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Characterization and investigation of submarine groundwater discharge from a coastal aquifer into the nearshore environment Caulkins, Joshua Lee

Abstract

The current trend of global populations moving increasingly to high density, coastal cities places a greater emphasis upon the water quality of aquifers supplying those cities. Problems that affect coastal aquifers (e.g. saltwater intrusion, non-point source pollution) will be amplified as this trend increases. The goal of this research project is to understand the processes that control freshwater/saltwater interactions in the coastal zone, specifically in areas of high submarine groundwater discharge (SGD). A site in NW Florida along the Gulf Coast is a well-documented SGD locale and provides an excellent opportunity to examine how tidal fluctuations, differential pressure in the seabed and groundwater seepage rates are interrelated. Experiments at the site focus on characterization of the nearshore aquifer, submarine groundwater discharge at the seabed and numerical modeling of the system. New onshore wells at the field site show that hydraulic conductivities in onshore region are similar to those in the offshore region. Slug tests and water level monitoring of the onshore wells are used to measure a seaward, horizontal hydraulic gradient. Direct measurements of discharge are conducted with an automated seepage meter, which shows that peak discharge rates tend to occur at the transition between high and low tides. A new apparatus called a differential piezometer system (DPS) is designed and used to measure differential hydraulic head in the seabed created by seepage and tidal interactions. This system failed to accurately portray differential head fluctuations in the seabed as a result o f calibration error. Salinity samples are collected from on and offshore wells and from a newly installed multi-level well, the data of which are made into salinity profiles. These profiles define the boundaries of a saline wedge and mixing zone in the nearshore region. FRAC3DVS is flow and transport groundwater model that is used to design and run a 1-dimensional numerical model. The model results confirm the temporal effects of tidal elevation on discharge rates observed in the seepage meter.

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