Small water supplies using groundwater often encounter significant levels of
hardness, but some larger surface water supplies also have the same issue. Calcium
concentrations up to and exceeding 100 mg/l are common in natural sources of water,
particularly groundwater. Magnesium is present in natural groundwater usually at
lower concentrations (from negligible to about 50 mg/l and rarely above 100 mg/l), so
calcium-based hardness usually predominates