Not sure it actually changes the pH, but water is the universal solvent. Take all impurities out of it, and it tries to dissolve things. It becomes moderately aggressive. If you have water of this type, and it is constantly replenished, you need to be very careful with what materials you run it through. This is a guess, but if you were circulating it in a closed loop, it would quickly reach an equilibrium, and you probably wouldn't care much. Water can only dissolve just so much of a material, but it will try its hardest to reach that point until it is saturated.