This offense is going to be a matter of state law or municipal ordinance, so it is impossible for me to give you a definitive answer. However, as far as I can tell, this is a form of trespass. Trepass, in common law, is a tort, an offense against person or property. Even if someone has not stepped foot upon your property, the materials placed upon your property constitute a trespass. Generally, trespass is a civil matter, for which one might be sued, and could be a criminal matter as well, depending on state law and local ordinances. You may want to take some photos, call the police and file a report, and speak to your local magistrate or district attorney. Certainly, consulting an attorney is always a good idea. If the dumped materials are creating a danger to you or others, take some photos and then clean up. You do not want to compound your problems by failing to clean up and causing someone to be hurt. Good luck with your problem.