Cisterns.... Keepin' it wet.
Doesn't really matter. If they get sick, they get sick and they're not in the states, drinking tap water any longer.
Yours doesn't come out of a tap?
The only long term ill effects I remember hearing about cistern water was years ago there was a report of older residents who were very mineral deficient because of drinking only rain water for years had depleted the trace elements normally replenished and present in groundwater and a weekly OTC mineral supplemented was recommended.
I personally do take a mineral supplement because of that idea which seemed to be a reasonable theory.