For the first time, a Mexican factory yesterday was fined for violating a U.S. environmental law. Maquiladora Chambers de Mexico, a leather belt manufacturer south of the Arizona border, consented to pay a $3,164 penalty for improperly disposing of hazardous waste in the U.S. and agreed to train its personnel in waste disposal laws. Two Arizona companies, which illegally accepted and stored the hazardous waste from the Mexican company, are being fined almost $41,000. The U.S. EPA said the fine against the belt maker, though small, set an important precedent for battling pollution at the border.