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Water Hardness

Hardness in water is usually observed by its ability to form scale and softness, and is observed by its ability to make suds with soaps and detergents. If your water leaves deposits in sinks and tubs, then it is moderately hard to hard. If you get lots of suds with soap, then the water is soft. Water hardness varies across the country and around the world. Denver’s water can be classified as soft to moderately hard.

Denver’s water is slightly harder in the winter when our waterways and reservoirs freeze, allowing the water to absorb more minerals. It’s softer during late spring through fall when the runoff dilutes down some of the minerals as the water makes its way into our reservoirs.


Hardness in water is defined as the sum of the calcium and magnesium concentrations, expressed as calcium carbonate. The hardness of the water varies with the amounts of these salts.

The U.S. Geological Service classifies soft water as anything under 60 milligrams per liter of calcium carbonate. Hard water is identified in the range of 121 to 180 mg/L. There are no health concerns associated with calcium carbonate levels, but hard water can be a household nuisance and leave a chalky or mineral aftertaste. 

Denver Water uses two main collection systems for its supply. The southern collection system is considered moderately hard and the northern collection system is considered somewhat soft. For information on Denver Water’s hardness concentrations please visit our Water Quality Reports webpage.


The earth's terrain is rich in mineral content. Subterranean and surface waters absorb some of these minerals. Absorbed minerals include compounds of calcium and magnesium carbonates, bicarbonates, sulfates and chlorides. These naturally occurring minerals give water its hardness.


Very hard water can leave a chalky or mineral aftertaste. Water that is too soft leaves a flat, unpleasant taste. A balance needs to be maintained. Hardness versus softness are not health concerns, but aesthetic ones. Our goal is not only to maintain a safe product, but also one that tastes, smells and looks good.


Denver's water tends to form scale. An initial scale usually takes two to five years to form and continues to build up over time. In older areas, while thick scale reduces rate of flow, it prevents water from being in contact with the plumbing. This protects pipes from corrosion and customers from potentially harmful metals such as lead. When using a cold water humidifier, hardness in the water can cause residue deposits on furniture. Deposits can be minimized by using half tap water and half distilled water in the humidifier.

It is no longer an issue with laundry since most modern detergents contain water softeners.