California Sea Grant College Program

Currently about 4500 million liters of treated sewage effluent are discharged daily in the Southern California Bight, mostly through four major submarine outfalls; an input greater than the natural input from rivers, runoff and storms. It is however unclear whether these artificial rivers are affecting the Bight water quality. Because oceanic properties vary naturally, it is essential to distinguish natural from man-induced variability, prior to assessing a human impact.

In this research I examine the temporal and spatial variability of four water quality properties (Secchi disc transparency, percent transmissivity, temperature, dissolved oxygen), measured monthly for 15 years at stations near three of the major outfalls, and of two anthropogenic properties (sewage flow and suspended solid discharge), measured daily for 15 years inside the sewage treatment plants.

The purposes of this research are: I) to examine the variability of these water quality and anthropogenic properties; 2) to identify whether this variability is caused by natural or anthropogenic factors, and 3) to identify problems inherent in the monitoring programs and to suggest possible solutions.

Chapter 1 gives the overall introduction to the research.

Chapter 2 describes the temporal and spatial variability of Secchi depth transparency between stations within one area, Point Loma, and its relation to the local sewage variables.

Chapter 3 describes the time-series analyses performed on transparency, percent light transmissivity at 15 m, sewage flow and suspended solid discharge. Information on the history of the three treatment plants and on the physical oceanography of the Bight is also included here.

Chapter 4 identifies the problems encountered while using these historical water quality data sets, and uses this knowledge to make recommendations for ocean water quality monitoring programs. This chapter also provides some of the background on which many data handling decisions were made.

Chapter 5 summarizes the results for oxygen and temperature.

Chapter 6 draws together the overall conclusions of this research.