The investigation of the landfill under Kettle Park, St Kilda, has also provided more information on its contents — which include asbestos, old gasworks waste and demolition waste.
The Dunedin City Council is stepping up its investigation of the site, which operated next to Ocean Beach until the early 1950s, as part of its long-term St Clair-St Kilda Coastal Plan.
A report by environmental and engineering consultants Tonkin+Taylor is contained in the agenda of next Tuesday’s council meeting.
A series of storms over the past 10-15 years had eroded the dune system that provided seaward protection of the landfill, and continued erosion risked exposure of the waste itself, the report said.
The landfill had been thought to cover about 17,000sqm based on an earlier investigation.
However, the recent study identified the presence of landfill waste over an area of about 48,000sqm to the east of Moana Rua Rd, while the waste area identified west of Moana Rua Rd covered about 3000sqm.
Sixty boreholes were made and soil samples analysed.
"Contaminants including asbestos have been detected in the landfill and capping material at concentrations that present a potential risk to human health," the report said.
"However, these materials are typically covered by a topsoil/turf layer in which asbestos was not detected and in which other contaminants were detected below levels that are likely to present a risk to human health."
Users of the sports field were therefore unlikely to be at risk, but if the landfill material was disturbed, health and safety controls would be needed to protect workers and the public from exposure to contaminants.
Some potential remained for exposure though small amounts of material at the seaward dune face, although this had previously been assessed as low, in part due to the council’s signage and periodic removal of material from the beach.
"We note that a more significant potential risk to human health could occur if a large volume of landfill material was exposed."
Contaminants above environmental assessment criteria were detected, but further work would be required to assess the actual effect on ecological systems in the marine environment from the current low discharge of landfill material and leachate.
"The risk of more significant adverse environmental effects exists in the event of a large discharge of landfill material into the marine environment."
"Significant" uncertainties remained, including possible areas with higher concentrations of contaminants, potential munitions and unexploded ordnance from the former army camp to the north of Kettle Park, and the possibility the landfill was even larger than the study identified.
Council infrastructure and development general manager Simon Drew said the study helped to decide the council’s next steps and response, with work already under way.
"We’ve been monitoring this site and taking steps to protect it from coastal erosion in recent years, and we are well placed to respond should another significant weather event threaten this winter," he said.
"We also now know the site is both bigger than we thought and closer to the sea."
The council now seeks to investigate the dune face, to help decide how how the area is managed and identify any other short-term options needed to protect the dunes.
Options and timeframes for erosion mitigation and dune remediation work, as well as the approach across the wider landfill site, will be considered by councillors as part of the 2024-34 10-year plan.
