Collaborative Research: Freshwater and Saltwater 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会阻止新鲜地下水排入沿海沃茨。这一观测结果对营养物质向沿海海洋的输送量和时间以及内陆地下水通量具有潜在的重要意义。然而，物理mechanismsdriving这种含盐地下水循环知之甚少，事实上，很少salineinflows已被发现，以平衡所观察到的排放。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-盐水界面可能会以地下水位季节性移动的40倍之多移动，从而推动大量含盐地下水交换。然而，在假定的冬季盐水流入期间，几乎没有直接测量过沉积物界面的通量，而且这种季节性通量的实际大小及其对河口营养盐循环的影响尚不清楚。其他的盐水循环机制包括潮汐泵送、淡水排放中的盐水夹带和海滩上的波浪起伏。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，或者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、海岸过程或补给能力，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响应的过程，发生在不同的时间尺度，如内陆补给和潮汐联合强迫？(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，我们将把高频测量的海底流量和流入量与详细的密度耦合瞬态沿海地下水系统的数值模拟相结合。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，以发展对驱动沿海含水层中的海水交换和地下水流的过程的理解。含水层和沿海水域之间的水交换的精确的物理模型可以帮助解决存在于水文学和沿海海洋学界面的各种问题。首先，通过沿海含水层的海水循环影响陆地淡水系统，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，穿越海岸线的通量是一个重要的不确定性来源。第二，海水循环通过将化学品输送到海洋和从海洋中输送出来而影响沿海沃茨。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力学可能驱动沿海含水层内重要的生物地球化学反应。

0548691Mullingan.Field studies have shown that seawater circulation through coastal aquifers may exceedfresh groundwater discharge into coastal waters. This observation has potentiallyimportant implications for the quantity and timing of nutrient transport into the coastalocean, as well as for inland groundwater fluxes. However, the physical mechanismsdriving this saline groundwater circulation are poorly understood, and, in fact, little salineinflow has been found to balance the observed discharge. We hypothesize thatseawater exchange is often driven by seasonal oscillations in inland recharge. Becauseof the density difference between seawater and freshwater, the freshwater-saltwaterinterface can potentially move by as much as forty times the seasonal movement of thewater table, driving large quantities of saline groundwater exchange. However, fewdirect measurements of flux across the sediment interface have been taken during thehypothesized winter saline inflow period, and the actual magnitude of this seasonal fluxand its influence on estuarine nutrient cycles is unknown. Other mechanisms of salinecirculation include tidal pumping, saline entrainment in freshwater discharge, and waverun-up on the beach. These processes enhance mixing at the interface and maydominate seawater circulation in some regions. It is unknown how dispersive processesinteract with seasonal cycles, or how the relative magnitude of seasonal exchange anddispersive circulation may depend on aquifer properties, coastal processes, or rechargepatterns. We will address the following questions: (1) How do saline groundwatercirculation and freshwater discharge respond to combined forcing from processes thatoccur over different time-scales such as inland recharge and tides? (2) How do theseprocesses affect nutrient delivery to coastal waters? To answer these questions, we willcombine high-frequency measurements of submarine discharge and inflow with detailednumerical simulation of the density-coupled transient coastal groundwater system.Measurements will rely on a network of flux stations connected by wireless links; verticalhydraulic gradients will be measured automatically and relayed to shore. We willcombine detailed field measurements of water and chemical fluxes with simulationmodels of groundwater flow to develop an understanding of the processes that driveseawater exchange and groundwater flow in coastal aquifers.Accurate physically based models of water exchange between aquifers and coastalwaters could help solve a variety of problems that exist at the interface of hydrology andcoastal oceanography. First, seawater circulation through coastal aquifers impactsinland freshwater systems by changing the store of fresh groundwater, potentiallymodulating seasonal water table cycles, and affecting saltwater intrusion and up-coningin regions where groundwater resources are overdrawn. Flux across the shoreline is asignificant source of uncertainty in coastal hydrologic water budgets and groundwatermodels. Second, seawater circulation affects coastal waters by transporting chemicalsinto and out of the ocean. For example, excess nutrient inputs can adversely affectfisheries through eutrophication caused by excessive algal growth. The dynamics of freshwater-saltwater interactions in coastal aquifers control the timing of these solute fluxes to coastal waters. The spatial patterns and dynamics of saline water cirulation may drive important biogeochemical reactions within coastal aquifers.