwater, and a denitrification hot spot near the river-groundwater interface where surface water exchanges through oxygen depleted sediments; 2) as tidal range increases in the downstream direction within the TFZ, nitrate production via nitrification increases more than nitrate removal via denitrification. These hypotheses will be tested using a combination of field observations within a TFZ, laboratory experiments, and numerical models. The field component will characterize fluid and nitrogen fluxes across the river-aquifer interface and identify non-conservative nitrogen transport in the riparian aquifer of a TFZ within the Christina River Basin (Delaware). Sediment cores will be used in laboratory column experiments to explore relationships between water table fluctuations, groundwater redox conditions, and nitrogen transformation. Numerical models will be used to upscale nitrogen fate along the TFZ of the Christina River Basin.