Non-volatile hydrocarbon chemistry studies around a production platform onAustralia's northwest shelf

In September 1994 and 1995, scientists from the Australian Institute of Mar ine Science (AIMS) and the Australian Geological Sun ey Organization (AGSO) conducted surveys aboard the RV Lady Basten to determine the dispersion, f ates and effects of produced formation water (PFW) discharged from the 'Har riet A' oil production platform near the Montebello Islands. This report is one of four related papers and describes the non-volatile hydrocarbon chem istry studies. The dispersion of the PFW into dissolved and particulate fra ctions of seawater were measured using moored high volume water samplers, s urface screen samplers and moored and drifting sediment traps. Bio-accumula tion was studied using transplanted oysters, and dispersion measured into s ediment with benthic grabs. Results showed enrichment in non-volatile hydrocarbons in surface microlaye r samples to a distance of 1.8 km in the direction of tidal flow. Concentra tions in surface microlayers near the platform varied by an order of magnit ude and corresponded to when a surface slick was visible or not visible. Co ncentrations of oil in seawater ranged from 2.0 to 8.5 mu g l(-1) at near s tations to 1.3 mu g l(-1) at 1.8 km. Water column samples showed the proces ses of desorption from particles for soluble components occur within the ra nge of 1.8 km. Most particulate hydrocarbons drop out of suspension within c. 1 to 2 km from the platform. Fluxes of particulate hydrocarbons through the water column at c. 1 km, as estimated by moored sediment traps in 1995, were 138 to 148 ng cm(-2) day(-1). A decrease in sediment concentrations w ithin c. 1 km of the platform was measured as 2.45 +/- 1.29 mu g g(-1) dry wt (n = 15) in 1994 to 0.86 +/- 0.54 mu g g(-1) dry wt (n = 21) in 1995, af ter the platform installed a centrifugal separator in the discharge treatme nt process. Thus the residence time of this relatively low molecular weight oil was estimated in the coarse aerobic sands surrounding the platform to be less than one year. Oysters suspended near the platform bio-accumulated hydrocarbons and other lipophilic organics in their tissues. Uptake rates a nd bio-concentration factors of hydrocarbons indicated potential toxicity a t the near-field stations within c. 1 km radius. A mass balance was constructed to show the partitioning of the input of hyd rocarbons from the PFW into the surrounding marine ecosystem. The rates of dissipation processes were estimated as follows: dilution from tidal curren ts>degradation in the water column>sedimentation>evaporation. The calculati ons based on maximum concentrations measured in the environmental samples a ccounted for 85% of the daily input suspended within a 1 km radius. It is estimated that the potential zone of toxic influence in the water col umn extends to a distance of approximately 1 km. Concentrations of oil in s ediments were too low to indicate potential toxicity. By the collaborative application of oceanographic and geochemical techniques to marine environme ntal problems, we endeavour to provide effective feedback to the oil indust ry to gauge the effectiveness of their operational strategies in minimizing impact in these pristine regions. (C) 1999 Academic Press.