If it is a genuine whiskey barrel, Daisy, remember its previous life. When new the inside of the barrel was charred and then for ten or twenty years, not necessarily continuously, it has been filled with high strength spirits. This has soaked into the wood and will now slowley seep back out into the pond water. This is most likely the reason for the black sludge in the barrel and it would probably be best to line the barrel.
ps. if you choose to drink the contents of the barrel you don't need to add extra water, hic!

Hi thanks for that, did suspect that but also thought because it had whiskey in it would be pure and clean but see what you mean. Have liner from previous barrel but far too small for this one. Would black plastic be OK do you know? and what is best way of finishing off at the top of barrel don't want it to look ugly. The lady who delivered barrel said she had been smelling whiskey all day and I know what she meant, for days it was lovely just to stand in front of empty barrel and breathe in the fumes!

Just lined two barrels for exactly this reason. Plants don't appreciate the extra organic content!. A cheaper quality liner is fine, as it doesn't need to be something as strong as butyl. I just paid £23 for a 3m by 2m piece (it only came in 3m widths). You'll need about 1.5m square to fold in and fully line the barrel. I then used the upholstery dome topped pins to fix the liner after I'd folded it all in, pulled it fairly taut over the rim - fastening it about an inch or so from the top on the outside.

Thanks for that. Good idea about tacks will look much better than the staples I thought would have to use. Cheers.