Barrels mostly seal by moisture swelling and internal pressure from the liquid inside.What's the goop around the rim of the lid? is it some kind of sealant? I was under the impression that barrels were made leak proof by pressure and swelling.
Unfortunately, wood being an organic material, a perfect seal is nearly impossible, so for centuries , barrelmakers have used beeswax to seal drips on seams.
In modern times, commercially available barrel seal material is available, which is far more expensive and mostly made from beeswax.
Another tip from an old timer, if you don't nearly fill your barrel, you may want to rotate your barrel so that the seams not normally in contact with liquid don't dry out and leak.
I learned this stuff back in the 20th Century when my wife worked for Colonial Williamsburg and I got to hang out in the various craft shops.