From New Orleans to the Gulf of Mexico, a massive oil slick has spread over 100 miles of the Mississippi River. It's the result of yesterday's collision between a double-hulled tanker and a barge filled with heavy diesel. At least 200 people are working to contain the hundreds of thousands of gallons of diesel fuel so think and heavy it could sink to the bottom of the river. What are the risks to the environment and the water supply? How long might it take to clean up? Mark Schleifstein is environmental reporter at the New Orleans Times-Picayune.