Hi Tony, it is mainly rain water that has collected through grounds ,fields,rocks. It will contain chemicals used by farmers on their fields etc...but will contain less calc than tap water . it is the same as well water . depending on where you live it will contain different things. Tap water is sewage water that has been treated and in fact not really the best for human consumption..if you are very concerned you can filter your water .I use it for everything except food ,drink or to brush teeth.I also have dry compost toilets.
You can always ask for the water to be tested by the local sanitation inspection services - although they will charge you for it. Unless the source of the water is an underground spring, or a pristine well, it is unlikely to come back as fit for human consumption though. The biggest issue about using it to plug into your washing machine or toilet system is that, as the water used is unmetered, you would essentially be subverting your local assainissement tax assessment because the quantity of waste water used potentially would exceed what you are consuming in domestic water supply, or at least would be obscured. I have been told various things as to the legality of this, ranging from "what they don't know can't hurt them" to "if they catch you, you'll get a massive fine". YMMV
We still have one from too, from when this was a dairy farm. Like our well, it is non-potable but that refers to human consumption only so don't worry about your dog, after all they (ours anyway) drink dirty puddle water and worse. We do know from a reliable source that neither farm nor agricultural water can be used for household connection of any kind. If you inform your mairie about them, for instance by enquiring, they will be recorded and you will have charges, in the form of extra taxe foncière I believe. We use both to water plants and things like that but very discretely.