It’s likely that this happens more than we care to know.
From Environment News Service:
HOUSTON, Texas, November 20, 2007 (ENS) – The owner of a hazardous waste transport company and his operations manager face federal charges for their roles in a conspiracy to illegally transport and dispose of hazardous waste underground. Their alleged misuse of an underground injection well may have contaminated drinking water.
The men were arraigned Friday in U.S. District Court in Houston. They are charged with 14 felony counts including conspiracy, violating the Safe Drinking Water Act and violating the Resource Conservation and Recovery Act, which regulates storage, transportation and disposal of hazardous wastes.
The cliff note version: They dump hazardous waste into class II injection wells because it saves money.
This happens in the Barnett Shale area. Neighbors have witnessed Class I waste trucks dumping their waste into Class II wells but it’s almost impossible for regular citizens to prove.
Update: I just remembered this recent Star-Telegram article by Mike Lee
In 2006, the Railroad Commission fined another Wise County injection well operator $4,000. Investigators said the operator dumped water from a firefighting operation and “25 barrels of an unknown chemical” into an injection well.