Legal Question in Real Estate Law in California

Yes. Assuming the tank is in California, after five years there is probably a prescriptive easement for it. The easement holder might also argue successfully that the tank(s) were within the purpose and scope of the basic easement grant. Your chances would be improved significantly if you could show that the presence of the tank(s) interfered significantly with your use of the property -- for example, if in addition to the "septic tank(s)" there were a large leachfield which prevented your cultivating or building upon a significant portion of your land. There are limits on the extent and purpose for which an easement may be obtained by prescription. However, if there is only a tank or tanks, mostly below the surface, my guess is that a court would find a rule under which the tank(s) could remain, undisturbed.

I disagree with Mr. Whipple in part. You cannot obtain a prescriptive easement in secret. So unless you have known about the tank for five years, they don't get very far with the prescriptive easement. Also, again unless you knew about it and did nothing, if the tank extends outside the easement, that is an encroachment which you have a good case for having removed. If the tank is within the easement, however, I agree that the holder might persuade a judge that the tanks are a part of the sewer line system and therefore are within the scope of the easement as granted. Then again they might not. Much more factual analysis would be required to give any kind of informed opinion on that issue.