Enbridge Energy filed a petition with the National Energy Board in August to reverse the flow of oil through a segment of its so-called Line 9 pipeline from its terminal in southwestern Ontario. The reversal would mean oil would flow to the U.S. East Coast.
Environmental advocates told the NEB there was a potential for environmental damage from the pipeline's reversal.
Senior Counsel at the National Wildlife Federation Jim Murphy said he was worried Canadian authorities would rubber-stamp the project.
"A more responsible approach would be for the Canadian Energy Board to order an investigation into the full environmental impact of the larger project, including the safety implications of shipping tar sands and impacts on the environment, waterways and communities and carbon pollution from burning tar sands oil," he said in a statement.
Environment Maine Director Emily Figdor said tar sands are more corrosive than conventional crude oil, which increases the risk of a pipeline leak. The line in question, the environmental groups say, is 62 years old.
The NEB said Line 9 construction would be limited to areas already disturbed and no new land would be required.
Oral arguments for the Line 9 hearing are to start May 23.