She tells the newspaper that Pruitt is generally pretty private about his comings and goings, so when she saw the Trump administration official in the same restaurant as her, she says she knew she needed to act.
Deputy chief Andrew Wheeler will take over as acting administrator Monday, Trump said.
Last month, while praising Mr Pruitt's "fantastic job" at the EPA, the president admitted, "I'm not happy about certain things, I'll be honest".
"We have made tremendous progress and the future of the EPA is very bright!", Trump concluded his statement on the matter. The announcement comes after months of controversy and ethical questions.
Late last week, the EPA's chief ethics official said he is pushing for a series of independent investigations into Pruitt's actions, citing a number of "potential issues" that have surfaced from internal sources and media reports.
Pruitt attended a July 4 picnic at the White House.
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Despite the scandals, Trump has been approving of Pruitt's efforts to reduce regulations that the administration says curb business growth.
The fresh allegation that Pruitt enlisted a subordinate to perform personal duties comes on top of reports that he asked an aide to seek a business opportunity for his wife from the fast-food franchise Chick-fil-A, and that she received $2,000 from Concordia, a New York City-based nonprofit that had asked Pruitt to speak at an event a year ago.
The prospect of Pruitt having any additional involvement in government affairs at this time is severely unprecedented. In the post, Mink encourages people to support the Sierra Club's online campaign aimed at pressuring senators to call for Pruitt's resignation. Former aide Kevin Chmielewski told CNN that Pruitt and his staff kept "secret" calendars and schedules to hide controversial contacts with industry representatives and The Washington Post and The New York Times revealed aspects of testimony Pruitt aides gave to Congress about the EPA administrator enlisted them for personal errands. Republicans from Iowa, Senators Joni Ernst and Chuck Grassley, have also spoken harshly against Pruitt's for a proposal to change rules regarding ethanol and renewable fuels.
Pruitt was one of the more politically ambitious administrators to run EPA.