Field and Model Studies of the Boston Outfall

Near- and far-field oceanographic measurements of the plume from the Boston outfall were made over two days in April 2001. The results were consistent with the original physical model studies on which the diffuser design was based. New laboratory experiments were run to simulate the actual conditions at the time of the field tests using three-dimensional laser-induced florescence (3DLIF) to measure dilution. The 3DLIF results were very close to the field observations and provided considerable additional insight into the near-field mixing processes: the plumes from each port of the multiport risers merge, first with other ports on the same riser, and then with similar plumes from adjacent risers; mixing transverse to the current direction then gradually erases any vestiges of lateral concentration variations. The field results were consistent with predictions of the mathematical model NRFIELD. Measurements were also made in the far field on the second sampling day. For the observed travel times of up to 24 h, the increase in dilution was small and was due to horizontal diffusion only; vertical diffusion was negligible due to the stable oceanic density stratification. The results, and those of tests conducted in July 2001, indicate that the main features of the near-field processes of ocean wastewater outfalls can be closely predicted by small-scale physical model experiments and the empirical mathematical models derived from them. The results confirm that diffusers with widely spaced risers can effectively dilute and disperse the effluent.