Calcium hardness/Total hardness.

Has anyone actually experienced a situation where they had high hardness but low calcium hardness? I know its theoretically possible but I suspect it's rare. I think the principal contributor to magnesium hardness in water is dolomite. As I understand it, very large concentrations are rare on Planet Earth. In my area, dolomite is associated--in a limited distribution--with lead and zinc mining.

When I filled my pool after resurfacing, total hardness was low. I can't remember the exact number but it was less than 100. It was probably 70 because that's what our lake water tests at. I suspect that hardness is largely calcium based because of limestone formations under the lake and in watersheds. I added calcium carbonate. The water hardness increased to about 220 and has since risen to 300 without any additional calcium carbonate.

Where did the additional hardness come from? As it happens, I harvest rainwater from a roof into the pool. Apparently, ground or pulverized limestone (calcium carbonate or GCC) or dolomite (calcium magnesium carbonate) are used extensively in asphalt shingles. So it looks like I have a situation that may call for calcium hardness testing. Thanks for alerting me to this. Rebecca