Some frackers have used some benzene in their suspensions/solutions. Many have said their stuff is proprietary, and they won't disclose what's in it. A lot of states want to make revealing the composition mandatory.
A company called FTK has a solution that is mostly citrus based..It is injected in wells that are way below the water table..the bigger problem is spillage at ground level
Benzene is used, as I understand it, to track the progress and recovery of the liguids and sands.