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

Abstract

Produced water, a pollutant associated with offshore oil production, has been 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 Barbara Channel near Carpinteria, CA. Biological effects were studied previously in a series of experiments which examined the toxicity of ocean waters near the outfall. To define the changing ocean conditions around the outfall, we obtained time series observations of currents and water properties from July, 1992 to January, 1994. Near-field dispersion of the produced water is simulated with a buoyant plume model. Measured currents and density profiles are used as model inputs. Far-field dispersion is simulated with the current statistics combined with an elementary solution to the diffusion equation. The modeled depth of the plume varies strongly with season due to changing stratification. In spring and summer, the modeled plume is trapped below the surface. In fall and winter it extends over most of the water column and occasionally surfaces. Minimum initial dilution is ∼100 in summer and ∼500 in winter. Far-field modeling indicates along-isobath symmetry in produced water dispersion in the mid water column. This pattern agrees with the distribution of toxic effects from the biological studies. At 1000 m from the outfall, the farthest test sites in the biological studies, minimum dilutions range from 4000 to 4×10 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 dilutions in the far-field are larger by factors of 10 2 to 10 3.