Wave-Driven Porewater-Seawater Exchange in Sandy Coastal Sediments

Shelf systems with permeable (sandy) seabeds are a focus of considerable recent interest. However, there is currently only a poor understanding of the exact interplay between physical forcing, sediment-seawater fluxes, sediment alteration, and pelagic ecosystem response in these dynamic systems. A series of laboratory experiments and field validations were performed to observe porewater-seawater mixing of a fluorescent dye tracer (fluorescein) using a fiber optic system in permeable sediments (~75% within 125-250 μm grain size, ~0.3 porosity) under enhanced hydraulic conditions. Preliminary laboratory results suggested the fiber optic probe was effective at measuring wave-enhanced tracer mixing. Mixing of dye, driven by paddle-generated standing waves, was several orders of magnitude greater than molecular diffusion (3.96±0.9 x 10-4 versus 8.4±0.8 x 10-6 cm2 s-1, respectively). Similarly, this approach has been adapted to a field site on the south shore of Oahu, Hawaii (21.29° N, 157.87° W) utilizing in situ instrumentation at 10 m water depth. A single fiber optic probe was used as a first approach for sampling porewater mixing rates, although multiple probes will be used in field measurements commencing in fall, 2005. In addition, sediment surface and subsurface pressure data were collected simultaneously to measure the effects of surface waves on porewater mixing rates. This was the first step of a series of experiments that will be integrated into the Kilo Nalo nearshore reef observatory for the measurement of porewater/seawater mixing rates over a range of surface wave conditions.