Calcium (Ca)
Magnesium (Mg)

The term hardness refers to the quantity of dissolved calcium and magnesium in water. These minerals, which come primarily from limestone type rock formations, are found to some degree in almost all natural waters. Calcium and magnesium cause problems for two principal reasons:

• When the water is warmed, they precipitate out of solution and form a hard, rock-like scale. This scale accelerates corrosion, restricts flow, and reduces heat transfer in water heaters and boilers.
• When they combine with soap, they react to form a curd, which interferes with cleaning, dries out skin, and leaves deposits on plumbing and clothes (bathtub ring; ring around the collar).

Hardness is measured in parts per million (or the equivalent mg/L) or in grains per gallon (gpg). Note: if the water analysis is given in ppm as CaCO3 then 1 gpg = 17.1 ppm. A common aspirin tablet weighs 5 grains). There is no established limit for the acceptable level of hardness in water, but it is generally considered to start to become problematic at around 3 gpg.

Levels of hardness are referred to as follows:

Soft Water…………………………..0-1 grains per gallon (gpg)

Slightly Hard Water……………….1-3.5 gpg

Moderately Hard Water…………..3.5-7 gpg

Hard Water………………………….7-10.5 gpg

Very Hard Water…………………..over 10.5 gpg

Waters which naturally contain very little hardness can also be problematic because they may be corrosive in some applications (see acidity