Hard water is water that contains high concentrations of minerals. The ions that are of concern are calcium (Ca²⁺) and magnesium (Mg²⁺). These minerals are naturally occurring and enter the water as calcium carbonate (limestone) or calcium sulfate. The source of magnesium is called dolomite and has the following formula: CaMg(CO₃)₂.You can easily determine if you have hard water in your home by doing a simple test called lather (or froth) test. Soap usually lathers (or creates foam) with soft water, which is difficult to do with hard water, hence the terminology “hard water”.

Water hardness can also be determined using analytical methods such as wet titration or using analytical instruments.

In order to have an idea of the different grades of water hardness, the following is a rough estimate of level hardness compared to mineral concentrations:

• Soft: 0 - 20 mg/L as calcium
• Moderately soft: 20 - 40 mg/L as calcium
• Slightly hard: 40 - 60 mg/L as calcium
• Moderately hard: 60 - 80 mg/L as calcium
• Hard: 80 - 120 mg/L as calcium
• Very Hard: 120 mg/L as calcium

The World Health Organization says, “There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans.”

Hard water in the US

According to the United States Geological Survey, 89.3% of US homes have hard water. The softest waters occur in parts of the New England, South Atlantic-Gulf, Pacific Northwest, and Hawaii regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes, Pacific Northwest, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. Hardest waters (greater than 1,000 mg/L) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.

Hard water in Canada

Prairie Provinces (mainly Saskatchewan and Manitoba) contain high quantities of calcium and magnesium, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide from the last glaciations. In these parts of Canada, the total hardness in mg/L calcium carbonate equivalent frequently exceeds 200 mg/L, if groundwater is the only source of potable water.