Reading the joint complaint (PDF), the DOJ and Arkansas allege that there was an "unlawful discharge of heavy crude oil" from the 850-mile Pegasus Pipeline, which was first built in the 1940s and runs from Illinois to Texas. It usually transports 95,000 barrels of heavy Canadian crude oil per day, but have been closed since the spill.
The spilled oil "caused and continues to cause pollution to waters of the State," the complaint says, and the parts that ExxonMobil was supposed to have cleaned up remain a mess. The state and the DOJ are seeking civil penalties of "$1,100 per barrel discharged ... or if the violation is the result of gross negligence or willful misconduct, no more than $4,300 per barrel discharged," among other fines. You can see a video of the spill below.