Water consisting of water molecules alone does not occur in nature. Undistilled water is always “polluted” with many salts that are dissolved in the water.
Brewers use groundwater that comes up spontaneously in a spring or else is pumped up. Water pumped from deep underground is the least polluted, which is why brewers prefer to extract water from the deep underground layers. The closer to the North Sea, the saltier the groundwater. Various West Flanders breweries thus brew with salt-rich water that gives the local beer a typical salty taste.