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

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沃茨的淡水地下水。 这一观测结果可能对营养物向沿海海洋的输送量和时间以及内陆地下水通量产生重要影响。 然而，驱动这种含盐地下水循环的物理机制知之甚少，事实上，很少有盐水流入已被发现，以平衡所观察到的排放。我们假设，海水交换往往是由内陆补给的季节性振荡。由于海水和淡水之间的密度差异，淡水-盐水界面的移动可能是地下水位季节性移动的40倍之多，从而导致大量含盐地下水交换。然而，在假设的冬季盐水流入期间，几乎没有直接测量跨沉积物界面的通量，这个季节性通量的实际大小及其对河口营养循环的影响是未知的。 盐水循环的其他机制包括潮汐泵送、淡水排放中的盐水夹带和海滩上的波浪爬高。这些过程增强了界面处的混合，并可能主导某些区域的海水循环。分散过程如何与季节性循环相互作用，或季节性交换和分散循环的相对大小如何取决于含水层特性、海岸过程或补给模式，目前尚不清楚。 我们将解决以下问题：（1）如何咸水地下水循环和淡水排放的组合强迫发生在不同的时间尺度，如内陆补给和潮汐？(2)这些过程如何影响营养物向沿海沃茨的输送？ 为了回答这些问题，我们将结合联合收割机高频测量的海底流量和流入的密度耦合瞬态沿海地下水系统的详细数值模拟。测量将依赖于通过无线链路连接的流量站网络;垂直水力梯度将自动测量并中继到海岸。我们将结合联合收割机详细的水和化学通量的实地测量与地下水流的模拟模型，以开发驱动沿海含水层的海水交换和地下水流的过程的理解。含水层和沿海沃茨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之间存在的各种问题。 首先，通过沿海含水层的海水循环影响内陆淡水系统，改变淡水地下水的储存，可能调节季节性地下水位循环，并影响地下水资源超采地区的盐水入侵和上锥。 在沿海水文水收支和地下水模型中，穿过海岸线的通量是不确定性的重要来源。 第二，海水环流通过将化学品运入和运出海洋而影响沿海沃茨。 例如，过多的营养物投入可能通过藻类过度生长造成的富营养化对渔业产生不利影响。在沿海含水层的淡水-盐水相互作用的动态控制这些溶质通量的沿海沃茨的时间。 盐水循环的空间格局和动态可能驱动沿海含水层内的重要生物地球化学反应。

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.