I thought it was the other way around, and that exposing it to air increased the acidity(lowers the PH). If i remember correctly from the Water book its due to the water absorbing more CO2 and making carbonic acid.
The water in question comes from a well in a mesic region (close enough to where I live that I can guess what it is like) and as such will be saturated with CO2 when brought to the surface probably at a pH of 6.4 or so. It will, therefore, lose CO2 to the atmosphere until it is at equilibrium with it and with the carbonate and bicarbonate in the water and any chalk that might precipitate. The partial pressure of CO2 in the water is higher than the partial pressure of CO2 in the air and CO2 will move to the air. The equilibrium will occur at a higher pH than the pH of the well.
Now if you take deionized water its pH is 7 and the partial pressure of CO2 in it is 0, less than that of the air so, CO2 will move into the water and the pH will drop.