Pollution Levels in Harbour Drop Dramatically

30 July 2009

Water quality in the harbour and around Wellington's coast has been steadily improving in recent years as a result of hundreds of millions of dollars' investment in infrastructure upgrades, says Wellington City Council Environment Portfolio Leader Celia Wade-Brown.

Cr Wade Brown says last week's Dominion Post lead story, headed 'Big stink over harbour', over-dramatised a well-documented situation. "It is no secret that Wellington Harbour - or indeed any body of water in an urban area regardless of where it is on the planet - will be liable to contamination.

"We may never be able to completely rid our local waters of pollution - but we are doing a lot to reduce it to a reasonable minimum. And any long-time resident of Wellington will have to agree that water quality is far better now than it was probably at any time in the 20th century."

Cr Wade-Brown says just some of the improvements put in place by the City Council in the past 15 years have included:

  • Opening the Moa Point and Karori West wastewater treatment plants.
  • Construction of 21 sewage holding tanks as part of a $72 million project to eliminate sewage from the harbour, streams and coast.
  • This includes construction of a giant tank under the Michael Fowler Centre car park in 2004. It can hold 850,000 litres of sewage and so take pressure off the wastewater system and pumping stations in the event of very heavy rain or disastrous power failure.
  • The progressive detection and removal of illegal private 'cross-connections' that allow stormwater into the sewage system (the stormwater and sewage systems are not supposed to be connected in any way). Generally these illegal cross-connections involve stormwater downpipes being connected to sewers on private property.

As a result, water monitoring has revealed dramatic reductions in contamination levels in the harbour and around the South Coast. In most cases faecal-coliform levels per 100ml of water have dropped from the tens of thousands to the hundreds.

Examples include (note: cfu = colony-forming units - a scientific measurement of bacteria):

  • Lyall Bay west culvert where faecal coliform levels reached 33,000 cfu/100ml in 1996 and are now down to 195 cfu/100ml
  • Taranaki Street culvert (which drains from Mt Cook, Brooklyn, the Aro Valley, parts of Kelburn and Te Aro) - from 20,500 cfu/100ml in 1995 to 395 cfu/100ml this year
  • Evans Bay culvert - from 22,000 cfu/100ml in 1992 to 260 cfu/100ml this year
  • Overseas Passenger Terminal culvert (which drains from Mt Victoria, Newtown and parts of Berhampore, Mt Cook and Te Aro) - from 40,000 cfu/100ml in 1992 to 590 cfu/100ml last year (though an increase this year has prompted a search for the source of the new contamination).    

The City Council is also investigating ways of preventing seawater from infiltrating into the sewers and so putting more pressure on the system. This is generally thought to occur on the low-lying reclaimed land in the central business district - where the water table fluctuates with the tides.

Along with Greater Wellington Regional Council (GWRC), the City Council is also involved in ongoing public education programmes designed to deter people from polluting the stormwater system - from August this will include the installation of cast-iron drain lids featuring fish or crabs to remind people of the direct and untreated route that rainwater takes from the street to the harbour.

Fines for littering have also recently been increased - including the introduction of $100 fines for discarding cigarette butts in public places (Cr Wade-Brown says the number of cigarette butts in the harbour has risen significantly since smoking was banned in bars and more people began smoking on the streets).

GWRC's 2008/09 Recreational Water Quality Monitoring Report, which profiles contamination in the region's rivers and coastal waters, found that only two of 22 sites around Wellington Harbour and the city's coastline temporarily exceeded the 'action guideline' which required 'no swimming' warning signs to be posted.

Cr Wade-Brown says in both cases (at Owhiro and Mahanga bays) the heightened levels of water contamination followed periods of rain. "This reinforces the message that it's never wise to swim in the harbour or at any urban beach for a couple of days after heavy rain.

"The problem is that rain washes all the nasties off our roads and footpaths - and everywhere else - into the stormwater system and into the sea. So that can include everything from dog poo to cigarette butts to petrol and oil and heavy metals - it's a very unhealthy cocktail."

Twelve resource consents were granted by GWRC in 1994 for discharges from the city's major stormwater outfalls.  "As these consents come up for renewal we are looking at the 'bigger picture' and what we want to achieve," says Cr Wade-Brown. 

"Consequently we have applied for discharge consents under the RMA (Resource Management Act) for all stormwater discharges. This includes conditions for further investigation of the effects of stormwater on the local marine environment and management of the risks associated with the stormwater discharges."

The City Council has been working closely with GWRC to review stormwater issues generally. Together with GWRC and other councils in the region, a stormwater action plan has been produced for the region. The plan is a voluntary document. It aims to be a guide for the region, to have consistency in approaches to the management of stormwater, and be a framework for the ongoing collaboration to improve stormwater management.

GWRC and the City Council also collaborated on a recent harbour sediment quality investigation. This investigation identified the nature, severity and extent of sediment contamination, much of which can be attributed to stormwater inputs. The work provides a scientific basis for future management response in relation to urban stormwater discharges, says Cr Wade-Brown.

She also takes issue with claims that Wellington's drainage systems are 'ageing' and that some of its pipes date back to the 1860s. "It is highly unlikely that there are any stretches of public pipe left in the city that date back that far. Any sections from the 19th century will have been retained because they are still in good condition. "We also have extensive maintenance and pipeline renewals programmes in our asset management plans. Each year we spend about $7 million replacing sewer pipes alone.

"We agree that any sewage overflow that occasionally enters the harbour in heavy rain is not good - but if the health or regional authorities think that there is a risk we are required to put temporary signs up warning the public. This doesn't happen often."

Over the past year in Wellington City the amount of overflow of sewage to the harbour in heavy rainfall situations (when the sewage is already diluted with rainwater) was estimated at 44 cubic metres in total. This should be compared to the average 69,500 cubic metres pumped to Moa Point and treated every day - or the 25 million cubic metres pumped per year.

Cr Wade-Brown says the public has a major role to play to minimise stormwater pollution. "People should remember that when paint, chemicals, oil, cigarette butts and dog droppings enter the drain outside someone's house they will go into our stormwater system and end up polluting our harbour.

"It's always far easier to stop pollutants going into our drains and into the stormwater system than to have to deal with the problem at the outfall and once they're in our streams and in the harbour. Please help us by thinking before you pour something into your stormwater drain."