
Hardness of Water - USGS
Hardness is caused by compounds of calcium and magnesium, and by a variety of other metals. General guidelines for classifiion of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard.