About 16,000 litres of diesel flowed out of the tank at Christchurch Men's Prison, and only about 2000 litres were able to be recovered. File photo. Photo: Luke McPake / The Wireless
About 14,000 litres of diesel has leached into the ground at Christchurch Men's Prison, after the hatch of a fuel tank was left open.
The mistake, first reported by RNZ on 6 June, has potentially affected the water supply of neighbouring properties
Corrections custodial services commissioner Leigh Marsh said the overflow was identified on 25 May, when diesel was found in cable pits and underground infrastructure, but there was no evidence it had seeped into the ground at that time.
Contractors had been investigating an issue with diesel heating and a backup system in a block at Christchurch Men's Prison.
"Both Christchurch Men's and Christchurch Women's prison run off bore water, so we were very concerned if it was to go to soil that that water would be impacted so we immediately started testing, and there was no evidence to say immediately that that had gone to ground," he said.
WorkSafe, Fire and Emergency and Canterbury Regional Council were notified.
Daily testing of the prison bore water supply and surrounding soil began on 26 May and was extended to a 200-metre radius outside the prison perimeter two days later.
The samples remained clear until 6 June, when a sample taken from a bore on 28 May from 200 metres outside the prison perimeter came back positive for dissolved hydrocarbon, while the other samples remained clear.
Corrections then visited 40 properties south east of the prison and advised eight to stop using water from their private bores for drinking, cooking or feeding stock or animals and use water from other sources until the potential contamination was addressed. It remained safe for showering and washing.
Marsh said the majority of properties visited were vacant or were using the town water supply, not bore water.
If a property had not been visited by Corrections, or had a letter left in their absence, they were able to continue using their bore water, he said.
No diesel had been detected in the bores which supplied the prison, or in the stock water in the prison farms and there was no impact on prison operations, Marsh said.
How did a diesel leak of that size occur?
There are two 20,000 litre fuel tanks at Christchurch Mens Prison, which hold the majority of its diesel, and a 1000-litre local tank which powers the heating of most of its buildings.
The large tanks were being replaced by Downer, with a temporary fuel tank installed while the work was carried out.
"It looks like that installation hasn't gone as planned and as a result, the diesel has gone from the big tank into the smaller tank, which feeds some of the prison systems.
"The large tank was able to continuously feed into the small one and it appears that an inspection hatch on the top of the smaller tank hadn't been attached properly."
Marsh said about 16,000 litres of diesel had flowed out of the tank, and only about 2000 litres were able to be recovered.
On 30 May, it was found diesel might have soaked into the ground through a crack in the floor of a switch room.
"It appeared that some of the diesel had gone into the ground at that point but our water testing continued to show that it was clear," he said.
Marsh said advice from the Canterbury Regional Council, Fire and Emergency and other scientific advisors was the diesel spill had not gone beyond that building, but as a precaution it began taking soil samples from a wider area, predominantly focused on where geologists said water would potentially flow underground.
The incident was being investigated and the exact cause was not yet known, but it appeared to be related to the installation and change over to a new fuel tank, he said.
Both tanks had since been lifted and checked for leaks, but the programme to replace the tanks had been stopped while the investigation was carried out.
A Downer spokesperson said it took immediate action following the diesel spill to determine the cause of the incident and to maintain site safety.
"We will continue to work closely with the New Zealand Department of Corrections, along with all relevant local stakeholders and regulatory authorities to manage the response, undertake thorough testing, and determine next steps."
Environmental management of the spill
Marsh said Corrections was focused on tracking any contamination and ensuring residents had the appropriate information if it appeared in their bores.
Advice it received from geologists and scientists was a layer of clay under the soil in the region would limit the spread of the fuel.
"It won't pass through the clay, so it's not going to hit down to the aquifer, but it's how much it moves along above that into what is quite a granitey, gravelly soil from the clay up."
The regional council said Corrections was leading the response to the diesel spill with its support and that of Health New Zealand.
General manager of regulatory implementation Paul Hulse said monitoring work was ongoing to understand what, if any, impact there might be from diesel entering the groundwater.
"As a precaution, on Friday 6 June Department of Corrections notified residents downgradient of the site with advice from Health New Zealand to not use water from their private bore for domestic use," he said.
Of the 40 properties visited, some were vacant and some were on town supply so were not affected.
Affected households should discontinue using their private supply for irrigation and stock use, he said.
Residents would be kept informed, including when they could resume use of their private bore.
WorkSafe said it was making enquiries to understand the circumstances of the incident.
Taumata Arowai did not respond to RNZ's request for comment.
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