Well, here's what I did:
Dug down several layers (7 exactly I think).
Dug out a 7x7 base, and laid down a mortared stone floor.
The outside ring also become walls, leaving a 5x5 interior floor. Built a stockpile on that.
Built the walls up 5 levels. A 6th level then became the mortared stone roof.
Laid a layer of dirt over that, completely covering the entire structure.
Tailings still leak.
@emmeck, unfortunately I'm not up to a furnace yet, just got the Bloomery working.
The problem is, there's basically zero documentation I've been able to find on precisely how to encase the tailings. For example, people say dig down, ok, but how far? Do I need to go all the way to bedrock? Does bedrock even exist in this game? Clearly what I've done isn't working as the pollution isn't clearing up - but then again, maybe it is, and I just haven't waited long enough - again, there's no documentation saying precisely how long pollution lasts.
I'm not trying to rip on the game overly here, I understand that it's still basically pre-alpha, and I've put up with the bugs, and the awful FPS I'm getting on the understanding I'm not playing a Gold-release game yet, but the lack of documentation is a real killjoy - many of the entries in the games wiki lead to blank pages, or indeed to no pages at all, and even where entries do exist, they're extraordinarily brief and, to be blunt, not at all helpful.
@stormoffires - thanks for the offer, and I guess I might end up having to join a server. I usually prefer to work things out for myself (I don't want to burden other players with too many in-game "how does [x][y][z] work), but, sans documentation, this has proven so far to be a frustrating experience.
@Reeyaz Sorry to hijack your thread to. Hope you got the research table working though. :-)