Added water to hydrofluoric acid and it made a mess?

I'd advise you neutralize the acid. I'm not sure of a great way of doing it with 15L of acid. I know bicarbonate of soda is alkaline (opposite of acid), but I don't think it's strong enough. Being a powder though, if you got hold of enough of it (I don't know how you'd do that in a short period of time) then it would soak up the acid and potentially neutralize it a little bit. Probably best to phone the appropriate company (I have no idea who) and explain the situation. It might be costly, but it could save your basement!