An initial investigation earlier this year found the waste in the landfill under the sports fields at Kettle Park was too hazardous to be disposed of in municipal landfills, and could even contain munitions and unexploded ordnance from a former army camp to the north of the park.
The council has now found landfill material extending into the beach’s dune system is larger than expected and about 33,430cu m of waste is at present at risk of being exposed by coastal erosion.
Councillors will be asked on Tuesday to sign off on sand sausages being immediately installed along 1.5km of sand dunes east of Moana Rua Rd deemed to be at high risk.
A report to be presented to councillors at next week’s meeting said there was no significant risk to human health at present, but the risk of contaminant discharge within the coastal environment would have implications for the Ōrau marine reserve, which is expected to be implemented in the middle of next year.
The report to be presented to councillors offers them no options to consider.
"Immediate works are needed to mitigate the risk of landfill exposure from erosion associated with the El Nino Southern Oscillation, therefore there are no options," it said.
A long-term solution for the landfill under Kettle Park, which operated from about 1900 to the 1950s, would still be sought, and a remediation strategy would be developed in collaboration with the Otago Regional Council and mana whenua, it said.
Waste within the sand dune system contained asbestos and elevated level of metals that would be too hazardous for disposal at a class A landfill.
In some parts the layer of waste reached up to 5m thick and 8m deep.
To the east of Moana Rua Rd the landfill material was "only metres away from being exposed".
"Presently, the landfill material is covered by soil [or] sand and is only sporadically exposed, which is unlikely to present a significant risk to the public and the environment.
"However, if significant volumes of landfill waste become exposed following erosion events, the risk to the public and the environment will be greater," the report said.
It said that covering the entire length of Kettle Park was estimated to cost $6m.
However, because the amount of landfill material west of Moana Rua Rd was less significant, $5m was considered sufficient to cover the high-risk areas.
The report also noted that options under consideration as a part of the council’s coastal plan for the beach and surf zone, or foreshore and nearshore, were not considered able to eliminate the risk presented by the landfill material.
Further, in another report to be considered at the Tuesday meeting councillors will be updated on a method to assess the value of other erosion prevention approaches now under development.
However, the report said the solution that work identified was unlikely to proceed before the Dunedin City Council put the preferred option into its 2026-36 long-term plan.
An "information gap analysis" had found a better understanding of the ocean’s movement close to shore, as well as the impacts of storms on the beach, was required to ensure the success of the city’s St Clair-St Kilda coastal plan (Whakahekerau — Rakiātea Rautaki Tai).
A technical assessment consisting of a shoreline modelling and monitoring programme was under way, it said.