WASTEWATER EFFLUENT PLUME TRACKING: TEMPORAL AND SPATIAL DYNAMICS

The largest Publicly Owned Treatment Works (POTW) in California emit wastewater effluent into the subsurface ocean waters off the coast. The outfalls of these wastewater treatment plants are designed to keep the discharge plumes offshore and below the surface waters to allow bacteria, heavy metals, and other pollutants to disperse with the least amount of human health risk and impact on the environment.This paper presents the dynamics of the wastewater plume from the Orange County Sanitation District (OCSD) outfall/diffuser system (serving a population of more than 2 million) in terms of the shape and extent of the plume as a function of ocean density stratification and of current direction and speed. The wastewater plume often extended at least 10 km longshore in the direction of the current regime in the lower part of the water column. It ranged from 1 to 4 km wide, with greater widths generally farther from the diffuser outfall. Maximum plume widths ranged from 4 to 8 km but were not highly correlated with the duration, speed, and direction of the currents. The plume resided between 20 and 60 m deep, with the particulate constituents (e.g., coliform bacteria) usually somewhat deeper than the dissolved constituents. The plume frequently followed topography and moved offshore in the upcoast direction and tended to move onshore following the Newport Canyon contours in the downcoast direction usually during slower current conditions. The length of time the effluent plume was likely to persist at detectable levels varied from 0.5 to 4 days. No relationship was found between proximity of plume to shore on 10 cruises and the levels of coliform bacteria on beaches within the OCSD beach sampling universe.