Ah yes, the freshness of the water is dependent on the source it originally comes from. So if that becomes contaminated, then sad to say, the water storage becomes contaminated. However, precautions can be made by installing filters, it would just depend on how much the homeowner is willing to go.
I personally try to keep a back up of the back up so if option 1 fails, I'm not complete left in the dark.
So yes, every time the home owner uses water in their house, it's renewed that often but if the water source is bad, then there may not be much we can do.