The Fairfield Landfill was closed in 2017 but continues to produce leachate and gas, requiring ongoing management.
Responsibility sits with Waste Management NZ Limited, which has asked that its application to the Otago Regional Council for seven new and renewed consents to facilitate the "aftercare" period for the landfill be publicly notified.
The application includes a consent to discharge leachate on to land "in a manner which may result in contaminants entering groundwater".
Submissions on the application closed this month.
An ecological assessment prepared by consultants for Waste Management NZ said the adjacent Kaikorai Lagoon supported a diverse range of wildlife, and measured high in ecological and cultural values.
However, it was influenced by an "intensively developed catchment" upstream, including stormwater discharges often linked with high contaminant loads, particularly heavy metals.
"This makes it difficult to ascertain the relative contribution of landfill effects for many water and sediment quality parameters, however low pH levels and high ammoniacal nitrogen concentrations were clearly greater near the landfill site," the report said.
"This suggests that ammonia in landfill leachate is likely having a toxic impact on aquatic communities locally within the estuarine environment."
Ongoing monitoring of the area was "essential" to understanding long-term impacts of surrounding activities and natural variability, the company said.
In a subsequent letter to the company’s consultants, council consents planner Brittany Watson asked for an assessment of the "cumulative impact" of leachate on the waterway.
At present it was not clear to what degree the contamination "observable in the degraded receiving environment" was attributable to the Fairfield Landfill, Ms Watson said.
Leachate contamination was also an issue during the Dunedin City Council’s resource consent application for the nearby Green Island Landfill.
Locals have raised concerns about the overall state of the waterway.
In a notification report last month, Ms Watson recommended the application be publicly notified, as requested by Waste Management NZ and because the company had "refused to provide further information".
In the report’s assessment of adverse environmental effects, Ms Watson said modelling and assessment on the application’s natural hazards and climate risk were requested by the council, but not supplied by the company.
Overall, the level of adverse effect of granting the application could not yet be determined, she said.
Beyond public notification, Ms Watson said Aukaha (on behalf on mana whenua), the Department of Conservation, Otago Fish and Game Council and Public Health South be directly notified.
There are three active or historic landfills in the catchment: Fairfield, the city council’s Green Island landfill and the privately operated Burnside Landfill.
