Small concentrations of calcium carbonate combat corrosion of metal pipes by laying down protective coatings.  High levels of calcium carbonate precipitate on heating to form scale in boilers, pipes and cooking utensils causing various plumbing problems.

Acceptable results:  No health standard, but 50 ppm or less is ideal.

Sources:  Limestone deposits, gypsum and gypisferous shale

Recommended testing:  If there is a scale build up in plumbing fixtures, suspect hard water or consider installing a water softener.