Non-volatile Hydrocarbon Chemistry Studies Around a Production Platform on Australia's Northwest Shelf

Abstract

In September 1994 and 1995, scientists from the Australian Institute of Marine Science (AIMS) and the Australian Geological Survey Organization (AGSO) conducted surveys aboard the RV Lady Basten to determine the dispersion, fates and effects of produced formation water (PFW) discharged from the ‘Harriet A’ oil production platform near the Montebello Islands. This report is one of four related papers and describes the non-volatile hydrocarbon chemistry studies. The dispersion of the PFW into dissolved and particulate fractions of seawater were measured using moored high volume water samplers, surface screen samplers and moored and drifting sediment traps. Bio-accumulation was studied using transplanted oysters, and dispersion measured into sediment with benthic grabs.Results showed enrichment in non-volatile hydrocarbons in surface microlayer samples to a distance of 1·8km in the direction of tidal flow. Concentrations in surface microlayers near the platform varied by an order of magnitude and corresponded to when a surface slick was visible or not visible. Concentrations of oil in seawater ranged from 2·0 to 8·5μg l−1at near stations to 1·3μg l−1at 1·8km. Water column samples showed the processes of desorption from particles for soluble components occur within the range of 1·8km. Most particulate hydrocarbons drop out of suspension within c. 1 to 2km from the platform. Fluxes of particulate hydrocarbons through the water column at c. 1km, as estimated by moored sediment traps in 1995, were 138 to 148ng cm−2day−1. A decrease in sediment concentrations within c.1km of the platform was measured as 2·45±1·29μg g−1dry wt (n=15) in 1994 to 0·86±0·54μg g−1dry wt (n=21) in 1995, after the platform installed a centrifugal separator in the discharge treatment 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 and bio-concentration factors of hydrocarbons indicated potential toxicity at the near-field stations within c.1km radius.A mass balance was constructed to show the partitioning of the input of hydrocarbons from the PFW into the surrounding marine ecosystem. The rates of dissipation processes were estimated as follows: dilution from tidal currents>degradation in the water column>sedimentation>evaporation. The calculations based on maximum concentrations measured in the environmental samples accounted for 85% of the daily input suspended within a 1km radius.It is estimated that the potential zone of toxic influence in the water column extends to a distance of approximately 1km. Concentrations of oil in sediments were too low to indicate potential toxicity. By the collaborative application of oceanographic and geochemical techniques to marine environmental problems, we endeavour to provide effective feedback to the oil industry to gauge the effectiveness of their operational strategies in minimizing impact in these pristine regions.