First, do not touch any dumped waste since there is a chance that it could injure you. There are several government agencies you can contact to take care of the waste.
Go to the regional homepage for your state to find links to the state's agency that oversees environmental issues. You will also find contact information for reporting spills and releases to your EPA regional office. These offices will help you determine if the waste you have found is hazardous, and how to get it cleaned up.