Drinking Water Contaminant - Hard water

Drinking Water and Human Health December 06, 2010 Print Friendly and PDF


Sources of hardness minerals in drinking water

As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. The two most common minerals that make water "hard" are calcium and magnesium. The degree of hardness becomes greater as the calcium and magnesium content increases. Other ions that produce hardness include iron, manganese, strontium, barium, zinc and aluminum but, as a rule, these are not present in significant quantities.

Hard water impacts nearly all household cleaning tasks: dish washing, clothes laundering, bathing and personal hygiene. The amount of dissolved minerals in water (degree of hardness) affects the amount of soap and detergent required to effectively clean. Soap used in hard water combines with the minerals to form a sticky soap curd. Some synthetic detergents are less effective in hard water because the active ingredient is partially inactivated by hardness. Bathing with soap in hard water leaves a film of sticky soap curd on the skin. Soap curd on hair may make it dull, lifeless and difficult to manage.

When doing laundry in hard water, soap curds lodge in fabric during washing to make fabric stiff and rough. Incomplete soil removal from laundry causes graying of white fabrics and the loss of brightness in colors. A sour odor can develop in clothes. Over time, laundering in hard water may reduce the life of clothes.

In addition, soap curds can deposit on dishes, bathtubs and showers, and all water and plumbing fixtures. Hard water also contributes to inefficient operation of water-using appliances. Heated hard water forms a scale of calcium and magnesium minerals that can contribute to the inefficient operation or failure of water-using appliances. Pipes can become clogged with scale that reduces water flow and ultimately requires pipe replacement.

Potential health effects of hard water

Hard water is not a health hazard. In fact, the National Research Council (NRC) states that drinking hard water can contribute a small amount toward total calcium and magnesium human dietary needs.

Much research has been done on the relationship between water hardness and cardiovascular disease mortality. Numerous studies suggest a correlation between hard water and lower cardiovascular disease mortality. The NRC has recommended further studies on this relationship. A worldwide study on the effect of changes in water supply hardness on cardiovascular disease is being considered by the World Health Organization.

Hard water treated with an ion exchange water softener has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water. For example, if water has a hardness of 10 grains per gallon, it will contain about 80 mg/liter of sodium after being softened with an ion exchange softener if all hardness minerals are removed. Because of the higher sodium content of softened water versus the potential benefits of drinking hard water, some individuals may be advised by their physician not to install water softeners, to soften only hot water, or to bypass the water softener with a cold water line (usually to a separate faucet at the kitchen sink) to provide non-softened water for drinking and cooking.

Softened water is also not recommended for watering plants, lawns, and gardens due to its sodium content.


Public water suppliers test for the hardness level of the water they deliver. While the U.S. Environmental Protection Agency (EPA) establishes standards for public drinking water, there is no standard for hardness.

Consumers of private well water can use a home-use, water testing kit to approximate the hardness of their water; accurate measurements can only be obtained, however, through laboratory analysis.

Water hardness is often expressed as grains of hardness per gallon of water (gpg) or milligrams of hardness per liter of water (mg/L). Classifications of water hardness expressed as calcium carbonate follow:

Classifications of Water Hardness
Classification Parts per million or mg/l Grains per gallon
Soft 0 - 17.1 0 - 1.0
Slightly hard 17.1 - 60 1.0 - 3.5
Moderately hard 60 - 120 3.5 - 7.0
Hard 120 - 180 7.0 - 10.5
Very hard 180 and above 10.5 and above

Options for Hard Drinking Water

Water softening units can be installed into the plumbing system to continuously remove calcium and magnesium. There are a variety of water treatment methods with corresponding differences in costs and effectiveness. A table comparing some of the most popular methods can be found at: http://www.hardwater.org/water_treatment.html.


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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.