An Oklahoma oil and gas company plugged a well that disposes of oil production’s toxic byproduct after an investigation by The Frontier and ProPublica revealed that regulators had approved the well in violation of state rules meant to protect public groundwater supplies.

The disposal well, named the Flying Monkey, injected oil field wastewater underground and sat roughly a quarter-mile from two municipal groundwater supply sites that served the small city of Enid in northwest Oklahoma. Wastewater injection operations are banned within a half-mile of public water wells unless regulators hold a hearing to ensure that this activity will not pollute groundwater. Regulators at the Oklahoma Corporation Commission did not hold a hearing before approving the well in 2018.

The city of Enid filed a complaint last year asking the state to revoke the Flying Monkey’s permit to operate.

On Sept. 14, with the complaint still active in the agency’s administrative law court, employees of the Flying Monkey’s current owner — BCE-Mach III Midstream Holdings LLC — dropped a cast iron plug into the bottom of the well, followed by more than 200 sacks of cement, to........

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