Here's my $0.02 on purchasing used barrels for potable water. It's going to be virtually impossible to remove all potential contaminants. You can wash, scrub, bleach and otherwise clean until your face is as blue as the barrel, but the simple fact is they'll never be "clean". Even the smallest particulate can turn into mold or bacteria. Be sure to filter and purify your water before you drink it if you store it in used barrels.