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COI Funded Project: A New Differential Pressure Data Logger for Measuring Fluid Flux Across the Sediment-Water Interface Exchange

Project Funded: 2005

Proposed Research

Seawater circulation through coastal sediments can have important impacts on geochemical cycling, the timing of geochemical fluxes to surface water, and the storage of freshwater in coastal aquifers. Recent studies have revealed a major uncertainty in seawater circulation across the sediment-water interface: although significant outflow of saline groundwater from sediments into the coastal ocean has been observed in many settings, comparable volumes of seawater inflow have not been measured. Clearly such inflow must occur, but the spatial and temporal characteristics of this inflow are poorly known and speculative. A new hypothesis to explain these observations suggests that a significant component of seawater circulation through coastal sediments occurs on a yearly time-scale that is driven by recharge to the terrestrial groundwater system (Michael et al., in review). As precipitation recharges aquifers, the water table rises and freshwater displaces saline water seaward, resulting in saline groundwater discharge (SGD). When recharge is reduced or negative, as might occur in regions with high summer-time evapotranspiration rates, the water table falls and seawater intrudes into the aquifer. This seasonal forcing hypothesis has recently been proposed and has not yet been tested in a thorough manner. Current technologies are insufficient for testing this hypothesis, which requires measuring SGD over a year, because of operational difficulties such as power and other maintenance requirements or well destruction by sea ice. We propose to develop a new data logger to overcome these difficulties that will measure differential pressure between sediment porewater and surface water from which we can infer water flow direction and velocity. This project consists of designing, building, and testing the new data logger. The new instrument will also be deployed for up to one year in Waquoit Bay , Massachusetts , to test the seasonal forcing hypothesis by observing if the tidally-averaged pressure gradient reverses direction during the year. This project will provide a useful tool for coastal hydrology and oceanography and will test a potentially major forcing mechanism in fluid exchange across the land-sea interface.

Originally published: January 1, 2005