My wacky tap water

After an extended absence (around 10 years) I have decided to start a new aquarium again, and want to do a planted tank. I live in a different part of the country from where I lived the last time I had a tank, and after looking at my municipal tap water reports and doing some testing I have found something quite unusual that I don't quite understand.

My tap water comes out with a PH of around 9.0-9.2, in spite of being quite soft, around 4.3 ppm of Ca and 0.8 ppm of Mg. If I then barrel the water and aerate or agitate the water for a few hours the PH will drop down to around 6.8, there is no detectable change in Ca, Mg, or alkalinity in the accuracy of my test kits. It looks like in the treatment process they are adding Alkalinity, Chlorine, Chloramine, Fluoride, Silica, and Sodium in addition to raising the PH by 2.5 points (http://www.mwra.state.ma.us/monthly/wqupdate/pdf/2008data/112008.pdf on page 9 for "Carroll Water" between the raw and treated water).

Why would the water have such a dramatic change of PH in such a short amount of time after coming out of the tap? This seems like it could be a problem for large or even moderate water changes unless I am willing to barrel the water perhaps overnight.

Thanks,
~Droid