A quick search came up with this explanation.
I heard that we're not supposed to store our water on concrete...why not, and how do you suggest storing it?"
According to preparedness lecturer Kenneth Moravec: "Concrete attracts fluids and 'bleeds.' Anything that has been on or in that concrete will find its way into your plastic water barrel. This includes the lime in the concrete, any hazardous materials (i.e. gasoline, oils, kerosene or anything a contractor used in construction), algae, etc. Usually it is not enough to make the water toxic but it will taint the water enough to make the taste unbearable. And no amount of pouring it from container to container will take that taste away."
The best way to store that water is on two by fours making a pallet out of it.