What makes water hard?

Water is referred to as the "universal solvent" because, over the course of time, water will dissove or erode almost any material that it is in contact with.  It is this natural occurance that attributes to the hardness of water.

Water hardness is normally referred to as a measure of the soap or detergent consuming power of water.  Technically, hard water is water having a high concentration of calcium and magnesium ions.  These, along with other minerals, are commonly present in all natural water.

When water that contains any degree of hardness evaporates or is heated in typical household water heating equipment, it can leave residual mineral deposits.  In the water industry hardness is expressed in terms of milligrams per liter (mg/l).  In the water treatment business, however, hardness is most often expressed in terms of grains per gallon (gr/gal).  The conversion factors is 17.1 mg/l equal 1gr/gal of hardness.

The table below describes the various textbook levels of hardness and their classifications:

Description Hardness (mg/l) Hardness (gr/gal)
Extremely Soft 0-45 0-2.6
Soft 46-90 2.6-5.2
Moderately Hard 91-130 5.2-7
Hard 131-170 7.6-9.9
Very Hard 171-250 10.0-14.6
Excessively Hard over 250 over 15

As water seeps through the ground (or percolates) to reach the aquifers, it is filtered and purified through the many layers of earth at the same time, water may dissolve and retain the naturally occuring minerals it comes in contact with.  This is why ground water (or well water) does not usually need to be treated or filtered.  Higher levels of dissolved solids, constant cool temperatures, and low levels of dissolved oxygen characterize ground water.  However, ground water may contain an abundance of the minerals that contribute to hardness.

Water that comes from streams, rivers and lakes is exactly the opposite. Surface water accumulates mainly as a result of direct rain or snow.  It does not percolate through the ground and does not pick up the elevated levels of dissolved minerals that attribute to water hardness.  For the most part, surface water is referred to as "naturally soft", although is is not mineral free.  In general, turbidity, suspended solids, rapid temperature fluctuations, and high levels of dissolved oxygen characterize surface water.