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Aquatic Environmental Assessment Program

Sydney Water will develop and implement a Water Quality and Aquatic Ecology Monitoring Program (the "Aquatic Environmental Assessment Program"), in accordance with the Conditions of Approval for the Replacement Flows Project.

The program aims to assess changes in the receiving water environment that may arise from the Replacement Flows Project and the St Marys Recycled Water Plant (also referred to as the Advanced Water Treatment Plant). The program involves monitoring the Hawkesbury-Nepean River and some of its tributaries before and after commissioning of the new plant. Ecological field monitoring activities will focus on:

• recycled water quality (as produced by the plant)
• local water quality as a whole in the surrounding creek and river
• macroinvertebrates (small aquatic organisms such as insect larvae that live in the river)
• fish communities, and
• macrophyte assemblages (floating and submerged aquatic plants).

In designing the program, Sydney Water consulted with representatives from various government departments including the Department of Primary Industries, Sydney Catchment Authority, Department of Water and Energy, Department of Environment and Climate Change and NSW Health. Sydney Water will prepare a report describing existing conditions that will be submitted to the Department of Planning after the recycled water plant is commissioned. Annual reports will also be prepared for the first three years of operation detailing any changes that occur in the Hawkesbury-Nepean River system resulting from the project.

Fish attraction study - Nepean River/Boundary Creek

In accordance with the Conditions of Approval for the Replacement Flows Project, Sydney Water will conduct a study to assess whether the discharge from the recycled water plant into Boundary Creek is likely to create flow conditions that attract fish from the Nepean River into Boundary Creek.

Fish species in the Nepean River migrate upstream at certain times of the year, and at certain stages of their lives. Migratory behaviour in fish is driven by a number of 'signals' in the river environment, including temperature and water quality.

To successfully migrate, fish in the Nepean River need to 'stay on course' and navigate through the weir at Penrith, using the existing fish ladder. Boundary Creek is a small tributary of the Nepean River, located a short distance downstream of Penrith Weir. The existing Penrith STP currently discharges highly (tertiary) treated effluent into Boundary Creek (which flows into the Nepean River), and the new recycled water will also be discharged into Boundary Creek. This means the flow in Boundary Creek will change from tertiary treated effluent to highly treated recycled water, and the volume discharged each day will almost double.

With these changes, it is important to investigate whether fish are attracted to the new Boundary Creek flows and thus diverted from their upstream migration along the Nepean River. The investigation will involve a laboratory study to assess fish movement patterns using water from the Nepean River and the recycled water plant. The study will also assess the presence of fish in Boundary Creek and at the weir fish ladder before and after commissioning. Results from both investigations will be used to determine if 'attraction flows' have been caused by the project. If so, Sydney Water will put in place appropriate measures to address this.

Permeate conductivity study

In accordance with the Conditions of Approval for the Replacement Flows Project, Sydney Water will evaluate the likely conductivity of the recycled water produced by the recycled water plant, and any potential impacts of this on aquatic life in Boundary Creek.

Conductivity is a measure of the salt concentration in a water sample. The highly treated water produced by the recycled water plant is expected to have low conductivity or background salt levels. In the aquatic freshwater environment, if background levels of dissolved salts are too low, some types of aquatic life may expend excessive amounts of energy and become stressed. Sydney Water will conduct a series of toxicity tests on the recycled water using a range of organisms (from algae to fish) to determine if the recycled water produced by the new plant has the potential to cause such stress to the aquatic life in Boundary Creek. This toxicity testing will be supplemented with a modelling component to investigate how the recycled water discharged into Boundary Creek will mix with Nepean River waters under a range of flow conditions. If any undesirable outcomes are identified through the study, Sydney Water will take steps to address these.