55 Gallon water storage questions

I picked up a couple of blue 55 Gallon water storage barrels recently at a garage sale. I've read online that it is not advisable to use these barrels for drinking water if you don't know their history as the plastic could contain a lot of containments rending the water unsuitable for drinking. As such, I have cleaned out these barrels and sterilized them with bleach and have now filled them up. My plan is to use them for sanitation purposes in the event our water goes down (I have also purchased 2 new 55 Gallon barrels I'll use for drinking).

My question: I'm storing these used 55 water storage barrels in my garage and have put them on 2x4's so they're not resting on the concrete directly (I've read in many places that by resting on the concrete directly, they can pick up more containments ... not sure if this is true, but better safe than sorry). Being stored in the garage, they're basically in the dark all the time since we don't leave the garage door open. I filled them up with tap water which has chlorine already, but I'm trying to figure out do I need to add more water chlorine to the water to prevent bacteria growth? They're both sealed off. During the summer, the garage can become an oven (easily 120F+ if not hotter) so I'm not sure what I can do prevent bacterial growth long term in these barrels. I'm willing to rotate the water out annually, but was wondering if anyone in the community as any advice on what I should be doing to extend the water life on these barrels. In a worse case scenario, I'd be open to drinking the water in these barrels after filtering the water of course. Should I be adding additional chlorine? From my understanding, chlorine begins to degrade over time and with increased temperature, the chlorine will break down even faster.