Spatial and temporal variability in receiving water toxicity near an oil effluent discharge site

Abstract

The distribution of point-source pollutants in the field can vary in both time and space. This study examined spatial and temporal patterns of toxicity from a produced water (an oil production effluent) discharge plume using a sea urchin fertilization toxicity test. Specifically, the sensitivity and response patterns of sea urchin gametes exposed to receiving waters sampled along a 1 Km transect near an active produced water outfall were tested. Fertilization success varied significantly with proximity to the outfall, with reduced fertilization found closer to the outfall. Although toxicity in receiving water samples, based 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 relatively constant. The discharge plume was well established in the westerly direction throughout the experimental period. Toxicity data from samples of effluent and receiving waters, which were collected simultaneously, were used to determine the effective plume concentrations of produced water at seven sampling stations along a 1 km transect down-field from an active outfall. Strong evidence that field toxicity was directly attributable to the presence of produced water was provided by sampling the discharge plume during a period while the produced water discharge was not operating. During this period, no toxicity was found at any of the field sites.