I've used water with a tds of up to about 25 - 30 with no problems. It makes me wonder why I change the resin when it gets above about 10. Maybe I fall for the hype that it has to be zero.
Also, Lee's paddling pool. Apart from I don't believe it had a tds of 0001 because to get 6" of rain water takes months and months and months, the amount of debris built up in that time would have contaminated the water as it rots and forget the actions of sunlight and green algae, etc etc, what about all the dirt and that in the water?
The purer the water, the more it will absorb contaminates because that's the nature of pure water. I tested my sisters water butt in her garden which gets used regularly and has a lid on it. TDS was 086 yet a paddling pool was 001.
I could be completely wrong though about the paddling pool but I don't think I am.