The Marsden Point oil refinery says it is sucking a lot less leaked fuel out of the ground and that the amount will fall further if it stops processing entirely.
Marsden Point oil refinery Photo: 123rf.com
Reports show that as recently as four years ago, between 500 and 2000 litres of light fuel and dissolved-phase hydrocarbons a day were being sucked up from the aquifer near Whangārei.
Refining NZ said that had now dropped to just 200-300 litres a day.
"The most significant impact of historic contamination on our site is from our processing plant," the company said.
"In the event that we cease processing, this will stop."
The company wants to stop refining, and become an import-only terminal next year if shareholders agree. The company's annual meeting is tomorrow in Auckland.
Amounts of leaked fuel pumped up fluctuate with the seasons, with more in summer.
"It is not possible to say definitely what might be coming from current operations, but we do expect the vast majority of what is being extracted to be legacy contamination," the company said.
"This is because in recent years we have significantly improved our practices, upgraded our equipment, and been focusing on works to reduce leaks overall, thus reducing the contamination, which is evidenced by the plume reduction."
A 2020 hydrogeological report shows the extent and thickness of a plume of light fuel (LNAPL) under the refinery shrinking between 1997 and 2013.
A graph showing pollution plume thickness at Marsden Point oil refinery in 1997. Photo: Supplied
This graph shows how the pollution plume thickness has reduced by 2013. Photo: Supplied
Monitoring at 140 wells showed the refinery was complying with its resource consents, it said.
If seawater intruded into the aquifer, that would actually help cut contamination, it added.
Alongside the leaked fuel underground, masses of water is pumped up daily, which has the effect of pulling the polluted groundwater in towards five wells, instead of it spreading into the sea.
This pumping also protects the refinery control room, which was built below the level of the groundwater, from explosive gases getting in.
More decline expected
The company renewed its resource consents for 35 years earlier this year; the consents had been due to expire next year.
"We have no intention of ceasing to operate our well network, and therefore, over time, we expect to see a further significant decline in the plume," it said.
Council reports show drainage networks are not completely lined, so leak fuel into the ground, as do storage areas.
The company used to bury contaminated sludge in a process it called "landfarming".
"Ultimately, the former landfarms were excavated and taken offsite or used to strengthen the sand dunes on the southeast coast surrounding the refinery."
The reports list the sources and types of contamination in the water and soil.
For instance, it's likely an area of soil contamination by polycyclic aromatic hydrocarbons exists; but transformer oil that once had now-banned polychlorinated biphenyls (PCBs) in it is no longer used.
At the boundary, 29 perimeter wells are routinely monitored.
The council reports on these and other wells showed generally low levels - or the absence - of a range of toxic contaminants, including phenols and solvents, in the groundwater.
However, there is contamination by PFAS (perfluoroalkyl and polyfluoroalkyl substances) from firefighting foam used previously in training exercises.
"Since 1 January, 2019, only training foams have been used, and these are fluorine-free compounds," council reports said.
The refinery's consents do not require it to produce a clean-up plan for the 114-hectare site, unless it is closing down entirely.
"We have a strong focus on reducing the extent of legacy contamination over time as part of our ongoing remediation of the site, and this has been recognised by a range of parties, including our iwi partners, and council," the company told RNZ.