Because water is a universal solvent, rainwater collects particles and chemicals even as it's falling. Geological strata only add more to the composition the minerals and trace elements of the local area give each water its distinct terroir. Underground geology may filter water for decades or even millennia; when the water finally emerges at the source it has terroir.
Calcium and magnesium levels combine to determine the mineral water’s “hardness”. Bottled water is naturally soft, thanks to low levels of calcium and magnesium. Higher levels are often found in municipal water, which is often “softened”—particularly in the United States—to be used at home. The taste of water is impacted heavily by softening.