DENVER (AP) – Colorado’s top environmental official is stepping down, months after he clashed with federal authorities over a massive spill of toxic wastewater from an inactive mine in southwest Colorado.

Mike King‘s resignation was announced Thursday and takes effect Dec. 31. The executive director of the Colorado Department of Natural Resources had led the agency since 2010. His departure comes on the heels of the resignation of Hickenlooper’s lieutenant governor and appears part of the regular string of exits of veteran officials as the administration moves into its final years.

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King clashed with the U.S. Environmental Protection Agency earlier this year over the Aug. 5 Gold King Mine disaster. The spill polluted rivers in Colorado, New Mexico and Utah, including on the Southern Ute Reservation and the Navajo Nation. The EPA came under intense criticism from Congress and from state and local officials for causing the blowout and for the way it responded.

The EPA said after the spill that that Colorado officials endorsed its cleanup operation at the mine and that state experts were at the Gold King Mine on the day of the spill in a supporting role for the cleanup operation.

PHOTO GALLERY: Gold King Mine Tour

But in a September letter to the EPA, King raised nine objections to the EPA’s account. He said that Colorado officials neither approved nor disapproved of the operation.

King’s letter was a blow to the EPA’s contention that outside technical experts supported its plan to push a drainage pipe through debris covering the entrance to the Gold King Mine. The letter also raised questions about an investigation of the spill by the federal Bureau of Reclamation, which said two mining experts from the state approved of the project

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“The investigation’s conclusions into the events surrounding the discharge were not consistent with our staff’s involvement and we felt it important to make sure the investigators were aware of our perspective,” King said in a statement about his letter.

The director did not immediately respond to a request from The Associated Press for additional comment Thursday.

A statement from Gov. John Hickenlooper on Thursday said that King plans to take a new job as director of planning with Denver Water.

Hickenlooper praised King as “wicked smart” with a ” casual charm,” and credited King for overseeing Colorado’s first statewide water plan, which came out earlier this year.

The governor gave no timeline for replacing King.

The governor has another notable vacancy to fill in his cabinet, Lt. Gov. Joe Garcia. Garcia announced last month that he is resigning to run the Western Interstate Commission for Higher Education.

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