You cannot use groundwater for plastering.
Do not try to plaster the house yourself unless you are skilled at it.
Do not mix plaster using groundwater or you will likely make a mess of it.
Plastering requires very clean water because the particles are "white" and very fine.
Any impurity at all, even a small amount of dirt on a mixing rod, will discolour the plaster and ruin the mix.
Plasterers are known for taking a long time to prepare their mix in order to ensure that there are no dry lumps in it.
This applies to a "clean" mix, but their task becomes impossible when they attempt to do this to a mix with impurities in it.
Small dirt particles do not dissolve readily, get taken up on the trowel and will cause local smears when the coat is applied to the wall.
Larger particles may corrupt the mix, creating lumps which in turn "bump" the trowel, making the coat uneven and impossible to "finish" smoothly.
High levels of salts or other chemical impurities in the water may disrupt the chemical bonding process turning the paster into something else entirely.
In this final case, the water must not only look clean, it must also be chemically clean to ensure the good appearances, durability and hardness of the finished plasterwork.
Hope this helps.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.