The tank sat at the Newington Board of Education's bus garage on Garfield Street.
Town Attorney Ben Ancona said a resident who smelled gas notified the town on December 27th, and excavation began immediately. More than 6,000 tons of contaminated soil have since been removed from the lot.
The leak happened in what should have been a sealed underground vault, but Ancona said multiple fail-safe technologies in the vault did not work.
"We know that there was an anti-siphon valve that didn't work," said Ancona. "There was an audible and visual alarm that did not initiate."
Ancona told FOX61 that the Board of Education didn't know it should have been inspecting the vault monthly. Monthly inspections, he said, would have avoided the problem. Ancona said the vault was not inspected since 2014.
"Everybody really just wants to get to the other side of this," said Ancona. "I'll say it, it's an environmental disaster. We just want to remediate as completely as possible."
The town said the leak is now contained, and environmental officials remain on scene, guiding the town. Oil remains visible on the site, as well as waterways immediately nearby.
"I'm not going to diminish it, oil is oil, it shouldn't be in the waterways," said Ancona. "But there's no additional contamination taking place."
The Town Council has voted to allocate $5 million for cleanup.
Tuesday night, Board of Education members met to discuss how they would contribute to the clean up.
Newington town manager Tonya Lane sent the board an invoice of $875,000 for their portion of the cleanup.
After a lengthy discussion, the Board voted to pay that invoice using money out of their capital improvement plan. It's money that will not affect students' education.