This paper proposes an assessment method for discharge of produced water from coal seam gas operations to streams in Australia. Coal seam gas (CSG) production has expanded rapidly in Australia over the last five years and management of produced water is a key issue for the sustainability of the industry. Significant volumes of water are produced during most CSG operations, and the industry is likely to continue expanding in future, creating pressure to find beneficial uses for this water. It has been suggested that some produced water, treated to an appropriate level, could be used to compensate for environmental flows in streams in catchments experiencing water stress. The flow assessment method proposed includes six main stages, from determining discharge requirements of the operator and existing environmental values, to predicting changes in stream condition under discharge scenarios and selecting key indicators for monitoring. It was found that the main changes that will be brought about to stream ecosystems from discharge of produced water are to the geomorphology, water quality and hydrology. The extent of these changes and the sensitivity of the environmental values within the system to these changes should be assessed prior to approval of stream discharge of produced water, while operational requirements, such as timing, volume and release points also need to be considered. A case study is presented to illustrate use of the method.

A scientifically defensible assessment method for stream discharge of produced water will be of benefit to the coal seam gas industry, for the regulators of this industry and for the environment and communities that value ecosystems that may be affected. The resulting assessment method has been based heavily on environmental flow assessment methods already in use in Australia and methods for assessment of wastewater discharge. The method was tailored to take into consideration specific issues related to produced water from coal seam gas projects, as most existing flow assessment methods were designed for different objectives, such as assessing impacts of reduced flows due to water extractions.