This weekend brought a large turnout of protestors against the Keystone XL pipeline. An crowd estimated at between 10,000 and 12,000 circled the White House, railing against the pipeline that would bring oil from Canadian tar sands to the Gulf Coast.

Opponents of the project are concerned both about the environmental damage done by extracting oil from the tar sands and about the potential for spills from the nation-crossing, 1,700-mile pipeline. Recent pipeline failures that dumped oil into the Yellowstone River in Montana and a spill last summer in which a ruptured pipeline gushed an estimate 820,000 gallons into Michigan's Kalamazoo River have only increased worries that Keystone may lead to an environmental disaster.

Even if the pipeline never leaks, there are additional concerns over the significant amount of processing it takes to free oil from bituminous (tar) sands. The oil trapped in these sands is generally removed by strip mining the sands and heating the material to release the oil, or by injecting steam into the reservoir rock. The amount of energy consumed is many times higher than that of conventional oil extraction and the land area affected is much greater. Even when separated, the oil extracted is heavy oil that requires extensive processing.

Those favoring the Keystone XL pipeline have noted that it would provide access to oil from a source that's consistently friendly and allied with the United States. Construction of the pipeline is expected to produce between 6,000 and 20,000 jobs, some of which would persist after construction is complete.

Questions have been raised about the official environmental impact statement that gave a go-ahead to the project, particularly the way in which companies involved in the project were allowed to influence the choice of contractors selected for the review. The inspector general of the State Department has called for an investigation into the process, and some legislators have urged President Obama to wait for the outcome of this review before allowing the project to move ahead. The people gathered around the White House this weekend made it clear where they stand.