Auckland Council has admitted it was at fault for not telling North Shore residents about contamination arising in part from Auckland Harbour Bridge maintenance.
Revelations by RNZ of heavy metal and hydrocarbon contamination discovered by the Transport Agency a decade ago have alarmed residents at Stokes Point/Te Onewa.
There was not enough certainty to put any contamination tag on households' LIM (Land Information Memorandum) reports now, the council said.
"The threshold for this is whether it is 'likely'" there was a risk to human health, council Regulatory Services director Craig Hobbs said in an internal email to the mayor, council chief executive, and two local councillors released to RNZ.
"Because there was no risk, there was no need to notify anyone or record anything on property files," Hobbs said.
However, the council should have told locals what had been found in two rounds of tests in 2010/11, he said.
"The biggest fault in my opinion ... is that neither council or NZTA spoke to the neighbours and shared the results of the testing at the time.
"While there was no human health issue identified, the fact that we remained silent has now opened us up for criticism as we are reacting after the fact."
NZTA documents show its own health risk assessment found it was likely the majority of properties were OK, but recommended more tests. This did not occur.
Households were not told, and residential testing never took place.
The agency gave its 2010 findings to the council and to regional public health officials, who urged NZTA to tell residents, test soil at their properties, and investigate the wider impacts on the marine environment, of high zinc, lead and copper levels.
No 'unacceptable' risk
The council had a health risk assessment done in 2011 based on sampling at the public reserve at Stokes Point; this followed a risk assessment by NZTA.
"In the summary on page 13 it states that 'as a whole, contaminant levels do not pose an unacceptable level to human health'," Hobbs said in the internal email.
"Based on the information we have, specifically the conclusion of the risk assessments, there is not enough certainty for council to put any type of 'tag' on the adjacent properties for contamination.
"The threshold for this is whether it is 'likely'."
Even after the 2011 risk assessment, the council kept talking to regional health officials through 2012 about the contamination, and undertook its own remediation of a small area of the reserve.
This was next to a pathway where the carcinogenic hydrocarbon BAP was found - samples rated this at up to 30 times permitted levels.
"This involved covering the area with some geotextile matting and 100mm of mulch," Hobbs said.
What was considered a safe level of BAP had been raised a lot since 2011, he added.
NZTA manager pushed for residents to be told
The Transport Agency's national environmental manager at the time, Carl Reller, told RNZ that simple remediation work could also have been undertaken by local residents in their gardens, had they been told, something that he urged for two years be done.
Hobbs in his email played down the bridge as a source of contamination, saying lead paint on houses and from leaded petrol could be a source.
However, lead levels rose at Stokes Point between the only two lots of sampling, in 2001 and 2010/11, well after leaded petrol and paint was phased out.
There were likely to be sources of contamination in the reserve that were not on private properties, notably old fill material and an old coal tar footpath, Hobbs said.
"Bridge-related contamination associated with maintenance and traffic are described as less important sources," he said.
"Lead measurements in the reserve may not be a result of bridge maintenance activity."
This is at odds with an NZTA document from 2012 which said the distribution of lead and zinc matched the bridge alignment, "suggesting either the bridge or the motorway as likely sources of lead and zinc", though the BAP findings were more dispersed and "do not correlate with the AHB as an exclusive source".
Hobbs said the council's documents showed that NZTA planned to undertake testing in residential properties adjacent to the bridge.
In July 2012, the agency "advised council that letters requesting permission to test residential properties had been sent".
But this did not happen.
NZTA documents show a mailout was cancelled in July 2012. "It appears to have not gone ahead," Hobbs said.
The council had told the agency in February 2011 that if it tested at houses, "the property owners need to be aware of the implications i.e. that council would record any confirmed contamination on property file / LIMs for these properties".
Regional public health officials had in 2011 also raised the prospect of future home buyers needing to be advised in the LIMs.
The health officials did not follow up beyond 2012 to see if the NZTA followed their recommendations - and it did not - according to Auckland Regional Public Health's statement to RNZ.
The Transport Agency wrote to residents this month after RNZ reported the contamination, saying current controls at the bridge were effective.
Its latest annual report summarising environmental monitoring data showed it was compliant with the council's Environmental Management Plan, the agency said.
RNZ has requested the release of the annual reports.
A 2011 internal NZTA document said the bridge maintenance operation had been in breach of half of the 15 consent conditions since 2001.
Neither the agency nor the council have addressed RNZ questions about those breaches.
The agency in its 2 October letter said that "specific reporting is required if an event occurs which results or has the potential to result in a breach of the conditions of consent or adverse effects on the environment, like a significant discharge into the air or water".
Sampling shows zinc and copper levels have been rising in the harbour water in the vicinity of the bridge, according to official reports.