State senators in Nebraska are putting pressure on Secretary of State Hillary Clinton to slow down the process of approving a massive pipeline connecting Canada's oil sands to the United States.

Five members of the Nebraska legislature penned a letter to Clinton, asking the State Department to delay a decision on the controversial Keystone XL pipeline until May 2012. The State Department is handling the approval process for the pipeline, and many expect a decision before the end of 2011.

The letter articulates serious concerns about the route of the pipeline in relation to public water supplies and some of Nebraska’s most fragile ecosystems. The Ogallala Aquifer sits under a large part of Nebraska and supplies people and agriculture with massive amounts of water. The proposed pipeline would also go through part of the Nebraska Sand Hills, which are ancient sand dunes and described as one of the nation’s most fragile environments. “TransCanada plans to run its proposed pipeline through a part of the Sand Hills where the Ogallala Aquifer is both deepest and closest to the surface, and most vulnerable to contamination,” wrote the five Nebraska state senators.

Since plans for it were announced, TransCanada’s Keystone XL pipeline has been controversial. The pipeline would carry huge amounts of unrefined oil from the Alberta tar sands to refineries in the United States. Tar sand oil is considered one of the most harmful fuels in the world because of the intensive refining process and the negative effects of extraction on northern Alberta.

These concerns have been amplified over the last year, as pipeline safety has become a growing issue in the United States. Pipeline company PG&E continues to face criticism for failing to produce a safety report in California after an explosion in 2010 killed eight people and destroyed 38 homes. In February 2011, a pipeline explosion in Allentown, Pa., took five lives. A similar explosion took place in Fairport, Ohio. On Jan. 18, a gas line explosion in Philadelphia killed one man and injured six. And in December 2010, a pipeline explosion in Wayne, Mich., killed two furniture factory employees. According to reports, between 30 and 60 people die every year because of pipeline failures.

There are also some growing political concerns. In recent weeks, Clinton has been singled out for a potential conflict of interest regarding TransCanada. The lobbyist representing TransCanada is a former high-level campaign operative from Clinton’s failed 2008 presidential campaign. This potential conflict has resulted in lawsuits against the State Department as well as Freedom of Information Act requests from environmental organizations.

A few months back, the new Keystone XL pipeline was viewed as inevitable, but challenges continue to pop up for TransCanada.