“We have had a septic system on our land for 10 years and owned the property for 20 years . The bourough wants to move the road into our property over our septic system. Is this legal? I thought the septic system has been there so many years it’s a permanent structure?”
The borough is a governmental agency, and as such has the power of eminent domain. That is, they can take all or part of your land for public use (such as a road). It doesn’t really matter what is on the land being taken.
Under the U.S. Constitution, when a governmental agency exercises the power of eminent domain, it is required to reimburse the owner of the land. If that is what they are doing, unfortunately, your only real argument is over how much you will get paid. You will most likely want to get an attorney to help you with that argument.