The Water Preservative Concentrate stuff that came with the barrel only has a one-year shelf life listed on the bottle before you have to use it. I will be keeping regular unscented bleach on-hand also but went ahead and dumped the WPC stuff in the barrel as I was filling it.

From what I've read so far, it's basically unscented bleach but with some kind of stabilizer to make it release slowly over time so it works for 5 years. I went ahead and dated the barrel for fill date and can decide as I go along how often I was to refill the barrel.