Environmental Protection Agency Acting Assistant Administrator Janet McCabe was the lone witness in a Senate Homeland Security Committee hearing Thursday on “Re-examining EPA’s Management of the Renewable Fuel Standard Program.”

Committee chairman James Lankford (R-OK), who favors repealing the RFS, grilled McCabe over the “aspirational” goal of breaching the blend wall and the potential that volumes of cellulosic ethanol targets under law will have to be “reset” by the EPA because not enough is being produced.

Committee Ranking Member Heidi Heitkamp, who is a strong supporter of the RFS, was more concerned that the volume obligations for biofuels proposed by EPA “ignore Congressional intent and reduces Congressionally-mandated blend volumes, citing availability of distribution capacity.”

McCabe continued to defend the EPA’s recently proposed volume obligations for 2014, 2015 and 2016 as “ambitious but responsible” in the face of criticism from both sides of the issue.