Only thing I can figure, Pure, is that the water source is a deep, soft rock (carbonate) aquifer of which the surface land use is largely forestland and the acidic rainwater percolating through the leaves and other detritus results in a higher concentration of carbonic acid being maintained in the ground water. Probably karst geology dominates the immediate recharge area.
Usually means cleaner water that is extremely hard, low in dissolved oxygen, cold, and oh-so good on a hot summer's day, straight from the tap and containing more calcium per glass than milk! Also means that one should age or rest their water overnight, with an aerator, before adding to a populated tank.
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Gotta say it today....I love this place (the forum)!@!