Unfortunately the term 'moderate to high carbonate hardness' was coined by BJCP style guideline committee members who apparently don't understand the difference between hardness and alkalinity. Hardness is not the primary concern in brewing, its alkalinity that is more important. Since a Porter is more likely to have a more acidic grist than a pale beer, the mashing water needs to have a little more alkalinity to avoid dropping the mash pH too low. The 89 ppm alkalinity shouldn't be too bad, but I'm not surprised that a touch of acid is needed to help out that Porter. I'm sure that even more acid malt is needed for pale beer brewing with that water.
Hardness has only to do with the total quantity of Ca and Mg ions in the water. Acidification only affects the alkalinity and not the hardness. The acid reacts with the bicarbonate and converts it to CO2 gas. The acid doesn't remove the Ca or Mg from the water and therefore the hardness is unchanged.
Given the hardness value quoted above, it does not appear that the water has a Ca content of 50 ppm or above. If that is the case, then adding Ca could be used to help offset the need for acid malt in the mash since hardening the water has the effect of increasing acid production from the malt. Just an idea for the future.
