In your first sentence I can tell you're translating from Spanish. 'Contaminación' is a false friend here. We would say 'pollution' or a variant of it in this context. It's also more normal to say what they are dumping, it seems a bit incomplete without it. They are dumping a lot of rubbish and polluting the countryside. Second numbers two and four are perfect. Not quite sure 'dump' is the best verb for number three. The 'in excess part' doesn't work. You're saying don't overfill the landfill, right?

Normally, the word ''dump'' is used as a normal verb with such meanings as to leave, to drop, to throw, to quit. For me, sentences above sound quite right.

All the sentences seem correct. I would only change: "Do not dump into the landfill..." ---- "Dump" can also be used as a noun, like "This place is a dump", which means "This place is messy". Does this help?