Question:

 Are there any concerns about drinking water with a pH outside the recommended range of 6.5 - 8.5? 

Answer:  

From a health effects standpoint, a wide range of pH values can be tolerated by persons consuming water.  However, values outside the range of the secondary standards of 6.5 to 8.5 can cause increased corrosivity.  Corrosive water tends to dissolve metals (e.g., lead, copper) with which it comes into contact.  Elevated levels of metals such as lead in drinking water are known to cause adverse health effects additionally, at high pH values, the ability of chlorine to provide disinfection diminishes and the formation of halogens (e.g., trihalomethanes) increases

(42 FR 17143; March 31, 1977)

Source: EPA Standard for Safe Drinking Water Website