Field Study of Pollutant Migration in the Vicinity of a Coastal Front

Discharge of estuarine water onto the shelf can create plumes with limits defined by fronts. The associated density field can produce a circulation with flow toward the front and back down along a density interface. In a field study, surface and dispersed pollutant simulators were released in the vicinity of such a front and tracked using aerial photographic techniques. Tracers tended to converge to the front at only a small portion (10%) of the front´s migration velocity. Most fixed-frame tracer velocities resulted from tidal currents and plume growth. Wind effects caused divergence between aluminum chips and dyes. At times, wind shear moved aluminum across the front and against the convergent surface currents. When shear was weaker, aluminum accumulated on a moving front. Dyes were sobducted along the density interface. Within the plume, vertical mixing and resultant dispersion were probably due to stronger vertical gradients in horizontal velocity than found in the ambient velocity field. Exchange between plume and ambient water was often inhibited by the strong stable pycnocline.