Collaborative Research: Surface water-groundwater connectivity in the Tidal Freshwater Zone and the fate of nitrogen in tidal rivers

合作研究：潮汐淡水区地表水与地下水的连通性以及潮汐河中氮的归宿

• 批准号: 1446763
• 负责人: Rebecca Barnes
• 金额: $ 9.49万
• 依托单位: Colorado College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-03-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1446763&HistoricalAwards=false
• 关键词: Collaborative; Research; Surface; water; groundwater

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.

人类通过食品和能源生产活动极大地改变了氮循环。氮气增加到景观通常会导致氮出口到海岸，从而导致藻华，死区和渔业下降。河岸含水层和河床中的硝酸盐清除提供了重要的生态系统服务，可减轻氮负荷到海岸。 然而，绝大多数氮从未到达海岸，而是将其保留在微生物的流域或转化。河岸和底部是保留和转型的主要位置，提供了宝贵的生态系统服务。在潮汐淡水区（TFZS）中，潮汐在河岸和河床中抽水，清除硝酸盐可能特别有效。这项研究将确定TFZ的氮去除效率。如果TFZ在流域内的氮中去除氮中起着不成比例的作用，则管理策略应通过维持河岸缓冲区和限制可能堵塞河床的沉积物来寻求保护TFZ，从而降低其去除效率。这项研究应提高我们在淡水中管理硝酸盐的能力，并更好地重视潮汐淡水区和河口的生态系统服务。 结果将通过两种方式与公众共享：（1）Creek Fest，这是一项推广分水岭的宣传活动，向1000多名与会者促进了管理人员，PIS将与当地利益相关者讨论对TFZ管理的影响； （2）PIS还将开发一次虚拟实地考察，以在潮汐环境中的生态系统服务上教育高中和大学生。 TFZ很少监测排放或营养通量，是河流和河口之间的动态过渡区。在TFZ中，半潮汐阶段波动应促进强烈的银行存储和释放。银行存储区可能是去除硝酸盐的有效地点。但是，相关的地下水位波动也可能使浅地下水充气并增强硝酸盐，从而将硝酸盐添加到地下水中。这两个潮汐引起的相反过程的净效应范围从硝酸盐净产量到TFZ内的去除。该提案询问潮汐诱导的流体动力学过程，例如银行存储和地下水位波动，控制着TFZ的河岸含水层内的氮转化，以及这些过程如何高档影响从陆地到海洋的氮气通量。假设：1）TFZ流体动力学促进了河岸含水层中氮转化的两个热点：在土壤 - 地面水界面上的硝化热点，在土壤 - 地面水界面上，半潮湿的地下水位的波动氧气氧气氧化了浅层地下水，并在地面水上散发着浅水层面的层次散发出层状的山顶山顶播放层。 2）随着TIDAL范围在TFZ内的下游方向上增加，硝酸盐通过硝酸盐通过反硝化而增加的硝酸盐增加了。这些假设将通过TFZ，实验室实验和数值模型中的现场观察结果进行测试。该田间成分将表征横跨河流界面的流体和氮通量，并在Christina River盆地（特拉华州）内的TFZ的河岸含水层中识别非保守的氮转运。沉积物核心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。数值模型将用于沿着克里斯蒂娜河流域的TFZ上升氮命运。