Geothermal activity has closed Hatupatu Drive behind the Rotorua Sportsdrome.
The failure of two bore casings in Rotorua within three weeks was just a coincidence, a volcanologist says.
On the weekend of October 9, a bore casing failed in Kuirau Park, pushing mud, water and steam up through the road and leading to its closure.
Efforts to quellthe geothermal activity using cold water and cement were unsuccessful due to the high pressure of the bore, so well-drilling equipment was used to inject mud and cement into the ground to suppress the activity.
At the time Rotorua Lakes Council said it was likely the bore would be unable to be repaired and be grouted shut permanently.
On October 27, less than three weeks later, geothermal activity from another bore forced the closure of part of Hatupatu Drive near the Sportsdrome. Water was used to cool and quench the bore that same day and it was grouted shut.
A geothermal bore casing failure caused mud pools at a Kuirau Park car park on October 9. Photo / Supplied
But on Tuesday Rotorua Energy Events Centre and the Sportsdrome were evacuated after the bore reheated, causing gases and steam to escape from the sealed area.
Higher than usual levels of hydrogen sulphide gas were detected, leading to the facilities' evacuations.
The facilities reopened on Wednesday while work to cool the bore continued.
GNS scientist Brad Scott said the two bore casing failures appeared to be a coincidence.
He said the most likely reasons a bore failed were corrosion of the casing wall or damage during maintenance cleaning.
When a bore is drilled the well drillers line the drill hole with a steel tube casing. If the wall of the casing is punctured by corrosion or cleaning the hot fluids will leak out of the bore and create a new path to the surface, creating the mud and water seen, he said.
Scott said the impacts of a bore failure were usually contained to a 50m area and bore and other geothermal infrastructure failures were very common in Rotorua.
"The hazards are the hot water and steam. Sometimes when the breakout first occurs there may be some rocks and mud produced.
"The fluids in the bore are normally not corrosive and have a neutral pH. However, the fluids in the ground around it can be acidic, so they impact the casing.
"The failure of a bore is often a gradual process. The heat plume arrives first, killing grass etc, then water and steam reach the surface to create the flooding of hot water and muds. This can take a while."
The cordoned-off area behind the Energy Events Centre and Sportsdrome on Tuesday. Photo / Ben Fraser
The council was asked how bore casings worked and what parts failed in these instances.
It said the recent issues in Hatupatu Drive and Kuirau Park were due to failures of the bores' casings, "not any known increase or change in naturally occurring geothermal activity".
"In this instance, it is suspected that the acidity of the geothermal water has corroded the bore casing over time, causing the failure," the post stated.
"It's important that people avoid the bore site due to potentially unstable ground, very high temperatures from steam and water, exposure to hydrogen sulphide and machinery and staff actively working on the site."
Te Arawa Lakes Trust climate change co-ordinator Lani Kereopa said they would know if there was "anything spooky going on".
"The people have been among it for so long that if there was something wrong we would know.
"Geothermal changes all the time, the hotspots are always moving. We're used to that. That's natural," she said.
"In the short time frame since those bores have gone off I wouldn't say I've heard there is more unusual geothermal activity happening."
