THE DALLES, Ore., Feb. 8 (UPI) -- Just weeks after passing a major energy bill, some congressional leaders are questioning whether certain portions of the law are achievable.

The biggest concerns hinge on the Renewable Fuel Standard established by the Energy Independence and Security Act of 2007, a massive piece of legislation signed by the president on Dec. 19. The provision mandates increasing amounts of biofuels be produced and mixed with domestic gasoline. The dictated volume grows steadily each year, requiring a nearly eight-fold increase overall from last year's production of 4.7 billion gallons to 36 billion gallons in 2022.

While the law as a whole has the potential to significantly decrease U.S. dependence on foreign oil and domestic greenhouse gas emissions, the RFS provision could prove difficult to enact as it is currently constructed, said Sen. Jeff Bingaman, D-N.M., at a hearing Thursday in the Senate Committee on Energy and Natural Resources.

"First, early year biofuel requirements could be too aggressive; second, mandates for specific technologies and feedstock could prove to be overly prescriptive; finally, the environmental restrictions may be too narrow," said Bingaman, chairman of the committee.

The RFS requires biofuel production to almost double in 2008 alone. While Bingaman said he's confident the biofuels industry can produce the 8.5 billion mandated gallons, getting it to consumers presents a daunting task.