Water makes up 90 - 95% of beer. The alkalinity and mineral content of the water interacts with other ingredients and impacts the flavour of the beer.
Common Minerals in Water
Calcium creates hardness in water (along with Magnesium) and is beneficial to your beer at the correct concentration. Though it will not have a great impact on your beer flavour, Calcium helps lower the pH of your mash and is beneficial to the health of your yeast during fermentation. The optimal Calcium concentration in your brewing water is 50 - 100 parts per million (ppm) in ales, and 40 - 50 ppm in lagers.
Chloride can enhance flavours from malt and give your beer a fuller body, as well as improve clarity in the finished beer. When present at higher levels, Chloride can impart a mineral off-flavour in beer. The optimal Chloride range is 10 - 100 ppm.
Chlorine is used in all municipal water sources as a disinfectant to keep disease-causing pathogens from developing in the water. Chlorine in brewing water can create chlorophenols in the beer, which have a medicinal off-flavour and aroma. Chlorine can be removed from your water by sterilizing (i.e. boiling) the water, filtering it through activated carbon, or by using a Campden Tablet (metabisulfite).
Iron is common at low levels in most groundwater but can be a pollutant in beer brewing at high concentrations. Iron is perceptible at 0.3 ppm and can lead to a metallic off-flavour in your beer. If your municipal water contains high levels of iron, using reverse osmosis (RO) water can help mitigate it.
Magnesium creates hardness in water (along with Calcium) and can enhance your beer flavour at low concentrations. Magnesium can also lead to astringency in your beer at higher concentrations. The recommended range of Magnesium concentration in brewing water is 10 - 30 ppm.
Sodium can enhance beer flavour if used at the correct concentration but can be detrimental to yeast and impart harsh flavours if it is too concentrated. The recommended range of Sodium in brewing water is 0 to 100 ppm.