Preliminary assessment of petroleum hydrocarbons in water and sediment at Yellow Water, Kakadu National Park

Department of the Environment, 1998

Internal Report 283
van Dam RA, Camilleri C & Turley C
Supervising Scientist Division

About the report

In 1996, the environmental impacts of a commercial tour boat operation at Yellow Water, a highly valued wetland ecosystem within the World Heritage listed Kakadu National Park were assessed. While not specifically addressing pollutant-related issues associated with boating, the report did recognise the potential for pollution from outboard motors to impact upon the Yellow Water environment.

Leaking fuel and oil from outboard motors results in the contamination of waterways with petroleum hydrocarbons. The period of high boating activity at Yellow Water coincides with that of low water levels and no freshwater input, suggesting that petroleum hydrocarbons could accumulate to potentially toxic concentrations. Therefore, the present study aimed to provide a preliminary assessment of outboard motor-derived petroleum hydrocarbon levels in water and sediment during the late-dry season (ie a worst-case-scenario). In the event of petroleum hydrocarbons being detected, an assessment was to be made regarding the potential impacts on the Yellow Water environment, and the requirement for further monitoring/toxicity/risk assessment programs.

Water and sediment samples were collected from three sites at Yellow Water on December 16, 1997. The sampling sites were chosen according to where boating activity was considered to be greatest. Samples were analysed for benzene, toluene, ethyl-benzene and xylene (BTEX), and total petroleum hydrocarbons (TPH).

Petroleum hydrocarbons were detected in water samples from one of the three sites, at a concentration of 200 /-lg/L. Analysis revealed that the hydrocarbons were relatively high molecular weight compounds, possessing carbon numbers of between 15 and 28. Petroleum hydrocarbons were not detected above the Practical Quantitation Limit in any of the sediment samples.

For the contaminated site, the fact that no low molecular weight, highly volatile hydrocarbons were detected indicated that i) the contaminant event was not particularly recent, and ii) the source was oil, not gasoline (ie unleaded petrol).

Based on previous literature on the aquatic toxicity of petroleum hydrocarbons, it appears unlikely that acute toxicity would be observed for the plant and animal species inhabiting Yellow Water. However, at the concentration detected, there appears to be a possibility of sub-lethal effects to aquatic organisms.

The present study did not represent the 'worst-case-scenario' situation initially anticipated. Boating activity in December is much lower compared to the July - September peak, while Yellow Water had already received significant freshwater input prior to sample collection, potentially diluting hydrocarbon concentrations in the water.

As a result of the potential for sub-lethal toxicity to aquatic organisms, and also for the presence of higher hydrocarbon concentrations during peak boating periods, the implementation of a more comprehensive monitoring program, incorporating sampling periods before, during, and after the peak boating activity, is recommended. Such a program will better characterise the input of petroleum hydrocarbons to Yellow Water throughout the dry season. In addition, more specific chemical analyses, and potentially, toxicity assessments, will assist in the prediction of adverse environmental effects associated with boating activities at Yellow Water.