neighbor dumping motor oil on my tree...?

So last night i sat in my car while my daughter slept for about an hour. During that time my neighbor came out holding this big metal pipe from his jet ski. He didn't see me sitting in my car and dumped the stuff right into the mulch at the base of my new tree. So this morning i go out and i touch the black stuff and smell it and it's oil... i go to my neighbor and ask him not to dump his chemicals on my tree and he looks confused like he doesn't know what I'm talking about. so after i explain to him that i watched him do it he said it's not chemicals but sud water.. i advised that he didn't dump his chemicals in my yard again but what do i do about the oil at the tree????? Also when the tree was planted the state said that the home owner would be responsible if any damage or death were to occur to the tree they've planted.