No, you can't stop them from building a fence on her own land, even if it interferes with your septic bed. In fact, she can require you to relocate the bed--and pay to remediate, or clean-up, restore, etc.--her property. The fact that the bed was on her property for "many  years" gives you right to have it there or over her property more generally. (Note: you do not meet the criteria to try to claim the land under "adverse possession" because a septic bed under her land does not qualify as "exclusive" use--preventing her from using the land, too--or sufficiently "open and notorious"--that is, an unmistakable, obvous possession of her land by you in derogation of her rights.)You can have the land re-surveyed--maybe you actually *do* own that part--but if it is her land, she can put up her fence and make your relocate the septic bed.Or you can negoitiate with her: maybe she'll sell you some of the land, or agree to not put up a fence and let you keep the bed there in exchange for licensing or rental payments.