Farfield Tracing of a Point Source Discharge Plume in the Coastal Ocean Using Sulfur Hexafluoride

摘要

Pathways and dilution of a point source ocean discharge in the farfield (approx= 10-66 km) were measured using the deliberate tracer sulfur hexafluoride (SF_6).The injection of SF_6 was performed by bubbling the gas over a period of 6 days into an ocean outfall pipe discharging into the southeast Florida coastal ocean.The surface SF_6 concentrations show that the discharged water flowed northward parallel to the coast with a broadening of the width of the plume to about 3 km at the farthest point sampled,66 km from the outfall.The discharge was fully mixed throughout the water column within 13 km of the outfall terminus.In the first 20 km from the outfall,SF_6 surface concentrations were highly variable,while beyond this the SF_6 concentrations decreased monotonically going northward.The currents were measured during the study with a bottom-mounted acoustic Doppler current profiler (ADCP) located 5.5 km from the outfall.Velocities were variable in magnitude and direction but showed a net northward flow during the 6-day study.Maximum concentrations decreased by about 200-fold per kilometer from the outfall to the northern end of the study area.The study shows that SF_6 is an effective method to trace point source releases far from their origin.