WASHINGTON — The U.S. State Department hasn’t responded to a request by seven Democratic U.S. senators that the agency reassess its initial review of a pipeline proposed by TransCanada.

The pipeline would carry oil from Canada to Texas. The legislators said they were concerned about the environmental damage from possible spills and the route of the pipeline.

In a letter to U.S. Secretary of State Hillary Clinton on July 15, Sens. Sheldon Whitehouse, D-R.I.; Patrick Leahy, D-Vt.; Barbara Boxer, D-Calif,; Ron Wyden, D-Ore.; Robert Menendez, D-N.J.; Frank Lautenberg, D-N.J.; and Ben Cardin, D-Md., urged the State Department to re-examine the pipeline’s potential dangers to the environment and alternative routes for the pipeline.

The first two phases of the Keystone Pipeline System went into service in June 2010 and February 2011, moving oil from Canada to the U.S. Midwest and from Nebraska to Oklahoma.

The proposed Keystone XL pipeline, which would incorporate part of the Nebraska-Oklahoma pipeline, would cost $7 billion and deliver oil to Texas. The 1,661-mile pipeline would run through Montana, South Dakota, Nebraska, Kansas, Oklahoma and Texas and have a daily capacity of about 1 million barrels.

The project is especially controversial because the pipeline will carry oil sands. Environmentalists claim oil sands crude is more corrosive than other crude oil because of its higher sulfur content and could cause more pipe leaks.

Susan Casey-Lefkowitz, director of the National Resources Defense Council’s international program, said because oil sands petroleum is more corrosive than other oils, a spill would have a devastating effect on the environment.

TransCanada and the oil industry cite studies saying that oil sands petroleum is no more corrosive than the oil the United States has been using for years. All crude is becoming more sulfurous, said John Kerekes, Midwest field operations manager for the American Petroleum Institute, the oil and gas industry trade association.

In their letter, the Democrats cited TransCanada’s existing Keystone pipeline which has leaked 12 times in the year since the start of its operation.

“These spills are troubling, as the Keystone XL pipeline will have similar characteristics, and underscore the need for careful assessment of both the spill risks and route of Keystone XL,” they said.

The State Department hasn’t responded to the senators.

Dan Clune, an official in the State Department’s Bureau of Oceans and International Environmental and Scientific Affairs, said Friday that the department is examining the issue but has made no decision on whether to further assess oil sands petroleum’s corrosiveness on pipes.

The State Department issued the first draft of its Environmental Impact Statement on April 16, 2010, and a supplemental draft on April 22, 2011. The first draft found that the Keystone XL pipeline “would result in limited adverse environmental impacts during both construction and operation” as long as it was built and operated in compliance with laws and regulations. The writers of the supplemental draft reached the same conclusion.

A final version of the study will be released in August, followed by 90 days of consultation with other agencies about whether the project is in the U.S. national interest.

The State Department is the lead agency in the analysis of the project. There are 10 other federal agencies involved, including the Environmental Protection Agency, the Bureau of Land Management, the National Park Service, the U.S. Fish and Wildlife Service, and the Department of Transportation. The Montana Department of Environmental Quality also is involved.

Casey-Lefkowitz said most of the work has been done by State Department staff instead of experts in oil sands and pollution from refineries and there must be specific studies from outside experts. She also wants the Department of Transportation to analyze the effect of oil sands on pipes and the risk of leaks resulting from corroded pipes.

“We shouldn’t be building new pipelines until we’ve done a thorough review and have new safety regulations in place,” she said.

But Kerekes said he has “confidence in TransCanada’s ability to construct the pipeline in a way that is sound and environmentally safe. They are staking their corporate well-being on its success.”

TransCanada spokesman Terry Cunha said the company will use the newest technologies to prevent leaks in the pipeline expansion. It is to be buried deeper than the industry mandates and use a thicker pipe in environmentally sensitive areas, such as the Yellowstone River area, the location of July 1 an ExxonMobil pipeline spill.

TransCanada will monitor the pipeline using satellites and 16,000 sensors along the pipeline 24/7, and if they detect a drop in flow because of a leak they will be able to shut down the system.

Environmentalists also say oil sands petroleum emits more greenhouse gases but TransCanada points to a December 2010 study by the Royal Society of Canada that said greenhouse gas emissions from oil sands are only 0.08 percent of estimated global greenhouse gas emissions, though greenhouse gas emissions from the oil sands industry pose a growing challenge to Canada meeting its greenhouse gas emission reduction targets.

In response to comments the State Department received about the first draft of its Environmental Impact Statement, the agency commissioned a study about greenhouse gases by a contractor. And the final draft of the Environmental Impact Statement will include a section about alternative routes for the pipeline.

U.S. Rep. Gene Green, D-Texas, a co-sponsor of the bipartisan North American-Made Security Act, which would require U.S. President Barack Obama to issue a final order granting or denying a Presidential Permit for the pipeline by November, said the State Department’s analysis was thorough. TransCanada filed its proposal for the pipeline with the State Department in 2008, and Green said this has been the longest review process by the department of any project between Canada and the United States.

The new pipeline will be built according to current safety standards and be safer than many of the decades-old pipelines covered in the Pipeline Safety Reauthorization Act, he said.

Houston Mayor Annise Parker, whose city would lie at the end of the Keystone pipeline, asked the State Department in March to further review the potential for increased greenhouse gas emissions at refineries, the effect on low-income communities surrounding the proposed pipeline site, and the safety and spill response preparation. Houston has five refineries, the highest concentration of petrochemical plants in any U.S. city.

Green, whose district includes Houston, said Canadian oil sands will give Texas refineries another option than offshore and foreign oil.

“I would have hoped (Mayor Parker) would consider that our refineries need a stable supply of crude oil,” Green said.