There are thousands of dirty old drill sites in Colorado. The state gave oil firms a $1bn pass

Investigation reveals regulator let firms off the hook on cleanup bonds despite backlog that will take decades to clear

When Christiaan van Woudenberg moved to Erie, Colorado , in 2007, he never imagined he would become an anti-fracking activist. He simply thought he was buying his dream home – a four-bedroom with a panoramic mountain view, 30 minutes north of downtown Denver .

Then, in 2014, the drilling started. Oil and gas rigs sprang up, some just 800ft (240m) from his bedroom window. The dream turned to nightmare: loud noises rumbled all night long, and the air stank like exhaust. Neighbors started getting headaches and nosebleeds , and van Woudenberg developed new respiratory issues. He kept his windows shut and worried about his daughters going outside.

“So I got mad,” he said. “Like, ‘Oh, if they can do this to me in my fancy house as an upper-middle-class white guy, they can do it to anybody.’”

Van Woudenberg looked for ways to visualize the scale of the industry’s pollution. A software developer, he sorted through vast data published by the state energy and carbon management commission (ECMC), Colorado’s oil and gas regulator. What he discovered shocked him. Chemical spills were turning up daily in Weld county, where he lives – sometimes at new drilling projects, but more often at old, defunct sites where contamination had gone undetected for years.

He started charting locations where wells, storage tanks and underground flow lines had leaked toxic material into the environment, bringing his detailed maps to anti-fracking protests and community meetings. Without highly specialized skills, the toll was nearly impossible to see.

“We’re trying to show the oil and gas infrastructure burden on this state,” he told the Denver Post in 2018 , a year when more than 11 spills a week were uncovered on average in Colorado. “There’s heaps and heaps of it. It is everywhere.”

An investigation by the Guardian and DeSmog examined thousands of state documents to create an unprecedented picture of that invisible public toll, as an ageing oil industry struggles to clean up – and pay for – its own decommissioning.