SHAPE AND INITIAL DILUTION OF SAND ISLAND, HAWAII SEWAGE PLUME

The wastewater plume discharged from the Sand Island Treatment Plant, Hawaii outfall diffuser was mapped several times during September 25-October 1, 1994. The deeply submerged plume was patchy at its center, thin on its edges, and displayed vertically separated layers on three out of five days of plume mapping. Complexity of the sewage plume shape was due to a combination of factors that include temporal and spatial variations in currents, temperature stratification, and internal tides. Equilibrium depth, thickness, and initial dilution of the sewage plume were derived from in-situ measurements proximal to the sewage outfall and were compared with simulation results from the Roberts, Snyder, and Baumgartner (RSB) model. Simulated equilibrium depths were within 6 m of their measured counterparts in all but two cases, and simulated thicknesses of the plume were larger than measured thicknesses in all but two cases (out of 11 cases). Simulated dilutions were 1.9 times the dilution values derived from in-situ data. Dilution differences are explained by lack of temporal resolution in velocity measurements and differences between the engineering definition and the oceanographic characterization of initial dilutions.