Super-retailer Walmart has been slapped with $110 million in environmental fines after pleading guilty to improperly dumping pesticides — all of it returned by customers — into public sewers and landfills, creating a toxic nightmare.

The Justice Department's Environmental Crimes Section told Secrets that it was the second-largest environmental criminal fine ever imposed in U.S. history after the $4 billion BP oil spill in the Gulf of Mexico.

Walmart pleaded guilty to an assortment of charges with fines totaling more than $110 million for violating state laws and the Federal Insecticide, Fungicide, and Rodenticide Act. The Arkansas firm will pay $81.6 million in fines in cases brought by Justice and the Environmental Protection Agency and the rest the result of cases brought by Missouri and California.

In announcing the guilty plea, Justice and the FBI said the case and huge fine were a warning to other retailers who take returns of pesticides and other poisons and simply toss them in the garbage or down the drain.

Justice charged that Walmart didn't have a program to train employees how to handle and dispose of the chemicals "As a result, hazardous wastes were either discarded improperly at the store level — including being put into municipal trash bins or, if a liquid, poured into the local sewer system — or they were improperly transported without proper safety documentation to one of six product return centers located throughout the United States," said Justice.

"By improperly handling hazardous waste, pesticides, and other materials in violation of federal laws, Walmart put the public and the environment at risk and gained an unfair economic advantage over other companies," said Ignacia S. Moreno, Assistant Attorney General for the Justice Department's Environment and Natural Resources Division.

In the Missouri case, more than 2 million pounds of pesticides were improperly disposed of. "Today's criminal fine should send a message to companies of all sizes that they will be held accountable to follow federal environmental laws," said Tammy Dickinson, U.S. attorney for the Western District of Missouri.

With more than 4,000 stores nationwide, Walmart is one of the country's largest suppliers of pest killers. As part of the plea deal, the company must make sure it trains employees how to handle the chemicals.