General Information: Water hardness is commonly defined as the sum of the polyvalent cations dissolved in the water. The most common such cations are calcium and magnesium, although iron, strontium, and manganese may contribute (AWWA, 1990; EPA, 1986). Hardness is usually reported as an equivalent quantity of calcium carbonate (CaCO3). Generally, waters are classified according to degree of hardness (EPA, 1986). The hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l), or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.

The Environmental Protection Agency (EPA) establishes standards for drinking water which fall into two categories -- Primary Standards and Secondary Standards.  Primary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosivity, foaming, and staining properties of water. There is no Primary or Secondary standard for water hardness.  Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:

Classification	mg/l or ppm	grains/gal
Soft		  0 - 17.1		   0 --1
Slightly hard	17.1 - 60		  1 - 3.5
Moderately hard	 60 - 120		3.5 - 7.0
Hard		120 - 180		7.0 - 10.5
Very hard		180 & over	10.5 & over

NOTE:  Other organizations may use slightly different classifications. This information can be found at

Hardness is primarily a function of the geology of the area with which the surface water is associated. Waters underlain by limestone are prone to hard water because rainfall, which is naturally acidic because it contains carbon dioxide gas, continually dissolves the rock and carries the dissolved cations to the water system.

Numerical Categories: 

Designated Use                         Limit (mg/l CaCO3) 
Industry (raw water source) (EPA, 1986)
     Textile			   120 max concentration
     Pulp and paper		   475 max concentration
     Chemical		1,000 max concentration
     Petroleum		   900 max concentration
     Electric Utilities		5,000 max concentration

Health Effects: Not applicable

Environmental Effects: The effects of hardness on aquatic life is a function of the cations comprising the hardness.

Irrigation Effects: Carbonate deposits may clog pipes and coat the inside of water holding tanks. Extreme hardness may interfere with chemical processes.




  1. Nonpoint source:
  2. Point source: Inorganic chemical industry may release dissolved cations in effluent waters (EPA, 1986).
Mode of Transport: Dissolved cations are carried via overland, unsaturated, or saturated flow.

Analytical Techniques:

(APHA, 1992; Nebraska Administrative Code, 1993)

  1. Calculation: Use following equation to compute hardness from results of separate determinations of Ca and Mg concentrations.
    Hardness, mg equivalent/L CaCO3 = ([Ca,mg/l]*2.497) + ([Mg,mg.l]*4.116)


  2. EDTA Titrimetric Method: EDTA forms a chelated soluble complex when added to metal cations. Dye added creates a wine-red solution. EDTA added as a titrant complexes with the Ca and Mg. Solution turns blue when all complexed. A pH of 10+/- 0.001 is recommended for sharpest endpoint. IMAGE

Additional Information: