One of the more common sources of sodium in water is from the dissolution of clay and felspar minerals.
Look now: you can see big crystals of felspar, and maybe the sparkle of mica.
British Dictionary definitions for felspar
any of a group of hard rock-forming minerals consisting of aluminium silicates of potassium, sodium, calcium, or barium: the principal constituents of igneous rocks. The group includes orthoclase, microcline, and the plagioclase minerals