In separate letters to Wheeler, the lawmakers said the EPA has yet to announce any measures that would mitigate the demand destruction caused by the agency's continued issuance of RVO waivers to oil refineries.

"We remain concerned about the demand destruction of 2.25 billion gallons due to the unprecendented number of small refinery "hardship" waivers granted in 2016 and 2017. The recently released 2019 proposed rule indicates there will be no accounting moving forward of gallons that might be waived under the small refinery waivers in order to maintain the statutory Renewable Volume Obligation," Iowa's congressional delegation said in their letter. 

Iowa's congressional delegation comprises of Sen. Chuck Grassley, Sen. Joni Ernst, Rep. Dave Loebsack, Rep. David Young, Rep. Rod Blum and Rep. Steve King.

Similarly, Heitkamp said "it is concerning that the uncertainty of regarding EPA's improper use of waivers for large refiners was not addressed" in her letter to Wheeler.

Both Heitkamp and the Iowa lawmakers also urged Wheeler to issue a Reid Vapor Pressure waiver for E15.

"I urge you to quickly and transparently resolve the issue surrounding improperly granted waivers and to formally propose granting E15 a Reid Vapor Pressure waiver allowing for sale year-round," Heitkamp said.