The composition of a brewing water has a big influence on the quality of your brew. Brewing water chemistry is rather complex and unfortunately the deep insight given in many brewing texts is very complicated and confusing. But that needn't be so. I'm water chemist and could make big efforts but I've learned that an easy approach will do to adjust almost all waters to any beerstyle.

Let's focus on the essential elements first. Depending on the underground formation the water arises from, it takes up more or less mineral salts. When this salts are dissolved, they form ions -- positive or negative charged elementar particles. Ions of primary concern for the brewer are Hydrogencarbonat (HCO3⁺), Calcium (Ca⁺⁺), Magnesium (Mg⁺⁺), Chlorid (Cl^-), Sulfat (SO4^--) and Hydrogen (H⁺, acidity -- given as pH-value).

Carbonate Hardness = Alkalinity = Temporary Hardness

HCO₃^- + H⁺ --> H₂O + CO₂

As can be seen from the equation hydrogencarbonates are binding acid and thus have the potential to rise pH-level. This leads to some negative effects:

1. As can be seen in the enzymes - table, glucanases, proteases and beta-amylases develop optimal activity at pH-values of about 5.0-5.4. Higher pH-levels are causing insufficiant protein degradation and starch conversion -noticeable by low final attenuation.
2. Extended leaching of high molecular polyphenols from the husks gives adstringent taste.
3. Too high pH levels during the wort boil are leading to insufficient precipitation of proteins (hot break) and extended solution of hop bittering resins. This in turn could be met by reduced hoppings. Mostly however, an unpleasantly scratchy, incessant bitterness will result in the final beer.
4. Finally, lowering acidity has negative effects even during fermentation. Typical signs are insufficient formation of cold break with poor sedimentation of yeast and trub as well as slow fermentings with poor attenuation.

Total Hardness

Calcium- and Magnesium- ions together form the total hardness. Due to precipitation of alkaline Phosphates in the mash these ions have an acidifying effect. Brewing scientist Kolbach has found, that the alkalinity of 1 equivalent Hydrogencarbonat can be neutralized with 3.5 equivalents Calcium or 7 equivalents Magnesium. Unfortunately such a high surplus can only be found seldom in natural well waters.

Residual alkalinity according to Kolbach

The residual alkalinity (RA) is a measure, that describes what remains of the Carbonate-based alkalinity after Calcium and Magnesium have reacted in the mash. RA is the most important parameter for brewing water. Today for pale lagers a balanced RA in the range of 0°dH (deg. German Hardness) is strived for, Pils brewers even go down to minus 5°dH. Melanoidines -- the coloring substances of dark Malts act acidifying -- dark beers go with higher RA's up to 10°dH, but balancing the RA will do no harm to the flavor quality either!

Chlorid

Chlorid adds to the fullness and creates a soft, mild flavor. In the mash Chlorid aids activity of a-Amylase.

Sulfat

Sulfat helps creating hop flavor and aroma. It also adds a somewhat dry bitterness, that however can get incompatible (harsh) with some beerstyles at levels higher than 180mg SO₄^2-/L.