Dispersion of produced water in a coastal environment and its biological implications

Produced water, a pollutant associated with offshore oil production, has be en shown to have adverse effects on marine organisms. We conducted a study of the dispersion of a produced water plume from an outfall in the Santa Ba rbara Channel near Carpinteria, CA. Biological effects were studied previou sly in a series of experiments which examined the toxicity of ocean waters near the outfall. To define the changing ocean conditions around the outfal l, we obtained time series observations of currents and water properties fr om July, 1992 to January, 1994. Near-field dispersion of the produced water is simulated with a buoyant plume model. measured currents and density pro files are used as model inputs. Far-field dispersion is simulated with the current statistics combined with an elementary solution to the diffusion eq uation. The modeled depth of the plume varies strongly with season due to c hanging stratification, In spring and summer, the modeled plume is trapped below the surface. In fall and winter it extends over most of the water col umn and occasionally surfaces. Minimum initial dilution is similar to 100 i n summer and similar to 500 in winter. Far-field modeling indicates along-i sobath symmetry in produced water dispersion in the mid water column. This pattern agrees with the distribution of toxic effects from the biological s tudies. At 1000 m from the outfall, the farthest test sites in the biologic al studies, minimum dilutions range from 4000 to 4x10(5) when the plume is present. These estimates exceed the threshold for sub-lethal effects found by Krause (1993) in a sea urchin fertilization bioassay. Time averaged dilu tions in the far-field are larger by factors of 10(2) to 10(3). (C) 1998 El sevier Science Ltd. All rights reserved.