In this Feb. 13, 2012 photo, six and half miles of canal 

In this Feb. 13, 2012 photo, six and half miles of canal bring water from the Rio Grande to the Donna Reservoir in Donna, Texas.  State and federal researchers have found the highest chemical contamination levels in a section of the canal. (AP Photo/Chris Sherman)
Associated Press
In this Feb. 13, 2012 photo, six and half miles of canal bring water from the Rio Grande to the Donna Reservoir in Donna, Texas. State and federal researchers have found the highest chemical contamination levels in a section of the canal. (AP Photo/Chris Sherman)
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