SPATIAL AND TEMPORAL VARIABILITY IN RECEIVING WATER TOXICITY NEAR AN OIL EFFLUENT DISCHARGE SITE

The distribution of point-source pollutants in the field can vary in b oth time and space. This study examined spatial and temporal patterns of toxicity from a produced water (an oil production effluent) dischar ge plume using a sea urchin fertilization toxicity test. Specifically, the sensitivity and response patterns of sea urchin gametes exposed t o receiving waters sampled along a 1 Km transect near an active produc ed water outfall were tested. Fertilization success varied significant ly with proximity to the outfall, with reduced fertilization found clo ser to the outfall. Although toxicity in receiving water samples, base d on fertilization success, was variable in time-perhaps responding to variation in the quantity or make-up of produced water discharges-the general spatial pattern of toxicity along the transect remained relat ively constant. The discharge plume was well established in the wester ly direction throughout the experimental period. Toxicity data from sa mples of effluent and receiving waters, which were collected simultane ously, were used to determine the effective plume concentrations of pr oduced water at seven sampling stations along a 1 km transect down-fie ld from an active outfall. Strong evidence that field toxicity was dir ectly attributable to the presence of produced water was provided by s ampling the discharge plume during a period while the produced water d ischarge was not operating. During this period, no toxicity was found at any of the field sites.