POTTER POINT OUTFALL

Click the photograph for a enlarged view

Physical Environment

Landscape

The existing Potter Point outfall is located at the cliff face on the southern side of the Kurnell Peninsula. The cliffs in this area are made up of Hawkesbury sandstone, which consists of sandstone and quartz, with some shale (Wollongong Geological Series Sheet SI 56-9).

Bathymetry & Substrate

The general seabed characteristics in the vicinity of the outfall are predominantly rocky near Kurnell Peninsula out to a water depth of 40 to 50 m, with fine sand and mud covering the seabed at greater depths. The south-eastern section of the Kurnell Peninsula, including Potter Point, has large sandstone cliffs at the waters edge, with a short, wave-cut, intertidal rock platform at the base. This platform is littered with large boulders and is actively eroded during storms. The sandstone substrate steps down from the intertidal region to a depth of 30 m and comprises bedrock covered with boulders and erosion debris. Beyond the 30 m depth contour, the seabed slopes offshore more gradually and comprises irregular exposed bedrock with the valleys filled with coarse sandy sediment. The sandy/silly sediment cover becomes more consistent from about 40 m depth and is about 10 m thick at the 80 m depth contour. Sediments become finer with distance from shore. To the south-west of Potter Point, the shoreline changes from cliffs to beach in Bate Bay, with Mercies Reef extending out into the northern end of Bate Bay (CEE & Woodwind-Clyde 1996).
Currents, Winds and Waves

AWT (1994) studies indicate:

- coastal currents are the major influence governing the path and impact of effluent currently discharged.

- the prevailing southerly flow generated by the East Australia Current (EAC) transports effluent south from Potter Point towards the Bate Bay/Cronulla region.

- occasionally these currents reverse and flow towards the north.

- local movements of currents are also influenced by local wind and wave conditions.

- Bate Bay is a 4km wide semi-circular indentation in the coast between Port Hacking and the Kurnell Peninsula. There is a large reef (Merries Reef) which projects from the northern shore into the Bay. This reef has a significant influence on the pattern of circulation in Bate Bay as it reduces currents and water movement in the north-western sector of the Bay.

- at times of strong onshore wave action, an across-reef current is created over Merries Reef, leading to an anti-clockwise circulation in Bate Bay. The transport of water across Merries Reef during high wave conditions is the most important mechanism for moving effluent from the Potter Point outfall to the Cronulla beaches.

- during periods of light wind and small waves, the waters in Bate Bay inshore of Merries Reef are relatively confined and there is less mixing with the rest of Bate Bay. As a result the waters in this area are often less saline than in other areas in the Bay and frequently have higher levels of faecal coliforms (attributed to Potter Point effluent) (AWT 1994, CEE & Woodward-Clyde 1996).

## Sediment studie

AWT (1994) studies indicate:

- concentrations of heavy metals in the Bate Bay region were generally lower than in other areas offshore of Sydney and Illawarra.

- the exception was chromium levels, which were higher than other areas, but appeared to be coming from Botany Bay.

- no widespread accumulation of heavy metals or organochlorine pesticides was detected within Bate Bay itself or in adjacent nearshore sediments.

- copper levels were locally higher at one site immediately adjacent to the Potter Point outfall (copper was expected in domestic waste water and the very local nature of the observed increase indicated that copper was probably being continually discharged, but was not accumulating widely).

- concentrations of organochlorine pesticides in offshore sediments on the continental shelf east of Bate Bay were generally higher than in sediments from other locations along the Sydney and Illawarra coastline, however, no widespread accumulation was detected in Bate Bay.

- several pesticides including aldrin, heptachlor and heptachlor epoxide were at slightly elevated levels adjacent to Potter Point outfall and at sites just to the west. This supported the oceanographic conclusions of effluent transport from Potter Point.

Outfall Site

Outfall Site

Outfall Site

Potter Pt.

(Pott_5) Last updated June 2000