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

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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.