KEY POINTS:
When council staff discovered contaminated soil around the bases of trees in Victoria Park, they faced an immediate dilemma.
The 90-year-old London planes were scheduled as heritage trees, requiring full protection, but they were located on a well-used path next to a children's playground - an immediate public health concern.
The soil, contaminated from fill sourced from the former Beaumont St gas works, would have to be removed from the trees without damaging their roots.
The clinker byproduct of treated coal produces the carcinogenic chemicals benzo(a)pyrene.
Marcus Herrmann, project manager of the Auckland City Council urban soils review, realised the urgent remediation work would require expert attention.
"We had two goals, and they did not mesh together."
The remediation company Living Earth was contracted for the project with environmental engineers Tonkin & Taylor, and specialist arborists Greenscene were subcontracted to tackle the tricky exercise of removing the soil without damaging the extensive network of tree roots.
The problem at Victoria Park was identified after the council began examining 1400 sites it owned for risks of historical soil contamination.
The most likely were former industrial, horticultural or commercial sites or those where pollution had been reported.
Up to 10 per cent of the council-owned sites were considered potentially contaminated.
Victoria Park was tested because of its proximity to the old gas works and because of anecdotal evidence the reclaimed land, once foreshore, had been filled with unsuitable materials.
Testing in 2004-05 showed contamination, but hotspots were about half a metre below the surface of the ground, so not an immediate threat. As a precaution, the council placed barrier materials over those areas in 2005.
However, the location of the playground in the park made further testing necessary to be sure there was no hazard, Mr Herrmann said.
In November, it was found that the surface soil under the plane trees, which line the path next to the playground, was highly contaminated.
An urgent health-risk assessment by Dr Tim Sprott recommended immediate closure of the treeline area and playground in preparation for remedial works.
Dr Sprott's assessment was supported by the Auckland Regional Public Health Service.
Unlike the rest of the park, the soil had been left undisturbed over the years to protect the trees' roots.
The area was fenced off early in December and signs put up warning people to stay out.
Other soils under trees throughout the park were tested and the council found another two surface hotspots along Halsey and Fanshawe Sts and also fenced them off.
Remedial works began about a month later.
Mr Herrmann said it was not a popular move at short notice, especially because the adjacent sports fields were also closed for sand recarpeting.
The park is a popular inner-city venue for people to walk their dogs, run, relax and play sport.
The council sent newsletters to households and businesses in the vicinity explaining the action and consulted local iwi, the Historic Places Trust and others.
Rather than use a digger, which might damage the root systems and possibly kill the trees, a prototype device, called an air spade, was used. It blew dirt from around the roots into receiving pans inside a sealed tent.
The arborists worked with masks and full body suits, the dust was controlled with a sprinkler system and the dirt was taken to a landfill that accepted contaminated soil.
Mr Herrmann said that after the soil was removed it had to be replaced with clean soil so the roots did not dry out.
After the contaminated soil within the trees' dripline area was removed, a barrier layer of bidim mat was put down and then a flexible plastic barrier sheet called geogrid.
A layer of scoria was added for drainage, more mat and geogrid, and then a stainless-steel mesh to prevent anyone digging down.
A deep drainage system to bring water to the trees' taproots was established and, in an innovative move, a ring of permeable concrete tailored to follow the root pattern of each tree was installed around its base.
Mr Herrmann said that aimed to allow water to reach the tree roots while stopping anyone coming into contact with any residual contaminated soil below the barrier layers.
Topsoil, which will later be grassed, was finally put on.
Mr Herrmann said the exercise had been successful, with no signs of stress to the trees.
"They are actually probably in better condition. We removed a lot of buried asphalt and metal."
In the area outside the dripline, where the soil was excavated using a digger, deep drainage was put in to divert water to the trees.
A layer of bidim, then reinforced concrete and metal was established where new play equipment is to be installed. In the remaining areas permeable concrete was poured to act as a barrier but allow rain in.
The $1.6 million remediation was finished at the end of last month.
