What rights do I have when my neighbor wants to use my land for a drain for their septic system?

Your neighbor doesn't have the right to put a drain on your property unless you give it to him/her. I would strongly advise against this. First, there is the issue of allowing a drain where you don't control the effluent that might enter the river - you could violate antipollution laws if his septic system isn't correctly installed and maintained. Second, this isn't rainwater we're talking about, this is toilet water with the risk of major yuck contamination if the drain fails. If you do decide to allow him/her to place a septic system drain across you're property. 1. require that he/she give you plans for the entire system which have been approved by a professional engineer (which means one who is licensed by the state for approving projects). 2. require that the work be inspected and approved by a testing firm or professional engineer prior to it's being buried.