» Home > Services > Storm and wastewater

Since the Tahuna Wastewater Outfall began operating, we have been monitoring the bacterial count around the outfall, from Second Beach to Maori Head, almost every day.

The 1100m length of the outfall means that the wastewater is being discharged into approximately 20-25 metres of water. Previously, it was discharged directly at Lawyers Head over the rocks before the plume entered the water.

This is the first time in 100 years that it has been safe to swim at Tomahawk Beach.

Outfall updates 20/02/10 - Ent (JPG, 288.8 kb, new window)
This image shows the readings of the Outfall updates - Improvement in bacteria counts since the new outfall

Outfall updates 20/02/10 - Coast (JPG, 254.9 kb, new window)
This image shows the readings of the Outfall updates - Otago Peninsula

Outfall updates 20/02/10 - Mussel (JPG, 284.1 kb, new window)
This image shows the readings of the Outfall updates - Bacteria counts in mussels

What do we measure?

We measure enterococci and faecal coliforms as indicator organisms. The more there are of these, the more likely it is that there will also be pathogens, such as viruses, present.

While faecal coliforms come from many sources, including farm runoff and wildlife, we also measure the amount of Fluorescent Whitening Agents (FWA) in stormwater discharges in the area. High levels of FWAs indicate that the faecal coliforms are more likely to come from human sources. (FWAs are the whitening agents used in many detergents and cleaning products.)

The monitoring is carried out by Ryders, an independent environmental consultant and the lab tests are also carried out independently, by the Water Quality Laboratory.

Flushing and Diffusion

We flush the outfall daily for about ten minutes so it continues to work properly. The results of this can be seen as a plume for about half an hour after the flushing is completed because of the much higher flows than normal.

The plume can also be seen during wet weather events, again due to the higher flows. This is despite the fact that Dunedin has carried out a lot of stormwater and wastewater separation, as stormwater still gets into the foul sewers during very rainy weather.

Near the end of the outfall, there are eight diffuser units. Each unit has four tideflex valves (32 in total) that diffuse the wastewater over a larger area rather than sending one big gush out of the end of the outfall. At this point at normal flows the wastewater and seawater mix together thoroughly.

Impact of stormwater on these results

It is important to note when reading the maps on this page that the green spots indicate stormwater outlets. These stormwater outlets affect the levels of indicator organisms in the water but they are in no way related to the effective operation of the wastewater outfall.

The stormwater system carries natural runoff from land, roads, gutters and roofs (rain) directly to the nearest watercourse. The likelihood of pollution getting to sea is related to the amount of rain and where the pollution comes from.

The four main stormwater outlets that affect these readings are situated at:

• Second Beach - stormwater outfall for much of St Clair
• Beside the St Clair Hot Salt Water Pool - a watercourse coming from above and through St Clair on the hillside above St Clair beach
• Tomahawk Lagoon - usually filtered through sand unless the lagoons are particularly high
• Tomahawk Creek at the base of Maori Head on Smaills Beach -  a watercourse draining much of Smaills valley