In case you haven't had your answer yet.
Try encasing it in mortared stone at least 10 blocks below the nearest dirt/water blocks. I have heard various advice on this matter and the general rule is deeper is better.
I have also heard that you can now encase tailings above ground now too. Has not been tried on the server i am on currently but If you are impatient then it is probable worth a try just be prepared for the problems that may follow.
Also make sure it is within a stockpile.