Award: OCE-1251547

In September 2012, the Orange County Sanitation District (OCSD) conducted a planned wastewater diversion in order to make repairs to their primary outfall. The diversion, lasting from 11-Sept. to 3-Oct. 2012, resulted in the discharge of 5.3x108 liters per day (138 million gallons per day) of highly treated effluent through their secondary outfall much closer to shore. The secondary outfall has both a diffuser and an open end; the result was that the highly buoyant plume rose to the surface quickly and violently, entraining adjacent oceanic water and leading to a turbulent surface ``boil" 50-100m in diameter, which was visible throughout the diversion. Plume monitoring during the OCSD diversion included shipboard, mooring, and autonomous assets. Surveys were designed to map the extent and position of the plume as well as its chemical, biological, and physical characteristics. Shipboard operations provided near-synoptic measurements over scales >10 km, although the sampling locations were necessarily sparse in space. Anticipating strong stratification during the 2012 Diversion, and given the demonstrated importance of small-scale physical flows on the fate and dispersion of the effluent plume in the area, we deployed a set of three wave-powered profiling moorings (the Wirewalker, a product of the Ocean Physics Group, Scripps Institution of Oceanography). The Wirewalker is capable of continuous profiling with a repeat profile every few minutes over the course of weeks, with vertical resolution on scales <1m. The Wirewalker moorings carried high resolution physical and optical instrumentation, and were deployed in association with moorings maintained by OCSD and the Monterey Bay Aquarium Research Institute. The rapid, continuous profiling resulted in a unique view of the distribution and evolution of the effluent plume and receiving waters over the course of the 2012 diversion. We collected a total of 39,759 profiles of temperature, salinity, denisty, pressure, and chlorophyll fluorescence our the course of the deployment. This corresponds to 1,940 km of profiles at the 3 moorings over the deployment (roughly the distance from Los Angeles to US/Canada border). These data gave us an unprecendented view of the evolution of the wastewater plume in the shallow, coastal waters of the Southern California Bight. These dynamics are critical to understanding and predicting the influence of wastewater discharge on the heavily utlized coastlines of the United States. We observed the fine-scale vertical variability of a wastewater plume discharged into shallow, stratified coastal waters. The effluent plume occupied a variable portion of the water column, but was typically near the surface. The plume was characterized by small vertical scales away from the surface, while complicated patterns of vertical temperature and salinity compensation were found in the plume above the pycnocline. Far from the outfall, the plume acted as a passive tracer. At times, we apparently observed the final stages of near-field mixing at the profiling mooring closest to the outfall. Complicated vertical patterns in plume water temperature and salinity compensation and near-surface mixing dynamics demonstrated the complexity of the plume dilution process even in a surfacing plume. The co-occurence of elevated chlorophyll fluorescence and plume waters was evident in the later part of the diversion period, but the overall response of the phytoplankton to the effluent diversion was weak. There were no apparent physical barriers between the plume and ambient waters, and, given the elevated nutrients available in the plume, processes other than the physics of plume evolution were likely inhibiting the phytoplankton response to the wastewater diversion. Last Modified: 11/07/2014 Submitted by: Andrew J Lucas