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A large natural fish kill which resulted in the deaths of a total of more than 3400 fish comprising eight different species occurred in a freshwater billabong (lagoon or small permanent waterbody) on the Magela Creek in the subtropical Northern Territory of Australia in January, 1980. The fish deaths were assessed to be the result of a combination of natural acid water runoff into the billabong and resultant elevated aquatic biotoxic aluminium levels. Details of water quality changes which occurred during the event and analytical data for the fish killed are provided, together with some explanation of the sequences of occurrence of the fish kill.

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Previous assessment of dispersion modelling, toxicity testing, and characterisation of produced formation water (PFW) discharges into Bass Strait indicated a very low environmental risk from PFW to the marine environment. Peak PFW concentrations can exceed the effect levels (EC50 or LC50) measured in 24-96 hr laboratory toxicity tests only within distances of tens of metres from the discharge point. In this assessment, the field monitoring of aromatic hydrocarbons in the water column (which are in low concentrations in PFW) was undertaken to directly assess dispersion and predicted fate mechanisms. Very low concentrations of both light and heavier aromatic hydrocarbons are likely in any PFW discharge. A high volume absorption sampler was deployed 20 m from the discharge point to continuously sample ocean concentrations of aromatic hydrocarbons for up to one week, providing large volume (1,000 L) water samples. Gas Chromatography/Mass Spectrometry (GCMS) was used to measure aromatic hydrocarbon concentrations. The concentrations measured with the ocean sampling device provide time integrated samples over approximately one week, and the results showed that the ratio of discharge concentration to ocean concentration was approximately 20,000:1. Compared to dispersion modelling predictions, the ocean sampler indicates lower environmental risk. This is because dispersion modelling predicts ocean concentrations within the plume whereas the sampler is measuring concentrations at a fixed point over the long term and is exposed to the plume only intermittently, similar to a sessile marine organism. Therefore the ocean concentrations provided by the large volume sampler are more representative of longer term ocean concentrations which can be experienced by marine organisms. Further assessment of prevailing operational and oceanographic conditions in Bass Strait suggests that there does not appear to be a water column accumulation of PFW aromatic hydrocarbons adjacent to the discharge. Introduction There are currently 12 offshore produced formation water (PFW) discharges from Esso/BHPP's oil and gas production facilities in Bass Strait, Australia (Figure 1). Total volumes of the discharges are approximately 90 ML.d-1. The volumes have increased significantly in the past five years from the minimal discharges in the early 1970s when oil production was brought on-line starting with Halibut in 1970. PFW discharge volumes are likely to increase further as fields mature. Environmental risk assessment of PFW discharged into Bass Strait suggests the discharges present a low potential for impact on marine organisms due to low acute toxicity and high dilution rates. Terrens and Tait reported discharge plume dispersion modelling predictions where the dilution of a typical PFW discharge in Bass Strait was 30:1 within 10 m of the discharge point, and that acceptable acute toxicity could be obtained with dilution of less than 4:1. The dispersing discharge plume is spatially limited as a narrow band, and mobile due to local tidal conditions. The length of any water column organism's exposure to concentrations in excess of acute effect levels measured in laboratory toxicity tests would be less than about 30 seconds for median current conditions, a period much less than that of the 24 hr or 96 hr acute toxicity tests. P. 739

Pre- and post-drilling environmental studies were conducted around an exploration well drilled in the south Caspian Sea in a water depth of 145 m using both water-based and synthetic-based drilling muds. Eight months after completion of the well, chemical analyses of surface sediment samples for linear alpha olefins, polycyclic aromatic hydrocarbons, saturated hydrocarbons, and selected metals confirmed that the spatial extent of the settled cuttings and mud was limited to an approximate 400-m radius. Cross-sectional photographic images of the seafloor showed significant oxygen demand and presence of sulfidic sediments within 50 m of the well. Coincident benthic samples were examined for total infaunal abundance, species richness, biomass, and distribution of major taxa. The macrofaunal groups Amphipoda, Cumacea and Gastropoda were present in pre-drilling and post-drilling reference samples, but were not present in post-drilling samples collected within 50 m of the well. However, these groups had enhanced abundances, likely in response to moderate organic enrichment at 200 and 400 m distances, where there was no evidence of excessive oxygen demand. Ostracod crustaceans were rare in pre-drilling samples, but were the most abundant post-drilling group in all sediments collected within 400 m of the well site, indicating that elevated organic matter may be a strong macrofaunal attractor. Overall results indicate that changes in sediment chemical concentrations and benthic community composition associated with drilling did not extend significantly beyond 400 m from the well site, and that recolonization of the site was underway, eight months after completion of the well. Introduction A range of studies on environmental impacts associated with drilling and discharge of drilling muds and cuttings have been conducted in aquatic environments over the past 30 years1–7. Many of these studies focus on impacts to the benthic macrofauna, which are recognized as effective sentinels or indicator organisms for the monitoring of point source discharges and temporal impacts due to their relative immobility, typically short life spans, and sensitivity to physical-chemical features of sediments8–13. Petroleum exploration can impact benthic macrofauna as a result of discharge and seafloor accumulation of drilling cuttings and adhered muds, and release of hydrocarbons2–4,14. Documented long term effects from the discharge of drilling muds and cuttings include smothering, organic enrichment and toxicity from heavy metals and hydrocarbons. Use of diesel-containing oil-based muds in the North Sea often resulted in toxic concentrations of petroleum hydrocarbons in near-platform sediments15,16. Conversion to synthetic-based, low-toxicity drilling fluids has been shown to reduce these impacts, although physical impacts such as smothering and habitat alteration remain17,18. Recently, exploration and production activities have increased in inland seas, which are less saline, and therefore differ in species composition from offshore continental shelf environments. Impacts from oil and gas exploration in many of these seas are not well documented; even so, they should not be assumed to mimic those in offshore marine ecosystems. Lacking knowledge of the effects of low-toxicity synthetic muds in inland sea environments, a comprehensive set of environmental studies, including physical oceanography as well as chemical and biological assessments, was undertaken at several exploration well sites in the south Caspian Sea. This paper reports some of these studies, and examines chemical and biological results for pre- and post-drilling sediments collected at and around a single exploration well site located in the Azerbaijan sector of the south Caspian Sea.