Fred,
Take a look at the "Primer on Plant Nutrition" by Bill Argo, PhD, the guy who came up with the MSU fertilizers, that I have on my free info page, particularly the segment on water quality.
For one, Bill mentions that gpg is an outdated measurement equal to 17.1 ppm. That means your water supply is starting at around 300 ppm, and as hardness is usually expressed in terms of CaCO3, it means your water has fairly high alkalinity.
It's interesting that "alkalinity" - the measure of the amount of acid it take to lower the pH to a certain level - is stronger than pH itself. For example, he states that an alkalinity level of 50 ppm is equivalent to a pH of 11!
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