The sewage plume resulting from the discharge of treated municipal waste water from the Sand Island Treatment Plant diffuser in Mamala Bay, Oahu, Hawaii was detected and mapped during a September 25 - October 1, 1994 cruise. The three main objectives of the study were to: (1) detect the plume and calculate its initial dilution; (2) optically characterize the plume; and (3) determine the plume dynamics.

The sewage plume was detected unambiguously in situ and in real-time by its high beam attenuation coefficient at 660 nanometers (nm) and its low salinity signatures. The plume was generally located west of the diffuser, spreading along the isobaths due to the forcing of the dominant currents. Identical but weaker signatures located deeper than the plume were interpreted as "old" plumes, relative to more recently discharged waste water with stronger signatures. The initial dilution of the plume was calculated using temperature/salinity diagrams and by plotting initial mixing lines between discharged effluent and ambient waters. The initial dilution was low (always below 250:1) compared to the dilution expected for surfacing plumes (at least 1000:1). These low dilutions were due to the trapping of the plume at depth, caused by an unusual temperature stratification of the water column during the cruise.

The optical characteristics of the plume included increases in absorption and beam attenuation coefficients, and increases in ultra-violet fluorescence relative to ambient waters. Fluorescence for the excitation/emission pair 228nm/340nm was associated with both recent and old plumes, but not with phytoplankton, and was interpreted as tryptophan-like fluorescence. This signal should be further investigated as a potential in situ waste water tracer.

Strong temporal variability in the temperature stratification was reflected in the plume dynamics. Oscillations in plume depth revealed the vertical forcing by an internal tide propagating westward along-shore. This type of physical process is generally not accounted for in three-dimensional circulation models and could explain discrepancies between in situ observations and plume modeling results. This vertical forcing by internal tides could lead to unpredicted contamination of sediments or surface waters.