Mixing, dispersion, and resuspension in vicinity of ocean wastewater plume

Buoyant plumes discharged from ocean outfalls are important for dispersing municipal wastewater into the marine environment in many coastal areas, although field studies of operating outfall systems are rare. Here, we report on an extensive field study of the effluent plume from one of the largest wastewater outfalls on the west coast of the United States. This study shows that the dispersion and mixing levels of the plume depend upon the local current speed and ambient density stratification. Under these highly stratified, late-summer conditions, we find that a combination of temperature, salinity, turbidity, and chlorophyll fluorescence measurements are required to unambiguously identify plume waters. Over the course of these observations, the lcoal current speed at plume depth varied from 0.01 m/s to 0.07 m/s, corresponding to a Froude number Fr range of 10⁻³ to 0.2. A limited number of dilution estimates are made for the lower Fr case, and these fall in the range 110 to 160, which is within the design range for this type of diffuser. The observed maximum height of rise and wastewater field thickness are in reasonable agreement with laboratory results for zero current speed (Fr = 0). At the higher current speed, turbidity layers originating at the sea floor are observed which result from resuspension of bottom sediments.