I guess this was not unexpected, given the rumors and reports by the Washington Post. This is rather sad.
Here's an excerpt from the NY Times story on her resignation, which encapsulates some of the challenges she have faced during her time:
"After Republicans seized control of the House in 2010, Ms. Jackson became a favored target of the new Republican majority’s aversion to what it termed “job-killing regulations.” One coal industry official accused her of waging “regulatory jihad,” and she was summoned to testify before hostile House committees dozens of times in 2011. She was frequently subjected to harsh questioning that at times bordered on the disrespectful.
Ms. Jackson, the first African-American to head the E.P.A., brushed off that treatment as part of the territory and a reflection of the new partisan reality in Washington. More difficult for her was the lack of support she received at times from environmental groups, who saw every compromise as a betrayal, and from the White House, which was trying to balance worries about the economy and the president’s re-election campaign against the perceived costs of tough environmental policies."