Modeling Mamala Bay Outfall Plumes. II: Far Field

The far-field behavior and water quality impacts of the Sand Island, Hawaii, ocean outfall plume were predicted using two models, a statistical short-term model and a long-term model. The short-term model is coupled to a near-field model and uses measurements obtained from Acoustic Doppler Current Profilers. It predicted that the variable currents would cause the visitation frequency of the plume to decrease rapidly with distance. The long-term model predicted that flushing, horizontal diffusion, and decay would result in high dilutions with no significant buildup of contaminants. The frequencies of exceedence of various coliform densities were predicted to decrease rapidly with distance so that bacterial water quality standards should be satisfied very close to the diffuser. Any outfall impacts on the beaches should be very small, and other nonsewage sources are likely to contribute higher levels of bacteria. The effects of treatment upgrades on shoreline water quality will therefore probably be negligible. The results indicate that worst-case conditions are extremely improbable, and their use could lead to overly conservative outfall designs and treatment levels.