Carbonic acid forms alkaline bicarbonates with calcium, sodium, potassium, and other group 1A and 2A elements (and other cations). So the mere presence of these minerals (dietary) acts as a buffering system in the water. The amount of H2CO3 in the water will remain constant because the consistant partial pressuer of CO2 in the atmosphere (Henrey's Law) and the acid-base equilibria will remain undisturbed (Le Chatlier's principle). So mineral water will always remain BASIC and distilled water will become ACIDIC if exposed to the atmosphere. I think the type of water consumed may determine the pH of the blood to some extent.