Concrete ideally should have clean water for best strength, but it need not be potable. Salts, silt, organic matter, and small bits of trash all weaken the concrete to some, greater or lesser, extent. But the effects can be taken into account in the structural design. For that matter, there are additives that are commonly added to concrete under special conditions, for example to lower the freezing point in winter pouring, that also weaken it in similar ways.