Towed mapping of the effluent plume from a coastal ocean outfall

The dispersion of effluent from coastal ocean sewage outfalls has continued to be a difficult problem to study in situ. The spatial extent of the detectable effluent field is an important question for understanding the range of potential impacts from the effluent plume. Sampling around large southern California sewage outfalls is done using automated water column profilers (i.e., CTDs) at designated stations, typically centered on the outfall. Discrete water samples, if taken, are collected at predefined depths. While this method of sampling allows for analysis of the environmental impacts to the receiving water from these outfalls, the data have shown that the studies are inadequate for determining the vertical and horizontal spatial extents of these subsurface wastewater fields. To address this issue, the Orange County Sanitation District (OCSD), California supplemented its normal receiving water sampling by using a Guidline Minibat towed underwater vehicle, equipped with CTD and in situ pumping system, to map the distribution of physical, bio-optical, chemical, and bacterial variables in the vicinity of its outfall. A pumping system enabled the measurement of nitrate, ammonium and selected indicator simultaneously with the in situ physical and bio-optical CTD measurements providing more unambiguous mapping of the effluent plume over an extended area. Telemetered current and temperature observations were used for adaptive placement of the sampling transects to optimally sample the plume. We compare results from a late winter, weakly stratified period when the flow was weakly upcoast with a strongly stratified summer period when the currents were strongly downcoast. The mapping demonstrates that the plume is clearly present for distances of at least 12.5 km in either direction from the outfall, depending on the current direction. The mapping results allow comparison with results from traditional receiving water monitoring and plume models of initial height, dilution, and thickness.