It is also calling for a whole-of-site regulatory approach to give certainty to both the aluminium smelter and the Southland community.
The smelter said late yesterday it was committed to improving its environmental footprint and mitigating the impacts of the smelter.
In a statement yesterday, ES said the independent monitoring was undertaken by EHS Support (EHS), experts with international experience in decommissioning smelters. The monitoring focused on the coastal marine area around the smelter, landfill and wider site.”
ES’s position is that the site needs to be managed as a whole to meet contemporary expectations of industrial environmental performance.
NZAS operates under eight consents, most of which are 20 years old.
ES chief executive Wilma Falconer said a comprehensive regulatory approach would give certainty to both NZAS and Southlanders.
"A whole-of-site regulatory approach would give NZAS and the community certainty for the future operation of the smelter. It would require appropriate environmental standards to be met, the remediation of current contamination and ensure the site is cleaned up when NZAS eventually closes," she said.
"We want to work with NZAS ... to ensure the smelter is managed in a way that doesn’t leave Southland with a contaminated site and the costs to remediate it."
The independent monitoring was undertaken by EHS Support (EHS), which had international experience in decommissioning smelters.
The EHS report confirmed two years of studies that show manufacturing processes on the site produce waste with contaminants that escape into the surrounding environment via stormwater drainage and groundwater.
Many contaminants were found at levels above the ecological screening values deemed relevant and applicable to the Tiwai Point aluminium smelter site. These include fluoride, aluminium, zinc, nickel, and polycyclic aromatic hydrocarbons.
NZAS conducted similar monitoring, and the data and conclusions are broadly similar.
But EHS captured additional samples during a large flush event on February 2 this year when, after a dry period, heavy rain washed contaminants into site drainage and then into the sea. This led to the formation of a fluoride and aluminium plume extending well beyond the consented drain water mixing zone. Fluoride and aluminium contamination had the potential to impact marine life.
Fluoride in drain water discharging to the sea during this event was about 15 times higher than the ecological screening values, and aluminium in the water was 440 times higher than ecological screening values at a point 50m beyond the consented drain water mixing zone.
The EHS report also found contamination along foreshore areas at concentrations that could pose a potential land-based ecological risk. This included Department of Conservation land.
It was clear that concentrations exceed ecological screening values within the coastal marine area to the west of the landfill, in Bluff Harbour.
ES was asking NZAS to erect signage at discharge points and said measures should be implemented to warn the public.
NZAS said in a statement it was committed to improving its environmental footprint and mitigating the impacts of the smelter.
It would continue to work with partners, including Ngāi Tahu and ES to do so.
"We are pleased that the ES report confirms the findings in the extensive study by GHD, which we shared with ES and released last week," the statement said.
"That study found there was a very low probability of any negative health impacts from eating kaimoana in the coastal marine area, along with a low risk to marine ecology and no increased risk of health effects for people in contact with sediment and surface water."
NZAS acknowledged the ES report found some areas of contamination and concern.
