Mapping Ocean Outfall Plumes and their Mixing using Autonomous Underwater Vehicles

This paper reports on our experiences developing methods to survey an ocean outfall discharge plume using an Autonomous Underwater Vehicle (AUV). . The mobility of an AUV provides a significant advantage in surveying discharge plumes over traditional boat-based methods, and when combined with optical and oceanographic sensors, provides a capability for both detecting plumes and assessing their mixing in both the near and far-fields. Unique to this study is the optical sensing of of the presence of Colored Dissolved Organic Matter (CDOM) in the discharge plume for quantitatively assessing the dilution ratio of the plume water. This capability is demonstrated for two Publicly Operated Treatment Works (POTWs) located offshore San Diego, California that have significantly different environmental settings and operating parameters. The first is the South Bay Ocean Outfall (SBOO) operated by the International Boundary Water Commission (IBWC) whose discharge is located in approximately 30m deep water at a flow rate of 28 million gallons per day (MGD). The second discharge is the Point Loma Ocean Outfall (PLOO) which discharges at approximately 90m with a nominal volume flow rate of 250 MGD. The methodologies for planning the AUV missions to sample the discharge plume, characterizing the plume using the signals measured by the onboard oceanographic and optical sensors, and use of those data in predictive models is outline. The results suggest that even in variable oceanic conditions, properly planned missions for AUVs equipped with appropriate sensors can accurately characterize and track ocean outfall plumes with high spatial resolution more effectively than traditional methods.