Collaborative Research: Saltwater and Freshwater Fluxes through Coastal Aquifers: Multiple Time Scales of Terrestrial and Oceanic Forcing

合作研究：通过沿海含水层的咸水和淡水通量：陆地和海洋强迫的多个时间尺度

• 批准号: 0548706
• 负责人: Charles Harvey
• 金额: --
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-04-01 至 2010-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0548706&HistoricalAwards=false
• 关键词: Collaborative; Research; Saltwater; Freshwater; Fluxes

Field studies have shown that seawater circulation through coastal aquifers may exceed fresh groundwater discharge into coastal waters. This observation has potentially important implications for the quantity and timing of nutrient transport into the coastal ocean, as well as for inland groundwater fluxes. However, the physical mechanisms driving this saline groundwater circulation are poorly understood, and, in fact, little saline inflow has been found to balance the observed discharge. We hypothesize that seawater exchange is often driven by seasonal oscillations in inland recharge. Because of the density difference between seawater and freshwater, the freshwater-saltwater interface can potentially move by as much as forty times the seasonal movement of the water table, driving large quantities of saline groundwater exchange. However, few direct measurements of flux across the sediment interface have been taken during the hypothesized winter saline inflow period, and the actual magnitude of this seasonal flux and its influence on estuarine nutrient cycles is unknown. Other mechanisms of saline circulation include tidal pumping, saline entrainment in freshwater discharge, and wave run-up on the beach. These processes enhance mixing at the interface and may dominate seawater circulation in some regions. It is unknown how dispersive processes interact with seasonal cycles, or how the relative magnitude of seasonal exchange and dispersive circulation may depend on aquifer properties, coastal processes, or recharge patterns. We will address the following questions: (1) How do saline groundwater circulation and freshwater discharge respond to combined forcing from processes that occur over different time-scales such as inland recharge and tides? (2) How do these processes affect nutrient delivery to coastal waters? To answer these questions, we will combine high-frequency measurements of submarine discharge and inflow with detailed numerical simulation of the density-coupled transient coastal groundwater system. Measurements will rely on a network of flux stations connected by wireless links; vertical hydraulic gradients will be measured automatically and relayed to shore. We will combine detailed field measurements of water and chemical fluxes with simulation models of groundwater flow to develop an understanding of the processes that drive seawater exchange and groundwater flow in coastal aquifers. Accurate physically based models of water exchange between aquifers and coastal waters could help solve a variety of problems that exist at the interface of hydrology and coastal oceanography. First, seawater circulation through coastal aquifers impacts inland freshwater systems by changing the store of fresh groundwater, potentially modulating seasonal water table cycles, and affecting saltwater intrusion and up-coning in regions where groundwater resources are overdrawn. Flux across the shoreline is a significant source of uncertainty in coastal hydrologic water budgets and groundwater models. Second, seawater circulation affects coastal waters by transporting chemicals into and out of the ocean. For example, excess nutrient inputs can adversely affect fisheries through eutrophication caused by excessive algal growth. The dynamics of freshwater-saltwater interactions in coastal aquifers controls the timing of these solute fluxes to coastal waters. The spatial patterns and dynamics of saline water circulation may drive important biogeochemical reactions within coastal aquifers.

实地研究表明，通过沿海含水层的海水循环可能超过流入沿海水域的新鲜地下水排放量。这一观察结果对营养物质向沿海海洋输送的数量和时间以及内陆地下水通量具有潜在的重要影响。然而，驱动这种咸水地下水循环的物理机制还知之甚少，事实上，几乎没有发现咸水流入来平衡观测到的排放量。我们假设，海水交换通常是由内陆补给的季节性振荡驱动的。由于海水和淡水之间的密度差异，淡水-咸水界面的移动可能是地下水位季节性移动的40倍，从而驱动大量的咸水地下水交换。然而，在假想的冬季咸水流入期间，几乎没有对沉积物界面上的通量进行直接测量，而且这种季节性通量的实际大小及其对河口营养循环的影响尚不清楚。咸水循环的其他机制包括潮汐抽水、淡水排放中的咸水夹带和海滩上的波浪抬高。这些过程增强了界面上的混合，并可能主导某些区域的海水循环。目前尚不清楚弥散过程如何与季节循环相互作用，也不清楚季节交换和弥散循环的相对大小如何取决于含水层性质、沿海过程或补给模式。我们将解决以下问题：(1)咸水地下水循环和淡水排放如何响应不同时间尺度上发生的过程(如内陆补给和潮汐)的联合强迫？(2)这些过程如何影响营养物质向沿海水域的输送？为了回答这些问题，我们将把海底流量和流量的高频测量与密度耦合的滨海地下水系统的详细数值模拟相结合。测量将依赖于通过无线链路连接的通量站网络；垂直水力坡度将被自动测量并传递到岸上。我们将结合水和化学通量的详细现场测量与地下水流动的模拟模型，以加深对沿海含水层中驱动海水交换和地下水流动的过程的理解。建立含水层和沿海水域之间水交换的精确物理模型有助于解决水文学和沿海海洋学交界处存在的各种问题。首先，通过沿海含水层的海水循环改变了淡水地下水的储存，潜在地调节了季节性地下水位循环，并影响了地下水资源透支地区的咸水入侵和上升，从而影响了内陆淡水系统。穿过海岸线的流量是沿海水文水量平衡和地下水模型中不确定性的一个重要来源。其次，海水循环通过将化学品输送到海洋和将化学品输送出海洋来影响沿海水域。例如，过量的营养投入可能会通过藻类过度生长造成的富营养化对渔业造成不利影响。沿海含水层中淡水-咸水相互作用的动态控制着这些溶质流向沿海水域的时间。咸水循环的空间模式和动态可能推动沿海含水层内的重要生物地球化学反应。