As it leaves the skies in the form of rain, water is soft, meaning it is clear of impurities. But it picks up carbon dioxide and other impurities in the air. It also dissolves calcium and magnesium salts as it soaks through the earth and runs off into rivers or lakes and collects in reservoirs or wells. If the water, in these processes, picks up a substantial amount of invisible dissolved rock, calcium carbonate and other substances, it becomes "hard" a defined term.
Because the unique mixture of minerals dissolved in the water, its pH and temperature determine the effect of water hardness, a single scale somewhat over-simplifies, but hardness is generally defined using this scale:
- Soft 0-60 mg/L as calcium carbonate or < 3.5 grain hardness per gallon (gpg)
- Moderate 61-120 mg/L as calcium carbonate or 3.5 to 7.0 gpg
- Hard 121-180 mg/L as calcium carbonate or 7.0 to 10.5 gpg
- Very hard >180 mg/L as calcium carbonate or 10.5 gpg and higher
Hard water locations in the United States -- US Geological Survey map
U.S drinking water contains an average of 7.1 grains per gallon/121 mg/L of hardness. The U.S. Geological Survey calculates that 89.3% of U.S. homes have hard water with the hardest water found in the belt running from Kansas and Texas on the east to Southern California to the west with up to ten times the minimum defined concentration of calcium carbonate to be "hard." There are indications water is becoming even harder. The ever-increasing demand for water has lowered the water table, causing a greater draw-off at the surface which raises the natural hardness of water.
Canadians face the same challenges.
Hard water is an aesthetic and economic consideration, not a health concern. Even though modern diets are deficient in calcium and magnesium, such small amounts are available in drinking water that the better dietary solution is to drink a glass of milk. The World Health Organization studied the question and concluded that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."