We also had reserve septic locations here in my area of PA. Although I have a sand mound and I'm on 5 acres, I had enough space in the rear yard to construct the pool..
When the home was built (About 20 years ago) the board of health required an alternative septic location should a failure occur. Generally it's not advised to rebuild a sand mound in the same location that one has failed.
However, a few things have happened in the last 20 years. For one, a Sand Mound like I have can be rebuilt 1 time in the same location now. The thinking is: If it lasts (lasted) 20-40 years in the same spot, it will most likely perc again in that spot.
One other option (for me) is a drip irrigation system which (1) is allowed to be in the same spot, (2) is a bit cheaper to construct and (3) is a more current design than a sand mound.
I'm not sure about your area, but here in my area it wasn't until 2007 that my local municipality adopted their own zoning regarding sand mounds and septic systems. The general rule is: If locally there are no regulations, usually the Local Board of Health will oversee the construction of the septic systems and their requirements. The EPA has some guidelines that the local board of health usually follows.
One thing you could do is this:
Contact your local board of health. They will be happy to send or email the guidelines for your area. In my area, they no longer require a reserve location (I'm a commercial real estate broker but I run into this all the time with my residential agents. I spent about 100 hours this year alone at zoning hearings that usually consist of some sort of outdated septic "rule".). The idea of a reserve location (again... here) is an outdated and unneeded concept in most areas. Technology is much better now and frankly, they cram so many homes in a small area now, it's become a real pain to the developers.
I'm not a big fan of development myself but checking with the board of health may indeed help you get a clear picture of what your options in your particular area.
A "movement of earth" of 2 feet or so would (in most cases) not even effect your reserve location should you need it at a future date.
Hope that may help a little.